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.
Bassendean, W.A. ($50)
Microphone adaptor for a violin
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.