The values returned are the raw integers read from the ADC (analog to digital converter) on the output of the transimpedance amplifier (current to voltage converter). You will need to convert these integer values to current measurements e.g. uA. While it is possible to do a first principles calculation using the values of the feedback resistor in the transimpedance amplifier it is generally better to just do a simple calibration using a dummy cell consisting of a single resistor (of known value R).
The basic idea is to connect the counter and reference electrodes to one side of the resistor and the working electrode to the other side. You then want to sweep the voltages through a range of values - e.g. via a cyclic voltammetry test. Because the resistance R is known for each output voltage v(t) in the test you can calculate the current i(t) = v(t)/R through the resistor. At the same time you have the measured ADC integer values n(t) corresponding to this current. To get the calibration you can just perform a linear fit between the ADC integers n(t) and the know current values i(t).
When performing the calibration test make sure to select an appropriate resistor. You want to generate currents which span the input range (for current), but don't go outside of it. The transimpedance amplifier can't generate voltages higher then 3.3V or lower than 0V - so if you go outside of the current range you will get clipping (or saturation) of the output which will show up as a flat spot in your calibration data.
Oh gosh, you are right! I’m sorry for the delay but I was very busy. I have never thought to rotate the LED matrix because I thought I was right with that (otherwise, if not, no LED would have turned on). Thanks very much! I also take advantage of you for another issue. I have tried to load the arena configuration on the SD card but PControl doesn’t let me do that. Have you any suggestion? It seems that the controller doesn’t communicate to the SD nevertheless at the beginning (when the controller is switch on) PControl finds and initializes the SD card. I’m sorry because of all these questions!
I've swapped out the 15 Ohm resistor in R7 with two of the 50 Ohms in parallel ( 25 Ohms) to create a +/- 24 mA range. This allows us make use of almost the full range of the ADC. As before it looks good over +/-20 mA range.
It almost works over the full +/-24 mA range. It fails at the negative end right around -22mA.
Corrosion monitoring and analysis usually takes a large dynamic range (eg polarization curves etc.) or requires measurements with respect to open circuit (LPR, EIS , OCP-monitoring). These are not well-suited to the current version of cheapstat.
It looks like the RTL8195 has 3.3V IO. The potentiostat shield powers the MCP4261 with 5V. So you are connecting a 3.3V device to a 5V device - I think. I'm not sure if the IO pins on the RTL8195 are 5V tolerant, but you might want to check. Could be related to you issue.
Note, the MCP4261's could be powered off of 3.3V. They have a pretty broad allowed input range 2.7 - 5.5V. This would require cutting one trace - the one going to the 5.5V on the shield - and rerouting this trace to to the 3.3V pin.
Below are some images of the green LED transilluminator we designed for a custom application using the superbright green LEDs I mentioned earlier. The enclosure is the same as the blue version, we just swap the 470 nm LED for the superbright green LED. A diffuser and green filter is placed over the LEDs as I mentioned.
The red viewing filter that sits above the LED board is actually a thin film Roscolux #26 filter which we laser cut.
Because it is thin, we have to support it. We can do this using a black frame.
Let me know what you think and whether this would work for your application.
We would be able to ship approx. a week after ordering. The lead-time would be on purchasing the LEDs from Superbright. Once we receive the LEDs we would ship 1-2 days later. The cost would be $225 for the fully assembled version with the 18V power supply. So it would be ready to use. I can send you a formal quote if you want to send me an email with your shipping info to firstname.lastname@example.org.
I haven't modified the cheapstat to have a +/- 1uA range - however I'm pretty sure this should be doable.
Also, we have a new potentiostat design coming out soon which is basically a shield for the teensy 3.2 development boards (https://www.pjrc.com/teensy/teensy31.html). It has current ranges of +/- 1, 10, 100, 1000 uA and output voltage ranges of +/- 1, 2, 5, and 10V. We just got a batch of the new of the PCBs in - I've attached an image below. We should have them up on our website within the next 2-3 weeks.
Yes you could definitely use the Rodeostat's teensy 3.2 to control the LEDs. On the Rodeostat PCB there are two 5x2 expansion headers which expose various DIO pins on the teensy which you can use. The ability to toggle the expansion header IO pins via USB commands from the host PC is not included in the standard firmware . So you would need modify the firmware to toggle these pins - to control the LEDs - yourself.
The firmware for the Rodeostat is just an Arduino sketch. so if you are comfortable programming Arduinos modifying it should be pretty straight forward. You could probably quickly hack in this feature by adding a few lines to the SystemState's "updateTestOnTimer" method - just check the time and set desired digital outputs for the LEDs using digitalWrite based on time or the current measurement, etc. The "updateTestOnTimer" method can be found in firmware/libraries/potentiostat/ps_system_state.cpp starting on line 353. With the default settings it is called at 5kHz - so every 200us.