### Author Topic: IR LED and photodiode  (Read 609 times)

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#### Oaklander

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##### IR LED and photodiode
« on: April 08, 2018, 08:32:54 am »
I'm designing a device which is a bit like photointerrupter. There will be IR LED pointing at photodiode and I'm measuring the light getting to the photodiode.
Therefore I should calculate the reverse current of the photodiode when there is no obstruction between LED and the photodiode. The problem is I'm not enirely sure how to do it.

Power of the LED is given in mW/sr and photodiode reverse current in mW/cm2. How can I convert between the two? Obviously I need to know the distance between LED and photodiode. Is there something else to consider?

#### blueskull

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##### Re: IR LED and photodiode
« Reply #1 on: April 08, 2018, 09:14:15 am »
Just measure it. Don't bother to calculate, because LEDs and PDs both have fairly high range of drift and error right from factory. Also, sunlight can interfere with things.
The best way is to put a pot on the receiver so the customer can adjust accordingly.

#### StillTrying

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##### Re: IR LED and photodiode
« Reply #2 on: April 08, 2018, 02:09:18 pm »
"Is there something else to consider?"

Speed, AD/DC coupled, slower is easier, AC coupled is easier.

"I'm measuring the light getting to the photodiode."

Measuring the amount of light, and detecting blocked/notblocked can be quite different.

Too many variables, but at a wild guesstimate expect the photo current to be quite low, up to 2uA blocked, and 10 - 20uA unblocked.
CML+  That took much longer than I thought it would.

#### Kleinstein

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##### Re: IR LED and photodiode
« Reply #3 on: April 08, 2018, 02:17:07 pm »
The rating in mW/Sr gives the power for a full surface of a sphere. So the area is 4 Pi times distance square.

#### TechieTX

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##### Re: IR LED and photodiode
« Reply #4 on: April 09, 2018, 02:00:50 am »
Don't try to measure the received power of the photodiode, smack that IR LED with 30-100KHz and detect the frequency at the receiving end with an IR receiver IC.  It's *much* easier.  If you see 38KHz, then there's no obstruction.  If you don't, it's blocked.

I got stuck trying it your way (long ago) 'cos the design was already in production, and it never did work right in varying light conditions.  I could change the software to drive the LED with a pulsed signal, but had no way to detect it at the receive end.  It was a trivial change to the circuit but they didn't want to do a recall on all of the machines since they were scattered all across the USA.

If you're still in the design phase, try one of the IR receiver chips that has the ~40KHz bandpass filtering built-in, and your job is nearly done.  I'm looking at one right now that runs a 38KHz bandpass filter.  EDIT: a quick look at Digikey shows that today's frequency range is 20-60KHz with the majority of the parts running 30-40KHz bandpass, so that's your sweet spot for price.  Here's one for example:
https://www.digikey.com/product-detail/en/vishay-semiconductor-opto-division/TSSP4038/TSSP4038-ND/3789836
(datasheet is on that page, too)

As a side benefit, you can drive the LED much harder if it's 10-30% duty cycle, giving you extended range and/or better immunity to varying light.