I'm looking for help understanding a bizarre circuit failure. The function of the circuit is simply to activate an output signal and light an LED if the voltage from a Hall Effect sensor goes low enough (indicating that a magnet got close enough to the sensor.) We've built hundreds (maybe thousands?) of this circuit with no problems at all except for this one board. I've attached two schematics - one with the complete circuit (two sensors, using both inputs on a two channel comparator) and one simplified schematic for just one side of the circuit (this board is failing only on channel 2):
Normally the chip (Microchip Technology MCP6542-E/SN) operates as a comparator with crisp, clean switching and a small amount of deliberate hysteresis, but this one failing board outputs a variable voltage, inversely-proportional to the Hall Effect sensor voltage. It's as if the chip were running as a voltage-follower or amp of some sort instead of a comparator. The LED fades in and out as well, tracking with the output voltage. The sensitivity of the circuit also drifts slowly and gradually, tending towards making the circuit more and more sensitive until eventually the output is always on.
When the bad board was returned to us, I did a quick examination and couldn't find any obvious bad components, short circuits, or open circuits. Lacking any other explanation, I guessed that the chip itself was malfunctioning since it shouldn't output analog voltage when configured as a comparator. However, our board house replaced the chip with a new one, and the repaired board still behaves the same way, implying that something is wrong with the PCB itself or with one of the other components.
Is there any simple, plausible problem that could change this circuit and make it behave as a voltage follower instead of a comparator? Any short, open-circuit, or incorrect component value that would change this circuit so dramatically? I'm at a loss. I assume it can't be the Hall Effect sensor, simply because any analog voltage from the Hall Effect sensor should always result in a crisp on/off output from the comparator. My understanding of the myriad applications of op-amp circuits is still quite limited. I feel like there should be an obvious answer, but I'm at a loss.
Thanks in advance for any insights!
P.S. For this individual board, it would be cheaper to throw it away than to spend time diagnosing it, but I'd really like to figure this out in order to better understand these circuits and also so that we're prepared if similar issues come up again.
Normally the chip (Microchip Technology MCP6542-E/SN) operates as a comparator with crisp, clean switching and a small amount of deliberate hysteresis, but this one failing board outputs a variable voltage, inversely-proportional to the Hall Effect sensor voltage. It's as if the chip were running as a voltage-follower or amp of some sort instead of a comparator. The LED fades in and out as well, tracking with the output voltage. The sensitivity of the circuit also drifts slowly and gradually, tending towards making the circuit more and more sensitive until eventually the output is always on.
When the bad board was returned to us, I did a quick examination and couldn't find any obvious bad components, short circuits, or open circuits. Lacking any other explanation, I guessed that the chip itself was malfunctioning since it shouldn't output analog voltage when configured as a comparator. However, our board house replaced the chip with a new one, and the repaired board still behaves the same way, implying that something is wrong with the PCB itself or with one of the other components.
Is there any simple, plausible problem that could change this circuit and make it behave as a voltage follower instead of a comparator? Any short, open-circuit, or incorrect component value that would change this circuit so dramatically? I'm at a loss. I assume it can't be the Hall Effect sensor, simply because any analog voltage from the Hall Effect sensor should always result in a crisp on/off output from the comparator. My understanding of the myriad applications of op-amp circuits is still quite limited. I feel like there should be an obvious answer, but I'm at a loss.
Thanks in advance for any insights!
P.S. For this individual board, it would be cheaper to throw it away than to spend time diagnosing it, but I'd really like to figure this out in order to better understand these circuits and also so that we're prepared if similar issues come up again.