The signals produced by most percussion sensors will require some processing before they're useful as triggers. Usually this means converting an oscillatory signal into a single unambiguous pulse that is appropriately scaled to carry the widest possible range of velocity information to the circuit being controlled. The circuitry in the diagram below performs this function.
In this processing amp, the sensor signal is connected from the Tip of the jack to the potentiometer R3 which sets the sensitivity. The wiper of the pot connects to D6 and C4 which form a peak detector with a discharge time constant set by R4. The opamp IC2:B is used an voltage follower which buffers the sensor from the input requirements of the circuitry being driven (typically a trigger to MIDI converter such as the PAiA 9201 MIDI Drum Brain.
This circuit is somewhat more complex that is absolutely necessary because it adds a few interesting features. The power supply shown (PWR1, IC1, C1 and C2) also provides a 5V regulated supply that can power the Drum Brain so that no separate supply is needed for it. The LED D4 provides a power indication and also serves as an indicator that the sensitivity of the sensors has been set properly by blinking if the signal needs to be turned down.
R2 and C3 provide a source of power at the Ring of the jack which is useful when using Force Sensing Resistor or Electret Mic as sensors. This allows sensors such as those shown in the illustration below to be plugged into the processing circuitry interchangeably.
The requirements of the application will dictate the choice of sensors. The Electret Mic trigger is the only non-contact device in the group, but may be subject to audio cross-talk when trying to trigger from multiple drums. Piezo Disks have good sensitivity when placed on the drum head or shell and can be used with finger strikes, but they are too fragile to be struck directly with a stick unless encased in silicon rubber or some other suitable potting material. Force Sensing resistors are appropriate for finger strikes, but not much else. They are too fragile to be struck with mallets or sticks and not sensitive enough to use microphonically.