ferg
Posts: 32
Joined: Sun Jan 31, 2016 2:42 am

Recommendations for photodiode and signal conditioner??

Tue Mar 08, 2016 11:43 am

Hi,
I'm going round in circles trying to find a suitable photodiode for my project. I want to run it directly off my RPi 2B gpio and also feed into either a TIA or op amp to process a digital signal.
I am modulating an LED (dotstar with level shifter) at a frequency between 1-10MHz. I don't want to use an external power supply, just two pi's with the LED on one and the PD on the other.
Any recommendations of components?
Thanks, F

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experix
Posts: 204
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Location: Coquille OR
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Re: Recommendations for photodiode and signal conditioner??

Tue Mar 08, 2016 3:15 pm

So you seem to be trying to make one Pi use a LED to send to the other Pi, which picks up the signal by using a photodiode. I think you need to wire up the photodiode with the usual transconductance amplifier and put the output of that into a comparator to discriminate between background and LED signal. Often you can get good circuit ideas from application notes published by the company that makes the photodiode. If you don't have to overcome too much background light and the distance isn't too great you can use the PWM output of the transmitting Pi, with a current-amplifying transistor, to drive the LED, and not require any more power on that end. On the receiver side there's not much power required at all but you need to put some careful thought into designing the detection circuit. When you get it that far along, you are dealing with a high-rate logic signal that the Pi needs to handle somehow. Can you make it behave like a normal UART connection and use that feature of the peripheral block?
I seem to remember forum posts on doing IR communication with Pi. Try searching the forum for that sort of thing. In fact, there's a post today by evilkitty in this forum section that looks extremely relevant, with lots of links to follow.

ferg
Posts: 32
Joined: Sun Jan 31, 2016 2:42 am

Re: Recommendations for photodiode and signal conditioner??

Tue Mar 08, 2016 9:34 pm

Cheers, that's what I'm trying to do.
All that you've described I'm working on (except uart) and looking for anyone who has used a photodiode (not necessarily IR) on the gpio.
I think I might use a comparator to clean the signal up before sending to the decoding section.
It's just the devices that I'm looking for help with at the mo.

asandford
Posts: 1997
Joined: Mon Dec 31, 2012 12:54 pm
Location: Waterlooville

Re: Recommendations for photodiode and signal conditioner??

Tue Mar 08, 2016 9:56 pm

Using UV rather than IR and visible should cut down on noise.

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