How to Design Schematic Circuit of Audio Switch.

 This audio controlled relay has a large number of applications and is interesting to the experimenter who wishes to explore the working universe of relays

and audio or sound controlled circuits.

An audio signal in the amplitude range between 1Vpp and 5 Vpp triggers the

relay, powering on the device, wired between points A and B.

Relay value is determined by power supply voltage. For a 12 volt power supply you must use a 12 volt relay. Gurrent drain depends upon the power supply voltage and also the relay used. Using a12Y x 38 mA (Radio Shack 275-

248'1relay, the current drain is low enough to allow the use of common cells in

the power supply.

Cl can range between 0.01 and 1.0 uF, and determines the sensitivity of the

circuit in the audio range. Large values give more sensitivity at low frequencies.

This circuit has a high-impedance input and needs a strong audio signal to

operate.

Figure I shows the schematic diagram of the Audio Switch. Observe that the

heart of the circuit is a common general-purpose NPN transistor that acts as

a DC amplifier.

ln Figure 2we show layout of components for this project. The components

can be soldered on a terminal strip used as chassis.The trminal strip can be

flxed on a wooden board.

Position of the polarized components, such as the diodes, power supply and

capacitor C2, must be observed.

To use this audio relay you have to wire ponts A and B to the output of an

audio amplifier (minimum power required is 5 watts) and adjusthe volume

control for best operation.

A small transistor output transformer should be used to operate the relay with

low-impedance signals, such as the ones from the output of audio amplifier,

Figure 3 shows how to use this transformer.

R, depends on the output power of the amplifier and is given below:

Output Power

0 to 1 watt        

1 to 10 watts     47 ohms, 1/4 watt

10 to 25 watts   100 ohms, 1/4watI

25 to 50 watts    220 ohms,1/2watt