Sunday, July 14, 2013

Building a UV Index recorder for MSP430, Arduino, Raspberry Pi. Part 1 - the sensor

A couple of weeks ago I found this is Lidl.

I did what any right minded person would do - took it apart to see how it works.

OK, so modern electronic gadgets aren't easy to disassemble and reassemble. Thankfully I had no great desire to put it all back together again. One nice thing was that the UV photodiode was on its own board. One interesting thing is that there's a skeleton potentiometer so presumably these things are calibrated/adjusted at the factory.

Google led me to this manufacturer as the likely source of the photodiode and here's the datasheet for a device designed for UV Index monitoring
Good news is that device current increases linearly with UV Index,  bad news is it's measured in nano amperes.  Well I suppose that does mean the resulting device could be very low current; given the application it might as well be solar powered!
First things first,  I'm going to need an amplifier, so I went for a low power device I bought a few of a while ago but lacked the frequency response I needed, the TLV2252. Hooking this up on a breadboard with a circuit based on this one from  I used 10M resistors and a 3.3V supply.

Covering and exposing the photodiode did nothing, so I started thinking I'd damaged it or the circuit wasn't right.  I swapped the UV photodiode for an LED and it worked like magic throwing the output to max when I directed a bright torch at it, so I swapped back the UV photodiode and went outside.  Yes it worked giving about 20mV per UVI.  OK, that's not a lot, but there's a second OP amp in the package so I used a 10K and 100K resistor to give extra gain of 10 and hey presto it's ready to interface to a ADC.   As it is the circuit draws less than 0.2 mA.

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