The last meetup of the year was quiet: Only Bouwer arrived, so we experimented on his project.
Bouwer needed a circuit to buzz when pressure is applied to a sensor mounted on a stick, basically. He had some Velostat, which is a good choice in this case for building the sensor.
The idea is to build a circuit with an op-amp set up as a comparator to then drive the output buzzer.
The velostat sensor is put in series with a resistor that’s roughly the same resistance as the velostat at the desired force. This is mounted between VCC and ground, and the center point is tapped for the voltage.
The other input of the op-amp is simply connected to the center pin of a potentiometer, also connected from VCC to ground. That provides a fixed reference voltage divider that is settable to adjust the switching point.
He had a prototype with velostat, aluminium on either side, and some copper foil tape. We quickly found out that the copper foil adhesive is nonconductive, which is why this did not work.
We rebuilt the sensor, sticking the foil to some Depron foam, and sandwiching the velostat between these electrodes. Much more solid and repeatable.
We built the circuit and powered it with 2 coin cells. It worked perfectly!
Further improvements would be to add some resistors to increase the sensitivity of the potentiometer, and using a plain comparator instead of the op-amp.
It turned out to be a fun evening. It’s really nice to take a bunch of components and debug your way to a working setup.