How to build an electronic drum pad
If you want a basic electronic drum sound with no frills at a minimum cost here it is.
This circuit makes use of an active filter's transient response to produce the sound. This is a circuit commonly used in older rhythm boxes.
Lengthening the decay time and adding a pitch sweep produces a basic drum synthesizer sound.
The circuit is simple and construction is not at all critical. Parts values are pretty flexible too, so you can probably build it with parts on hand.
How it works.
IC1 and IC2 are common 741 type op amps. Using a 4136 and built three separate drums on one board, with IC2 acting as a mixer for the three drum sounds; however, the schematic in figure 1 shows the pinout for 741-type amps.
To mix in additional drums, feed the drum outputs into the inverting (-) input of IC2 through additional volume controls (R5) and 10k resistors (R3).
IC3 is one-sixth of a 4049 hex inverter, and is used as a voltage controlled resistor.
Note that power is not applied to the 4049. Remember too that this is a CMOS chip, so observe proper handling procedures.
Testing. Once the circuit is wired, set RI for minimum resistance and R7 for maximum resistance. Apply power and connect the output to an amplifier (the circuit will also drive high-sensitivity earphones directly): Turn up the volume with R5. Now set R9 to maximum and adjust R14 until the circuit oscillates; next, back off on R14 until the oscillation just stops.
Now press switch SI and experiment with the controls:
R9 controls the decay (length of the tone)
R1 controls the pitch
R7 control* the sweep rate, and
R5 controls the volume.
The range of the pitch control is a bit narrow and the sweep range suffers at lower pitch settings. If you want a different pitch range, experiment with Cl's value.
You can use any momentary switch for S1, ad example you could use a Remo practice drum pad (the kind with a real drum head on it) for SI. See figure 2.
Glued aluminum foil to the bottom of the drum head, and another piece of foil to a block inside the practice pad. Connect the switch wires to each piece of foil with a screw.
When the head is struck by a stick the pieces of foil contact each other, thus closing the "switch" and triggering the drum.
Although this circuit doesn't give touch sensitivity, noise, upward sweep option, LFO, or similar extras, the basic drum sound is as good as most analog commercial equivalents.
Resistors (10%, 1/4 Watt except as noted)
RI 500 Ohm pot
R5 10k audio or linear pot
R7 50k linear taper pot
R9 100k linear taper pot
R14 500k trimpot
Cl 0.05 to 0.1 uF (see text)
C2-C3 0.1 uF mylar
C4 0.22 uF mylar
C5 2.2 uF electrolytic
C6 10 uF electrolytic
D1-D3 1N914 or equivalent diode
ICI, IC2 741 op amp (see text)
IC3 4049 CMOS hex inverter