Unable to get single-supply inverting circuit to work to trigger a BJT

Hi Alan,

Welcome to e2e.
Have you tried each half of the circuit separately?
Perhaps you can first try to ensure that the first amp is providing the expected bias. Then check the signal coming from the piezo with R1 and R5 connected. Perhaps R1 in parallel is causing the voltage to drop.

Best regards,
-Ivan Salazar
Applications Engineer - Low Power Audio & Actuators

Well I just identified part of my problem. I built this circuit up using a salvaged opamp from an old board cus I didn't have one to test with on hand at the time. When I knocked it out of the board, a small sliver of solder bridged the second amp's in-/in+/out pins. I just cleaned it up and I'm going to test again. I'll update with the behavior of the circuit once I do

OK, Update after cleaning the amp and rebuilding the circuit:

The inverting behavior is different depending if I couple the inverting input to the piezo vs an actual DC voltage.

Now I'm actually getting the expected 2.5v on out2 with no activity, rather than the ~4.1V I was seeing before, so that's good. When I attach the piezo, and flex it, the voltage actually climbs about 20mV now instead of dropping as I expected, however, if I tap the in2- pin with a +5V wire, Out2 does drop down to ~1.34V. So I'm getting that negative edge behavior I want only from a dc source at the moment.

Also even when using a higher gain ratio, or a lower DC offset, I just can't get the output pin to go lower than that 1.34V, which isn't low enough to trigger the BJT or microcontroller pins.

Anyhoo, Amp1 is definitely doing it's job of voltage following, and keeping the In2+ pin at 2.5V, but I think Amp2 is having some issues with how it interacts with the Piezo disk. Maybe not enough current from the piezo?

I'm going to try a different configuration using Amp1 as a buffer to give it a bit more oomf, and just use a divider on In2+ instead and see if that makes a difference

Ok so I'm getting some good results from the buffer, but not the inverting circuit.

For kicks I set up the BJT on the output for the buffer alone, and was able to get it to trigger that way, but I don't always know exactly what voltage this circuit will be using, and I also don't always know which piezo disks will be used either so I'm still trying to integrate the inverting circuit to address that uncertainty.

anyhow, the circuit I built that is able to drive the BJT:

By adjusting that voltage divider on the BJT's base, I was able to get the amp's output below the trigger threshold of the BJT, and the output is just high enough to trigger properly.

However, if I attach Out1 back to the amp2 inverting circuit, I get nearly the same problem as before with like ~20mV change on out2

Edit: to clarify, I'm still using the same IC, I just built the schematic using a different one that's easier to read for a single amp