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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Phone mute for PA sound system

This circuit was developed to mute the sound from a PA system in a warehouse, so that when the phone rings, the workers can hear it. This was accomplished without any need for a physical connection to the phone line. Instead, it makes use of the fact that a LED lights on the phone when there is an incoming call. So whenever the LED flashes, the sound circuit is muted.

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The circuit works as follows: with no incoming calls, the LED on the phone is unlit and so the adjacent light dependent resistor (LDR1) is in darkness and its resistance is very high. LDR1 is connected in the base circuit of transistor Q1 so that when LDR1 is high in resistance, the transistor is off.

When the LED of the phone flashes to indicate an incoming call, its light falls on LDR1 and causes its resistance to drop markedly. This turns on Q1 which then pulls the gate of SCR1 high to trigger it into conduction. This energises relay RLY1 to mute the stereo signal to the sound system via its DPDT contacts. Multi-turn trimpot VR1 is used as a sensitivity control.

The LDR is mounted in a piece of heatshrink tubing and held in place over the phone LED with some hot-melt glue. Most phones have a light of some sort which comes on when it rings. Sometimes it’s an LCD screen which lights up and this circuit is sensitive enough to work from that as well.

The system stays muted until the "mute off" is pressed to break the circuit and return the SCR to the non-conducting state. The "mute on" switch can also be pressed at any time to mute the system, feeding a small gate current to the SCR via a 10kÙ resistor. A high-intensity red LED (LED2) indicates when it is actually muted.

Dual-gang potentiometers VR2 and VR3 are used to set the level of the sound system when it is operating and when muted, respectively.

Ron Russo,
Townsville, Qld. ($50)

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