IR circuit issues

Except there's no circuit?.

Nigel Goodwin said:

Except there's no circuit?.

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I'm sorry, it appeared in the draft earlier. I think it's too big.

Updated.

To make circuit immune to power supply V's use a Vref diode or Zener :




Power reference diode from supply, R3 connected diode output to ground.

Use a 7805 voltage regulator to ensure a consistent 5V supply.

You are meeting the CM range for the inputs ?



Meeting the power pin bypassing recommendations ?



Regards, Dana.

Unless your speaker has a very high resistance it will act almost as a short circuit on the LM358 output and will drag the output voltage down, probably below the point at which D1 (if that is meant to be an LED) will light.

1) Add positive feedback round the opamp, to make it 'snap', this is very important in comparator circuits.

2) Add PSU decoupling capacitors, essential in all electronic circuits.

3) Use a 5V regulator (7805 or 78L05) to power it, again with the required capacitors.

4) Remove the speaker - as already suggested you can't just hang a speaker on an opamp.

You can drive a speaker, at low levels, by using a series R with speaker to decouple
the low Z load a speaker represents. You sacrifice efficiency and audio volume.
Start by using 1K ohm and drop it, maybe not < 100 ohms.



Or use a LM386 kind of part, take a look at this thread -


Read the LM386 datasheet carefully as that part can oscillate and is not tolerant of HV (generated
by L loads).

Hysteresis calculator for comparator : https://www.allaboutcircuits.com/tools/hysteresis-comparator-calculator/




Regards, Dana.

Another approach is to remove the preset pot and connect the opamp positive input to a capacitor to 0V, and a fairly high value resistor (10k?) to opamp negative in.

Then add a much! higher value resistor from positive in to positive power, so the positive in is offset by just a few millivolts higher than the negative.

That should make it self-adaptive regardless of the exact photodiode or supply voltage - though it will not see a permanent interruption to the light beam. Some overall supply regulation would still improve it though.

Edit - just noticed that the opamp inputs appear reversed? As shown, the photodiode would be conducting and pulling the negative input low when there was no interruption, so setting the output high? Or is it supposed to be sensing reflection rather than interruption?

If it's for reflective sensing, the high value resistor should be from positive in to ground, rather than positive in to power. That will also be less sensitive to supply variations.


If you want to keep the fixed bias, try adding a normal or shottky diode (or two or both) in series with the 0V end of the preset, so the wiper voltage changes less with supply variations.