Some promotional material from the work I did with the EPMM group at Sheffield University…
New job!
Sorry it’s been so long! After finishing my contract at Sheffield University, I was out of work for a couple of months and desperate to get a job sorted. To this end, I’ve been working hard to improve my knowledge of C and it seems to have worked! I was offered a job as a graduate embedded software engineer at Cambridge Medical Robotics three weeks ago. So far so good – They’re a really friendly bunch of talented people and I’m learning a lot!
A big thanks to my mate Jan for putting me forward for the job and inspiring me to go for it and thanks to Al Kelley and Ira Pohl for their book on C. If you’re thinking about a move into software, then my advice would be to get stuck in. My experience was really positive at all the interviews I attended and hard work is rewarded. Put in the time, really learn your stuff and it will happen.
A solar simulator on a budget
Light source
To get the high light intensities that I needed for this project, I hunted around for a high-performance high colour temperature white LED and came up with this one. It’s a Cree XLamp CXA2520 high lumen output and efficacy LED array. I chose the 5000K version as I wanted something that would be closer to sunlight. The device delivers 2500Lm white light at 36V and draws 0.5A. I liked the fact that it was a chip-on-board assembly that was ready to mount. I tried a smaller device but cracked it when I tried to mount it on a heatsink.
Heat considerations
We really need a heatsink here because, even though LEDs are efficient, there is still quite a lot of heat to get rid of – 20W if we assume that all electrical power is converted to heat (obviously this is the worst case scenario given that a significant amount of power should be converted into light and radiated away*). Keeping the temperature down increases the efficiency of the system and the lifetime of the LED. More importantly, we don’t want to alter the environment around our solar cell too much as this would bring in an uncontrolled variable.
I found a CPU fan/heatsink laying around and looked into bonding it using adhesive thermal tape. Assuming the thermal resistance of the fan/heatsink is 0.4K/W, and that the ambient temperature is 20C, then the heatsink will run at 28C – hopefully, the LED will be in equilibrium with this so will also be at the same temperature. I checked the specs of the heat transfer adhesive and it seems its performance is predicted to be really good. To be able to transfer 20W heat power, it would need a temperature difference of only 0.001mK across it – so the LED would be at pretty much the same temperature as the heatsink surface we can assume.

Power output
This is the most important part: the power output calculation. We need to know how much light the LED actually is going to deliver to our solar cell – in the lifetime tester application, I envisage that each solar cell under test will be assigned its own LED and this way the system would be truly modular.
On to the calculations then…What we want to know is the light intensity (irradiance) on the solar cell front surface which is simply the light power per unit area. For instance, 1 Sun illumination has an intensity equal to 1kW/m2 which is itself a unit of irradiance. Here’s how we work this out:
- From the datasheet, the manufacturer told us the light output from the LED is 2500 Lumens at the rated operating condition. This is our starting point. Lumens are a measure of luminous flux or perceived light power. This is a measure of the total amount of visible light emitted – luminous flux is the total luminous energy emitted from the source per second.
- The thing is we want to know how much “real” power the LED emits in Watts. Basically, our eyes are setup to be sensitive to some wavelengths over others (the peak of the eye response happens to be tuned to the sun’s peak emission per nm which is green light at around 500nm – let’s not get drawn into a discussion about evolution here). Measuring the light output in Lumens tells us how bright the LED will be to our eyes but doesn’t tell us how much power there actually is. We need to convert units and to do this, we need to know what colour the light is. If you remember, I said that our eyes have a peak sensitivity to green light. So green light has the most number of Lumens per Watt, 683 Lm/W. Other wavelengths have less. This Lm/W number is referred to as luminous efficacy of radiation – it relates luminous to radiative flux and tells us...for a given amount of light energy, how much does this stimulate our eyes. Weird huh. Don’t get this confused with luminous efficacy of the source which is a measure of the overall efficiency of the LED in converting Watts of electrical input into Lumens of emitted light (126 Lm/W in this case). In fact, increasing luminous efficacy is one way to increase the LEDs apparent efficiency; if we made it green, it would be about twice as efficient.
- But we don’t have a green monochromatic light source, we have a white light source? So we need to average the contribution from all the different wavelengths that make up the emitted spectrum from the LED. This gets a bit complicated. Fortunately, we can make some assumptions. Let’s assume that the spectrum of the LED approximates a blackbody that has been truncated to the visible region (normally a blackbody emitter would radiate light in the NIR and UV that we can’t see so the luminous efficacy would be much lower overall). So the luminous efficacy of radiation will be 350 Lm/W. From this, we know the total radiant power output from the LED will be 2500 / 350 = 7.1 W. We’re getting there.
- The total radiant power is helpful but we need to know about intensity, or the number of Watts emitted over a given area. One way to go would be to assume that it’s distributed evenly over space but a better way is to assume that light emission follows Lambert’s cosine law; lambertian sources have the same brightness no matter at what angle you look at them even though their emission is not uniform. Let’s not get too drawn into the specifics here other than to say that the light intensity follows a cosine law with angle and LEDs are often approximated to lambertian emitters. So why break tradition? OK then we can now say that the peak intensity in the forward direction will be 7.1 W / π = 2.3 W / sr where sr stands for steradian (a unit of angle in 3D space. Imagine the surface of a sphere rather than the arc of a circle).
- To get the power on the front surface of our solar cell then, we just need to know how many steradians it covers and multiply. For a 2mm x 2mm (0.04 cm2) solar cell positioned 2 cm away from the LED (face on), I expect it to cover approximately 0.031 sr (using the formula for a cone with spherical cap) which would give us 71.3 mW incident flux and an intensity of 1781 mW / cm2 or 18 Suns! At a more reasonable distance of 5 cm, we would still have 3 Suns which would be plenty.
I’ve included the details of all these calculations in this sheet.
Mounting
When I mounted the LED, I was concerned about applying enough pressure to ensure a strong bond and good thermal contact. Here, they recommend pressures in excess of 100psi! I managed only 8psi. Basically, I was concerned about breaking the LED board. I had to rest a power supply on top of a toothpick box – it seemed to be just the right size to clear the LED optical surface which shouldn’t be touched. Everything was a bit unstable as you can see…


Driving circuit
I wired up a constant current LED driver to drive the LED with a potentiometer to control brightness (see schematics below). You can see from the chart that the output scales linearly with the voltage input to the dimmer pin – at 0V, you get the maximum output and at 4.2-4.3V the output has dropped right down to 0%.
This appeared to work well when I tested it out. It got fairly bright as I adjusted the dimmer voltage which you can see from the image above however, I don’t have a way of actually measuring this at present. What I need is a calibrated meter. Unfortunately, this is outside the price range of the shed right now but I intend to do this when I visit the labs in Sheffield again.
Mismatch factor
An important figure of merit when it comes to benchmarking solar simulators is the concept of mismatch factor. It’s basically a score that your light source gets on how well it represents the solar spectrum. To work it out, we need to sum up the power in wavelength intervals over the visible and near infra-red portions of the electromagnetic spectrum for the sun (reference) and simulator (LED). Have a look at this figure below…
Hopefully, you can see straight away that there’s a big difference in the shape of the two spectra. They have been area normalised – remember that the area under the spectra is the total power from the two sources. If we divide by area under the entire spectrum, then we’re effectively setting them to the same power for comparison which is what you would do when testing a solar cell. To get the mismatch, we then sum up the areas under the spectra between the intervals shown and compare (see table). You can see that the LED has a lot of its output in the visible range (400-700 nm) and none in the NIR compared to the sun.
To qualify as a class A solar simulator, the ratio (last column in the table) of all ranges needs to stay within 0.75 – 1.25 – we’re way off! Unfortunately, for this LED, the ratio even goes outside the allowed limits for class C (0.4 – 2.0). We need some NIR component to the spectrum to fix this which is possible. For the purposes of lifetime testing on a budget however, then I think we need to accept these limitations. It’s good to know what they are though.
How long do solar cells live? (Part 2)
Circuit design
In this post, I want to talk about the circuit that I developed to drive solar cells at their maximum power point – the main building block of a modular lifetime tester. At this point, I should credit Sarah Sofia at MIT for her article “Build Your Own Sourcemeter“. This is what really gave me the inspiration and got me thinking that this would actually be possible with an Arduino and simple electronics.
A schematic of the lifetime tester circuit is shown above. In essence, the system is composed of:
- a two-channel DAC (MCP4822) to give me the drive voltage across the solar cell. Because there are two channels I can run two solar cells at the same time. Typically, several subcells (6-8) are made on the same substrate so here, we can test two subcells of the same device at the same.
- solar cell output is dumped into separate small (10-100 Ohm) series resistors which allow us to measure the current from the voltage dropped across them (applying Ohm’s law). Since resistance values and currents are small, the voltage drop will be small (we don’t want to drop much voltage in our ammeter).
- an opamp is then needed to (on each channel) to bring the voltage to something that an ADC (ADS1286) can actually read. In fact, I’ve used an inverting op-amp with variable gain up to 1000x. To account for the fact that different solar cells under test might have different efficiencies and could therefore supply a different current, the gain is variable.
In this circuit, only the fourth quadrant (power generating region of the IV characteristic) can be accessed. Under operation, a solar cell will supply a current in the opposite direction to the applied bias. This means that the voltage across the series resistor will, in fact, be negative – one terminal is grounded, the other will be at some voltage below 0V. This signal gets fed into an inverting opamp which flips it positive again and amplifies it too. Any positive voltage at the input here will be rejected as it will be inverted to a negative output and will hit the 0V supply rail of the opamp. This means that if you try to run the solar cell in forward bias above the open-circuit voltage, giving you a forward current, you won’t get any output from the amplifier. I’ve tried to give you an illustration of this in the figure below (see lower panel).
So did it work?…
To answer this, I connected up a solar cell and just went for a simple voltage sweep from the DAC while monitoring the current using the ADC. Here’s what the data looked like…
I was pretty happy when I saw this data. As you can see, it looks almost exactly how we expected it to from simulations and from understanding how an inverting op amp should operate. Furthermore, the fact that we can get power output curve and see a clear maximum power point (MPP) means things are looking good for doing MPP tracking. There’s some noise but I think this might have been from the fact that I was using the torch on my phone as an illumination source and it was hard to hold it exactly still. Since the measurement takes several seconds to complete, shaky hands could well be the culprit.
If you’re interested in how I coded this, then please follow the link.
How long to solar cells live?
I recently introduced DACs and ADCs. The reason that I got into this in the first place was so that I could build a cheap system for testing solar cells and ultimately measure their stability (lifetime). Perovskite solar cells are notoriously unstable and this is an area of active research right now. Clearly, a system that could monitor the efficiency of many solar cells at the same time would be really useful here.
So I got to work thinking about how we might actually do this. At the moment, this kind of measurement is done with a handful of cells kept under constant illumination with the efficiency being sampled on a timescale of minutes to hours. In between measurements, the cells will be disconnected (held at open-circuit). The illumination is fixed at an intensity of 1 Sun (100mW/cm^2). This kind of measurement really limits the amount and quality of data that we can get.
Firstly, we can’t test many solar cells at the same time (around eight) and have to wait until we’ve finished measuring all devices until we can test any others – data acquisition has to be halted and restarted by the investigator.
Secondly, we’re limited to using the same illumination intensity for all devices and that can only ever be 1 Sun (or perhaps less if you were to stick a neutral density filter over individual solar cells). Increasing illumination intensity will accelerate the test. Naively, doubling the intensity will quarter the lifetime which would remove another bottleneck in solar cell testing.
Lastly, and most importantly, leaving solar cells at open-circuit between measurements is not representative of real-world operation; solar cells need to deliver current to a load ideally at their maximum power point (MPP). At open circuit, the cell does not supply power – if we’re not going to use the power from the cell, what’s the point! One might argue that testing this way is fine for telling us about stability. However, the electric field and charge distribution inside the cell will be different here to real operating conditions, where we actually extract charge by drawing a current, and degradation in these materials has already been linked to field assisted ion migration. Clearly, any learnings we might get using this approach would have limited practical application in developing highly stable solar cells for the real world.
Example I-V characteristics of a solar cell in the dark (black line) and under illumination (red line). Power output vs applied bias is also shown (dotted blue line) and the maximum power point (MPP) has been marked.So then the aim of the project is to build a system which can:
- provide high-intensity, controlled white light illumination
- monitor solar efficiency whilst the device is operated at MPP
- be modular and independent such that the number of channels can be expanded whenever the experimenter feels it’s necessary
- be manufactured for less than £20 per unit
System components
- High-intensity light source – A high-intensity LED light source seemed like the natural option here. They are cheap, efficient (important if we don’t want lots of heat) and capable of delivering lots of light power which is exactly what we want. On the downside, they may not match the solar spectrum all that well. Solar simulators are classified according to how well they can reproduce the Sun’s illumination.
- Basic source measurement unit (SMU) module – To characterise solar cells, a SMU is the instrument of choice. It allows us to precisely control the voltage and read off current in either direction so that we can see all four quadrants of the IV characteristic. Commercially available Keithley SMUs tend to cost in the £1000’s so will obviously be out of our price range for this project. Still, we’re going to need something that can fulfil the role of monitoring power output and maintaining MPP during the lifetime test. I found a really useful article here describing how to build your own SMU from an Arduino and a DAC which I adapted to suit my needs.
- Data acquisition and transfer to a central unit – As the solar cell is driven, the voltage, current and power output data as a function of time need to be transferred to a central unit that is interfaced with a computer (or SD card interface perhaps). This data will then be accessible to a user for further analysis offline.
In the coming series of posts, I’m going to detail what I did here including circuit design, testing and code. Watch this space…
…now for the digital-to-analogue converter
Now I’m dealing with the mirror case to the ADC that I just posted on. Instead of us reading an analogue voltage and converting that to a digital representation, we want an analogue voltage from a digital signal (DAC). In other words, I want the Arduino to give me a voltage from an int. But this is just like the AnalogWrite() function right?… Wrong!
- AnalogWrite() does not give us an analogue voltage. It gives us a pulse width modulated (PWM) digital signal ie. the output is either 0 or 5V and we change the ratio of on and off time to give us an analogue-like signal for driving motors for example. This is not truly analogue in that what we want is to be able to select any DC voltage between 0 and 5V that we want. We could apply a filter to the PWM signal to convert it to DC and this would be a cheap and fast trick but we would lose speed and voltage. Speed is a key figure of merit in a digital-to-analogue converter (DAC) systems as it determines the sampling rate.
- Resolution – if we were to go with option 1 plus filter, with the Arduino, we’re limited to a resolution of 8 bits ie. the output voltage would be scaled between 0-255*Vout. On the other hand with an external DAC, we could choose our resolution up to 32 bits ie. 0-4294967296!
So being able to use a DAC is going to be an important tool in our kit for attacking lots of projects. These things are commonplace in A/V equipment and mobile phones but I have another project in mind for this. More on that later. First, let’s discuss how to use a DAC – in this case the 12bit MCP4822. Se pinout here…
If you’ve not done so already, have a look back at my previous post on using an ADC which covers the basics of SPI connections, bit math and relevant code.
Same as before, we’re going to setup an SPI connection between the DAC and Arduino but this time we transfer data over the master-out-slave-in (MOSI) line. The process will look like this:
- Setup the SPI interface first given the settings in the chip datasheet.
- Take user input from the serial connection and store it as text.
- Separate the channel address, voltage and gain from the users command and store as char, int and int respectively.
- Convert our int representation of voltage to two binary bytes.
- Change the state of some of the leading bits to denote which channel of the DAC we’re addressing (A or B) and what gain we would like (1 or 2).
- Take the CS pin LOW and transfer two bytes via SPI (MSB first). Then return the CS pin to HIGH again.
- Print the result to the serial window.
- Repeat
Above you can see an extract from the datasheet that illustrates the SPI protocol. So how do we actually set the voltage that we want? Well, the MCP4822 has two channels, two gain settings and a 12-bit DAC register meaning and the voltage is calculated as follows…
Vout = Gain*Vref*(D/4095)
…where the gain is either 1 or 2, Vref=2.04V and is the internal voltage reference of the DAC, and D is the binary input that we send to the unit over SPI. Having an internal reference voltage means that even though the supply voltage may change slightly, the output is going to be stable since Vref is a stabilised internal reference…Nice! So this means we can choose whether we want to scale our voltage over 0.00-2.04V or 0.00-4.08V by the gain setting, either large/small range low/high resolution depending on our application, by using the gain setting.
Here is an extract from the MCP4822 datasheet for further information. It shows how the output is calculated and the writing process with example input. You can see that the before our binary input (left of the most significant bit) there are some extra bits referred to as config bits which in my code are inside the most significant byte (named MSB in the following code). We need to concentrate on bit 15, the channel selector, bit 13, the gain selector and bit 12, the shutdown bit…
Now let’s discuss how the code could look. The basic process is given in the numbered list above but let’s elaborate here (You’ll find a complete copy of the code at the bottom of the post):
Setup the SPI interface
// set the CS as an output: pinMode (DAC_CS, OUTPUT); Serial.begin(9600); // opens serial port, sets data rate to 9600 bps inputString.reserve(200); // reserve 200 bytes for the inputString: SPI.begin();
Take user input from the serial connection
// read the incoming string: inputString = Serial.readString(); // say what you got: Serial.print("I received: "); Serial.println(inputString);
Separate the channel address, voltage and gain from the user’s command and store as char, int and char respectively
//extract channel address channel = inputString.charAt(0); //and gain gain = inputString.charAt(1); //convert string to an int - binary voltage n = inputString.substring(2).toInt(); //set DAC state DAC_set(n, channel, gain, DAC_CS, errmsg); //print errors if there are any Serial.println(errmsg); //clear input string ready for the next command inputString = "";
Convert our int representation of voltage to two binary bytes
//convert decimal input to binary stored in two bytes MSB = (input >> 8) & 0xFF; //most sig byte LSB = input & 0xFF; //least sig byte
Set the config bits
You will see that setting config bits is done with bit manipulation. In essence, if you want to set bit 7 to a 1, you need to do an OR operation on your data with 10000000 (which is 0x80 in hex) – this is useful in setting the channel bit. If instead, you want to set the same bit to 0, you would do an AND operation with 01111111 (0x7F) so you would end up setting bit 7 to 0 and keeping any data that you already had. If you did, AND 00000000, all your bits would go to 0 of course.
//apply config bits to the front of MSB if (DAC_sel=='a' || DAC_sel=='A') MSB &= 0x7F; //writing a 0 to bit 7. else if (DAC_sel=='b' || DAC_sel=='B') MSB |= 0x80; //writing a 1 to bit 7. else errmsg += "DAC selection out of range. input A or B."; if (Gain_sel=='l' || Gain_sel=='L') MSB |= 0x20; else if (Gain_sel=='h' || Gain_sel=='H') MSB &= 0x1F; else errmsg += "Gain selection out of range. input H or L."; //get out of shutdown mode to active state MSB |= 0x10;
Take the CS pin LOW and transfer two bytes via SPI (MSB first). Then return the CS pin to HIGH again.
//now write to DAC // take the CS pin low to select the chip: digitalWrite(CS_pin,LOW); delay(10); // send in the address and value via SPI: SPI.transfer(MSB); SPI.transfer(LSB); delay(10); // take the CS pin high to de-select the chip: digitalWrite(CS_pin,HIGH);
Print the result to the serial window.
Serial.println("binary input to DAC: "); Serial.print(MSB,BIN); Serial.print(" "); Serial.println(LSB,BIN);
Repeat
In the serial monitor, you should see something like this…
I received: AL500 binary input to DAC: 110001 11110100 I received: AL0 binary input to DAC: 110000 0 I received: al1000 binary input to DAC: 110011 11101000 I received: al0 binary input to DAC: 110000 0 I received: AL1000 binary input to DAC: 110011 11101000
You get the command back that you wrote in then the binary input to the DAC and any errors. To check that the code and hardware were working, I measured the output from the DAC as a function of binary input. Here is what I got…
What follows is the final code that I used to check my DAC. It contains separate functions for setting the state of the DAC and reading user input from the Arduino Serial monitor. I hope this post is enough to give you the basics of DAC implementation using an Arduino. If you have any questions, please comment and I’ll do my best to answer. Note that this code is limited in terms of speed. I’ve included some short (10ms) delays in the code which would really slow the application of the DAC if we wanted to sample at high data rates. One to return to at a later date.
/* D. Mohamad 29/03/17 code to commumincate with MCP4822 12-bit DAC via SPI pin assignments as follows... Uno (master) MCP4822 (slave) CS 8 2 MOSI 11 4 SCK 13 3 Read output voltage with a multimeter Va/Vb on pin 8/6. send commands via serial interface eg. AL1000 would mean... channel=A gain=low D=1000 Va = gain*Vref*D/4095 = 1*2.04*1000/4095 = 0.498V see www.theonlineshed.com */ #include <SPI.h> const int DAC_CS = 8; //Chip select pin for the DAC String inputString = ""; //holds serial commands void setup() { // set the CS as an output: pinMode (DAC_CS, OUTPUT); Serial.begin(9600); // opens serial port, sets data rate to 9600 bps inputString.reserve(200); // reserve 200 bytes for the inputString: SPI.begin(); } //function to set state of DAC - input value between 0-4095 void DAC_set(unsigned int input, char DAC_sel, char Gain_sel, int CS_pin, String &errmsg) { //DAC_sel choose which DAC channel you want to write to A or B //Gain_sel choose your gain: H=2xVref and L=1xVref byte MSB,LSB;//most sig, least sig bytes and config info //clear error messages errmsg=""; Serial.flush(); // in case of garbage serial data, flush the buffer delay(10); //only run the rest of the code if binary is in range. if (input<0 || input >4095) errmsg += "input out of range. 0-4095."; else { //convert decimal input to binary stored in two bytes MSB = (input >> 8) & 0xFF; //most sig byte LSB = input & 0xFF; //least sig byte //apply config bits to the front of MSB if (DAC_sel=='a' || DAC_sel=='A') MSB &= 0x7F; //writing a 0 to bit 7. else if (DAC_sel=='b' || DAC_sel=='B') MSB |= 0x80; //writing a 1 to bit 7. else errmsg += "DAC selection out of range. input A or B."; if (Gain_sel=='l' || Gain_sel=='L') MSB |= 0x20; else if (Gain_sel=='h' || Gain_sel=='H') MSB &= 0x1F; else errmsg += "Gain selection out of range. input H or L."; //get out of shutdown mode to active state MSB |= 0x10; Serial.println("binary input to DAC: "); Serial.print(MSB,BIN); Serial.print(" "); Serial.println(LSB,BIN); //now write to DAC // take the CS pin low to select the chip: digitalWrite(CS_pin,LOW); delay(10); // send in the address and value via SPI: SPI.transfer(MSB); SPI.transfer(LSB); delay(10); // take the CS pin high to de-select the chip: digitalWrite(CS_pin,HIGH); } } //function to read user command from the serial command window and set the DAC output void serial_DAC_set(void) { unsigned int n; //number 2 bytes long unsigned char channel, gain; String errmsg; //errors returned from DAC_set // read the incoming string: inputString = Serial.readString(); // say what you got: Serial.print("I received: "); Serial.println(inputString); //extract channel address channel = inputString.charAt(0); //and gain gain = inputString.charAt(1); //convert string to an int - binary voltage n = inputString.substring(2).toInt(); //set DAC state DAC_set(n, channel, gain, DAC_CS, errmsg); //print errors if there are any Serial.println(errmsg); //clear input string ready for the next command inputString = ""; } void loop() { //only run when data is available if (Serial.available() > 0) serial_DAC_set(); }
My guide on using an analogue-to-digital converter
In this example, I’ve wired up an Analogue-to-digital converter (ADC) and showed you some code that will get this device talking to an Arduino using the SPI connection. There’s a potentiometer here that you can use to change the input voltage and it’s read once per second. Note that ADCs are capable of reading at much higher rates than this – according to the data sheet, the ADS1286 can sample at up to 20kHz but higher rates are possible.

SPI stands for Serial Peripheral Interface and is a really impressive invention that allows ICs to talk to one another over short distances (on a PCB) using three wires: (1) clock, (2) master-out-slave-in (MOSI)/master-in-slave-out (MISO) and (3) chip select. In our case, since we’re dealing with receiving data from the slave (ADC), then we only use the MISO line. Wikipedia and Arduino have really good entries explaining this. However, I think that the timing diagram on Wikipedia is really helpful. Basically, when we want to talk to a chip over SPI, we send the chip select pin low, then send/read data over the MOSI/MISO data line. The whole thing is synchronised by clock pulses on the clock line. Each time the clock switches state, we end up reading another bit. The exact protocol of whether we (a) read bits on a low-high or high-low clock pulse and (b) whether bits are read on the leading or trailing edge of the clock is set in the SPI settings in the code (see below). Here is the exact operating sequence from the datasheet:

In the code below, you can see that the process goes like this:
- Setup the SPI interface first given the settings discussed above and also set the clock speed as recommended by the manufacturers (It doesn’t seem to work if I don’t set this manually).
- Take the CS pin LOW and read out two bytes via SPI (MSB first). Then return the CS pin to HIGH again.
- Do some bit maths on these two bytes to convert them into a usable binary number.
- Cast this binary number into an int.
- Print the result to the serial window.
- Repeat.
Here’s some example raw data that we might read from the unit. The whole thing looks like this…
So what we need to do is keep only the data inside the box and get rid of everthing else. The key line in the code is this one…
ADCval = ((MSB & 0x3e)<<8 | LSB) >> 1;
Notice that we do a bitwise AND first with the MSB and 0x3e which is hex for 00011111 (step 1). This effectively does an AND operation with each bit in the MSB and this number. Therefore anything in the first three bits gets switched to a 0 because any bit AND 0 -> 0. Then we shift the MSB left 8 places (step 2) and do a bitwise OR with the LSB (step 3). This effectively moves the MSB into its correct position as the most significant data and stuffs the LSB onto the end. Lastly, we shift everything right one place to get rid of the junk hanging off the end (step 4). This effectively shifts it off the end. Note that this will only work because we have declared ADCval as an unsigned int. If not, instead of a 0 being moved in from the left as we shift everything right, a 1 would end up there instead.
And here’s the rest of the code which includes the steps for addressing the ADC.
#include <SPI.h> //pin connections //Arduino ADC //12 MISO 6 (with 10k pull-up) //10 CS 5 //13 SCK 7 const int CS = 8; void setup() { // set the CS as an output: pinMode (CS, OUTPUT); Serial.begin(9600); // opens serial port, sets data rate to 9600 bps SPI.begin(); SPI.beginTransaction(SPISettings(20000, MSBFIRST, SPI_MODE0)); } //function to read state of ADC int ADCread(void) { unsigned int ADCval=0; byte MSB,LSB; //now write to DAC // take the CS pin low to select the chip: digitalWrite(CS,LOW); delay(10); // send in the address and value via SPI: MSB = SPI.transfer(0x00); //most sig byte LSB = SPI.transfer(0x00); //least sig byte //print out the raw measurement for (int i=7;i>=0;i--) Serial.print(bitRead(MSB,i)); for (int i=7;i>=0;i--) Serial.print(bitRead(LSB,i)); Serial.println(); //now get rid of the first three digits (most sig bits) //combine with the LSB //shift everything right one to get rid of junk least sig bit ADCval = ((MSB & 0x3e)<<8 | LSB) >> 1; //print out data again for (int i=15;i>=0;i--) Serial.print(bitRead(ADCval,i)); Serial.println(); delay(10); // take the CS pin high to de-select the chip: digitalWrite(CS,HIGH); return(ADCval); } void loop() { int data = ADCread(); Serial.println(data); delay(1000); }
Here’s an example of the kind of thing that you get on the serial monitor…
1100110110101101 0000011001010110 1622 1100110110101010 0000011001010101 1621 1100110110101101 0000011001010110 1622 1100110110101101 0000011001010110 1622 1100110110101111 0000011001010111 1623 1100110110100111 0000011001010011 1619 1100011111001010 0000001101100101 869 1100010011110000 0000001001111000 632 1100000100101000 0000000000010100 20 1100000000000000 0000000000000000 0 1100000000000000 0000000000000000 0
…first, you get the raw data read from the ADC followed by the shifted bits after we do our maths. Then you get the decimal value representing the voltage which is Vin/Vref*4095.
I decided it might be a good idea to check the linearity of the device. I wanted to be able to relate the binary output from the ADC to a real voltage. Testing this is pretty easy. I monitored the wiper terminal (ADC input) using a digital voltmeter and compared it to the ADC output from the serial window. Here is what I got…
As you can see, everything looks rosy. The response is linear to a high degree of accuracy and the gradient is 812 V^-1 = 4095 binary / 5.042V. I hope this post will help explain the basic process of how to implement an ADC. Get in touch if I’ve missed anything.
Portable robotic spray-coater project
For the last couple of years I’ve been working on spray-coated solar cells at the University of Sheffield in the EPMM group. This group is doing lots of interesting work on thin-film next-generation low-cost solar cells and spray-coating is a great way to go about making them; it’s fast, versatile and scalable. Instead of using blocks of silicon, active (functional) materials are dissolved in solvents to make inks. Then inks can be processed using conventional printing techniques, or even sprayed, to leave thin-films that can harvest light or transport charge which, through careful optimisation, gives us a solar cell.
I’ve been used to using ultrasonic spray-coaters in my work but these instruments are really bulky and not at all portable. What I wanted was a system that we could use in Sheffield to make solar cells in the lab but would also be portable enough for us to take to an Xray light source. This would enable us to start to understand the way that semiconducting films form in real-time at the nanoscale.
So the brief was to make a portable spray-coater suitable for solar cell fabrication that was programmable and would give reasonably repeatable processes. To address this, my basic idea was to strip out a linear drive system from an inkjet printer and mount a simple artists airbrush on the carriage which I would control with a solenoid valve. With an Arduino microcontroller and some embedded software, I was able to control the drive system and solenoid valve to spray lines of water as you can see in the video. At this point, I finished my contract so I wasn’t able to take this any work any further. But now that the kit is built, it would be possible to do this project. Watch this space. Below I’m going to lay out the schematics and details of how I actually did it!
As a Christmas present to myself, I bought the Ardiuno starter kit. It really kick started things for me and took me from absolute ignorance to a “light-bulb moment”. Originally, I got the idea to use the H-Bridge project (Zeotrope project No. 10) form the kit. In the kit you get an L293 H-Bridge, a circuit and code to drive it among lots of other stuff. In a nutshell, an H-Bridge is an IC composed of several MOSFETs which you can use to control a small DC motor (speed/direction/on-off). I quickly realised that this wasn’t going to cut it however – the load requirement of the motor I was using was too much for the poor little chap. The peak output is only 1.2A and after some digging, I discovered that the motor in the drive unit would draw at least 1A at no load and up to 40A if stalled! You’d need some serious power electronics for this so made the assumption that I need to uprate things a little and I went for the Arduino motor shield which is rated at 2A (per channel). Turned out that this was enough to drive the motor and with pulse width modulation and some logic signals, I could control speed and direction of the motor.

To read the carriage position, you need to know about quadrature encoders. I found a nice tutorial on this here. Amazingly, it’s possible to get to an accuracy of 0.07mm! But if you want to print at high resolution then I guess that’s not all that surprising. For this application, we don’t need anywhere near this accuracy – cm accuracy is probably about good enough but it’s nice to have it all the same. The position is read by a light gate and encoder strip which is a piece of acetate with lots of lines printed really close together. As the carriage moves along, we see a square wave from the light gate as the lines break the beam and if we count these, then we can follow the position of the printer head. Neat! The code is relatively straightforward. So, all that was left was to integrate these two functions and sort out some of the details such as an interface, power supplies, solenoid valve and some hardware to mount everything.
Linear drive system



Final schematics and boards


Here (see video below), you can see me testing out the printer carriage with the boards that I made. Note that the movement is a bit jerky because I removed a retaining strip from above the carriage so I could have electrical access – this makes the carriage droop downwards and seems to add a bit of friction.
The code
//yellow is Q line? //blue is I line? // Interrupt information // 0 on pin 2 // 1 on pin 3 #define encoderI 2 #define encoderQ 3 // Only use one interrupt in this example volatile int count; void setup() { Serial.begin(9600); count=0; pinMode(encoderI, INPUT); pinMode(encoderQ, INPUT); attachInterrupt(0, handleEncoder, CHANGE); } void loop() { Serial.println(count); delay(10); } void handleEncoder() { if(digitalRead(encoderI) == digitalRead(encoderQ)) { count++; } else { count--; } }
Above is the code that I used to check that I could read positions from the quadrature encoder based on this youtube resource. It worked really nicely and so I moved on to controlling the motor and solenoid valve using a serial interface.
//130217 code for controlling printer head spray coater //commands issued as ascii commands via serial bus (USB) //version uses the motor shield /* COMMANDS AXXXX start position BXXXX finish position VXXX speed DXXXX delay in ms */ //yellow is Q line - pin3 //blue is I line - pin2 // Interrupt information // 0 on pin 2 blue // 1 on pin 3 yellow #define encoderI 2 #define encoderQ 3 // Only use one interrupt in this example volatile int count; //current position index int startPos = 0; //start of dep int finPos = 0; //end of dep int xPos = 0; int myDelay = 0; //delay after starting spray but before move String inputString = ""; //holds serial commands const int stopDistance = 2; //factor to determine stopping distance const int controlPin = 13; // channel B direction const int enablePin = 11; // channel B PWM input const int solenoidPin = 4; //solenoid control pin for starting spray const int solenoidOverride = 7; // manually turn solenoid on const int solenoidLED = 5; const int motorLED = 6; int motorSpeed = 220; int homeSpeed = 220; int approachSpeed = 200; //slower speed for approaching target position so you don't overshoot void setup() { Serial.begin(9600); count=0; pinMode(encoderI, INPUT); pinMode(encoderQ, INPUT); attachInterrupt(0, handleEncoder, CHANGE); // intialize the inputs and outputs pinMode(controlPin, OUTPUT); pinMode(enablePin, OUTPUT); pinMode(solenoidPin, OUTPUT); pinMode(solenoidOverride, INPUT); pinMode(solenoidLED, OUTPUT); pinMode(motorLED, OUTPUT); // pull outputs LOW to start digitalWrite(enablePin, LOW); digitalWrite(solenoidPin, LOW); digitalWrite(controlPin, LOW); digitalWrite(solenoidLED, LOW); digitalWrite(motorLED, LOW); // reserve 200 bytes for the inputString: inputString.reserve(200); } void loop() { int readUserSpeed; //decide which function to do with the switch case switch (inputString.charAt(0)){ case 'X': xPos = inputString.substring(1).toInt(); if (xPos < 0 || xPos > 4100){ Serial.println("Outside allowed range 0-4100"); xPos = 0; } Serial.print("Moving to "); Serial.println(xPos); moveStage(xPos); break; case 'A': startPos = inputString.substring(1).toInt(); if (startPos < 0 || startPos > 4100){ Serial.println("Outside allowed range 0-4100"); startPos = 0; } Serial.print("Start deposition at "); Serial.println(startPos); break; case 'B': finPos = inputString.substring(1).toInt(); if (finPos < 0 || finPos > 4100){ Serial.println("Outside allowed range 0-4100"); finPos = 0; } else { Serial.print("Finish deposition at "); Serial.println(finPos); } break; case 'D': myDelay = inputString.substring(1).toInt(); if (myDelay < 0 || myDelay > 9999){ Serial.println("Outside allowed range 0-9999"); myDelay = 0; } else { Serial.print("delay (ms) "); Serial.println(myDelay); } break; case 'V': readUserSpeed = inputString.substring(1).toInt(); //need to make sure this is between 0-255 if (readUserSpeed < 0 || readUserSpeed > 255){ Serial.println("Outside allowed range 0-255"); } else { motorSpeed = readUserSpeed; } Serial.print("Motor speed set to "); Serial.println(motorSpeed); break; case 'H': homeStage(); break; case 'Q': //query Serial.println("Current settings:"); Serial.print("Motor speed set to "); Serial.println(motorSpeed); Serial.print("Start deposition at "); Serial.println(startPos); Serial.print("Finish deposition at "); Serial.println(finPos); Serial.print("delay (ms) "); Serial.println(myDelay); Serial.print("current position "); Serial.println(count); break; case 'R': //run recipe Serial.println("Run recipe..."); moveStage(startPos); digitalWrite(solenoidPin, HIGH); digitalWrite(solenoidLED, HIGH); delay(myDelay); moveStage(finPos); digitalWrite(solenoidPin, LOW); digitalWrite(solenoidLED, LOW); Serial.println("recipe done."); break; default: digitalWrite(enablePin, LOW); } //override the solenoid when you push the button down if (digitalRead(solenoidOverride)){ digitalWrite(solenoidPin, HIGH); digitalWrite(solenoidLED, HIGH); } else { digitalWrite(solenoidPin, LOW); digitalWrite(solenoidLED, LOW); } inputString = ""; delay(10); } //update position counter void handleEncoder() { if(digitalRead(encoderI) == digitalRead(encoderQ)) { count--; } else { count++; } } //int dirState = 1; // 0/1 for R-to-L (count++)/L-to-R (count--) void motorDirection(int dirState) { // change the direction the motor spins by talking // to the control pins on the H-Bridge if (dirState == 1) { digitalWrite(controlPin, LOW); } else { digitalWrite(controlPin, HIGH); } } //read serial data if there is any void serialEvent() { if (Serial.available() > 1) { inputString = Serial.readString(); Serial.println(inputString); } } //better way to write things is to separate into functions for different things and then call within the serial.event void homeStage(){ //initialise homing - //go left until you stop moving Serial.println("Homing..."); int homePos = count + 1; //force into loop motorDirection(0); analogWrite(enablePin, homeSpeed); digitalWrite(motorLED, HIGH); while (homePos > count) { homePos = count; delay(50); Serial.println(count); } count = 0; analogWrite(enablePin, 0); digitalWrite(motorLED, LOW); Serial.println("Done"); } void moveStage(int targetPos) { //move stage function Serial.print("Moving stage to "); Serial.println(targetPos); while (abs(targetPos-count)>5){ Serial.println(count); // work out direction - 0/1 for R-to-L (count--)/L-to-R (count++) if (targetPos > count) { motorDirection(1); } else if (targetPos < count) { motorDirection(0); } //check how far we are away from the target and set motor speed if (abs(targetPos-count)<((motorSpeed-180)*stopDistance)) { // PWM the enable pin to vary the speed //going faster means you need to slow down earlier //use motorspeed or similar to manage stopping distance analogWrite(enablePin, approachSpeed); digitalWrite(motorLED, HIGH); } else { analogWrite(enablePin, motorSpeed); //otherwise go full speed digitalWrite(motorLED, HIGH); } delay(1); //think the delay is important to stop it checking position too often } analogWrite(enablePin, 0); digitalWrite(motorLED, LOW); Serial.println("Done"); //this doesn't print out. Why? }
Welcome to my blog
Thanks for visiting my blog. I’m keeping a record of my work on hobby projects relating to my personal interests that cover instrumentation, embedded software, electronics and general tinkering around with stuff. I am passionate about technology and making stuff work but I really enjoy learning and playing with things. This is my chance to give back and share what I’ve learned from helpful online resources and people that have contributed to my knowledge and development. I hope you like it!
Dave