Circuit Schematic Diagram of Voice Operated Switch using LM346 IC for Public Address Amplifier System

In public address amplifier, feedback is something that need to be avoided. Of course adjusting the speaker position and the microphone will solved the problem, however this solution is not always possible in many situations. Another solution is to change the frequency 10Hz lower than the input frequency using frequency shifter. However, this solution is too costly for a low cost public address system. Then, another alternative which doesn’t cost much is using the voice operated switch like what shown in below schematic diagram :

Voice operated switch circuit schematic

voice operated switch circuit schematic diagram

IC LM346 is a programmable op-amp. The op-amp shuts down and its output goes to a high impedance state by pulling the SET pin(s) to V-. With this property, this IC dan be used as a low speed analog switch for VOX application. From the figure above, the first op-amp is the buffer for the microphone with gain=2. The second op-amp and the diode 1N914 is configured as peak follower, follow immediately the signal peak but falls down slowly because the charge of the 10uF capacitor can’t be sinked by op-amp output (blocked by the diode) and must be discharged slowly through 10M resistor. The slow discharging will ensure the voice signal is not muted at short silence between words and sentences. And then, the third op-amp is configured as a comparator, which its reference is adjustable through the 10k potentiometer which also needs to be adjusted to set the proper level where the sound activate this circuit. The last op-amp doing the voice muting, when the control voltage output drop to zero because no sufficient sound level detected on the microphone. Thus, connect your mic to MIC IN, and the AUDIO OUT to your microphone amplifier input. CONTROL output is provided to control the other equipment such as lamps or relays. [Source : National Semiconductor Application Notes]

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