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I'm totally new to Raspberry Pi and I want to use it for an important school project.

My aim is to build a laser-seismograph using a laser pointer and 4 photodiodes which basically measure the laser's light intensity. The photodiodes (TSL257-LF) have a built-in amplifier, with the result that they convert light intensity to voltage. The output range of the photodiodes is 0-5 DC Volt. Now I need to measure simultaneously all four voltages of the photodiodes with a rather good accuracy (maybe ±10 mV, if possible ±1 mV) in order to know the current position of the laser dot.

I have read on several forums that I'd need a digital to analog converter such as the MCP3008, but I have no idea whether it is true, nor how I should connect it to the different elements. Also it has been said that the range would be 0-3.3 V without any supplementary circuitry.

It would be a great help if you explained step by step what I need to do in your opinion to measure the voltage of each photodiode.

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The MCP3008 is quite suitable. It has 8 channels (you can measure 8 things at once) with 10 bits of resolution full-scale (between 0 and the voltage reference).

If you have a stable voltage reference of 5V that will give a resolution of 4.88 millivolts. The similar MCP3208 has 12 bits of resolution which would give a resolution of 1.22 millivolts. It's probably pointless using a higher resolution chip as you would need careful design and a very stable voltage reference to make use of its abilities.

You do need an external reference 5V. The Pi's 5V power supply is unregulated.

Given that you need to use a 5V reference voltage you will get 5V level outputs from the ADC. You need to use a resistor divider on the output MISO line to the Pi to drop the voltage from 5V to a Pi safe 3V3.

All the other information you need to actually implement the solution is widely available on the www.

  • thank you very much joan i think i understand better now what i'm supposed to do. I think you did not finish your last sentence, could you please repeat under what URL the other information needed is found? – Duniel Oct 16 '17 at 20:11

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