infrared Help Troubleshooting This Circuit

Posted on Feb 6, 2014

This is a follow up to an earlier post I made here. Below is the relevant portion of the schematic for this issue (stuff to the left of Y1 is in original post if necessary). I have finally gotten this circuit working somewhat reliably with one exception. The original device (read layout) is intended to work at 5V. I have the original device and c

infrared Help Troubleshooting This Circuit
Click here to download the full size of the above Circuit.

an verify that it does work properly at 5V. I just got my PCB back which has a copy of the circuit. I populated the board and fired it up and it seemed to work like a champ. I had both (original and copy) wired up on the bench and was looking at the output of each channel on the scope. Things looked great. Then, I happened to notice that I was powering up the two from 3. 3V as I`ve been switching between projects and did not change the power supply voltage when I last switched. I begin to adjust the voltage up toward 5. 0V and I notice that right about 3. 8V the output of the copy goes low and stays low up to 5. 0V. I lower the voltage and it goes high (but toggles low when IR present) at about 3. 6V. So, the circuit works at a supply voltage < ~ 3. 6 V. With the voltage set at 5. 0V I start to probe around comparing all signals to the original. Following the individual inverter stages things look identical right up to the input of U1E (pin 11). As I vary the voltage across that threshold of roughly 3. 6V that pin toggles. So, next I want to take a look at the output of U1D (pin 8). I found it quite difficult to probe this and watch the scope at the same time as the probe would slip off the contact point. So, I decided to take a piece of 3" wire wrap and tack it onto the pin so that I could grab a hold of it with the probe. I did that, fired it up and the circuit behaves quite nicely! It operates as expected. At this point...

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