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Replacing infrared photodiode with a resistor

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I have an infrared (IR) position detector which consists of an IR LED and an IR photodiode with a rotating slotted disk between them - rather like in a computer mouse. I want the output to be permanently on, which should enable me to simplify the circuit, reducing the number of wires and components and increasing reliability.

I believe that the photodiode should be reverse biased, with the result that little current flows in (IR) darkness and more flows in light. It seems to me that this is equivalent to saying that there is high 'off' resistance which changes to a low 'on' resistance. I think the off resistance is very high (100s of KOhms), what would be the on resistance? I can test this, but would value your opinion.

I aim to dump the IR LED and replace the IR photodiode with a resistor which will simulate the on condition. Is this practical?
 
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JimB

Super Moderator
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I can test this, but would value your opinion.
A test trumps the various conflicting guesses which you will get on here.
Test it, measure the voltages and calculate the value of the fixed resistor to do the same thing.

JimB
 

alec_t

Well-Known Member
Most Helpful Member
Yes, it's practical. The resistor value required would depend on what circuit the photodiode connects to.
 
Yes, it's practical. The resistor value required would depend on what circuit the photodiode connects to.
Thanks, I've measured the 'on' resistance out of circuit so have a rough idea of what resistance to use. I'll try something higher, then reduce it until it works, then a bit more.
 
Yes, it's practical. The resistor value required would depend on what circuit the photodiode connects to.
I've had a go at this and it looks like this thing works in a more complicated way than I thought. It may require relative movement between two of these sensor pairs. I'm going to check the power components* elsewhere in the circuit in case there is a problem there, but I think it's less likely. I'll probably just buy a new device. Thanks for your help anyway.

(*I'll have to start another thread for a tip on how to check them...)
 
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