So, research time. I found some information on the RockBand forums
with several circuits proposed. The first one I found used an op-amp (LM386) and a 555 to handle the transaction. This seemed reasonable at first but just looked more and more complicated as I thought about in. On page 11
, someone posted a much streamlined circuit that had what I was looking for. However, this also seemed like it had a few too many parts, and frankly, I didn’t have that stuff lying around. I had 741s lying around, but not in a battery circuit. I had some LMP2011 op-amps lying around, but at $2.75 each, I was not going to use two of them.
Effectively, this is just a circuit that amplifies the piezo output, detects the peak level, and shorts a signal to ground for a certain amount of time. My op-amp can run at 2 volts, but not more than 5.5V. Using a BJT to short the output seemed ineffective since they require more current to operate than a MOSFET… plus I have more MOSFETs around than BJTs. Finally, the diode selection was important. I prototyped with 1N4009, but the .6V drop limited sensitivity at the desired voltage ranges, so I figured that a Shottkey diode rated at about .2V would make this very sensative. I came up with the design above.
Since everything I chose can run down to 2V, we can use a 3V Lithium coin cell to drive this. The battery is good for 230mAh, and since this circuit has a quiescent requirement of 900µA, this should be more than enough for months of normal play.
I also didn’t feel the need to put in a sensitivity control. Most triggers have one already and getting the cutout on the case correctly with the parts I had was going to be a nightmare.