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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.

Wireless doorbell
relay driver

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Fig.1: this circuit alternately toggles the relay each time the doorbell button is pressed. Note that you must use a battery-powered doorbell receiver (ie, NOT directly mains-powered) to provide the input signal.

This circuit is a novel way of using a cheap wireless doorbell (available from eBay or from a discount store) to control a relay.

In operation, a battery-powered doorbell receiver is used to trigger a 4027 flipflop (IC1a) that’s connected to toggle its outputs on each clock input.

The doorbell receiver has an output signal that changes state on receipt of a doorbell transmission. It’s usually high when the doorbell pushbutton transmitter is pressed and low otherwise. This signal clocks flipflop IC1a to turn the relay on or off via transistors Q2 & Q3.

The circuit is powered from a 5V rail which is reduced to a nominal 4.5V via diode D1 to also power the doorbell receiver. The circuit controls a 24V relay which is powered from a separate 24V supply rail.

Thomas Drage,
Bassendean, W.A. ($50)

Microphone adaptor for a violin

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This adaptor was devised to enable me to play violin with a small dance band. It amplifies my instrument using one of the microphone inputs on their sound equipment.

As shown, an electret microphone is coupled to a 5kΩ potentiometer via a 22µF electrolytic capacitor. The potentiometer sets the signal level into a 600Ω isolating transformer which then drives a balanced microphone input via an XLR socket.

The electret insert was secured within some sponge foam under the fingerboard and connected to a 3mm jack socket fitted in the rear of the violin. A long audio cable connects the violin socket to the adapter which is housed in a small jiffy box. It runs from a single 1.5V AA cell in a battery holder attached inside the box with double-sided adhesive tape, as is the transformer.

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