Sensitivity is controlled by R1, and you can use small dynamic microphones with impedances ranging from 4 to 200 ohm. A small loudspeaker can be used as a sensitive microphone in this experimental project. The impedance of microphones can be increased by using a small transistorized output transformer. The circuit can also be used to detect vibrations or mechanical shocks. In this case, the transducer can be a loudspeaker.
The circuit is powered from AA cells or a battery, and current drain is very low when the LED is off.
IC1 is a JFET operational amplifier (op amp) that drives the 4093. The gates of the 4093 are used as inverters (IC 1-a) and buffers (IC 1-b, c, and d). To drive a relay, you should use a transistorized output stage as described in several other projects in this site.
A schematic diagram of the Sound-Activated LED is shown in Fig. 1.
Proper positioning of the polarized components (diode, LED, and electrolytic capacitor) must be observed.
Capacitor C3 adds inertia to the circuit. Using large values (between 1 and 10 µF), any short duration sound Will turn on the LED for a time period ranging from some seconds to some minutes. A capacitor (C3) in this position is indicated if you want to modify the circuit to drive a relay.
C1 controls the frequency response. R1 adjusts gain and therefore the sensitivity of the circuit. The microphone can be installed at a distance from the circuit, and you can use common Wires.
IC1 CA3140 JFET operational amplifier
IC2 4093 CMOS integrated circuit
LED1 Red common LED
D1 1N 4148 general purpose silicon diode
MIC 4 to 200 ohm dynamic microphone or small loudspeaker (see text)
S1 SPST toggle or slide switch
B1 6 or 9 V, four AA cells or battery
R1 1,000,000 ohm potentiometer
R2 100,000 ohm, 1/4 W, 5% resistor
R3 1,000,000 ohm2, 1/4 W, 5% resistor
R4 1,000 ohm, 1/4 W, 5% resistor
C1 0.01 µF ceramic or metal film capacitor
C2 100 µF, 12 WVDC electrolytic capacitor
