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This simple circuit tests speakers, microphones, transformers and voltage. It's basically a very low frequency oscillator that produces extremely short 'fruity' pulses. The type of sound produced is very easy to hear and to determine the precise direction it is coming from, thus making it ideal for checking the phasing in multiple speaker installations. It is also very useful for car stereo installations as well as public address systems where it can drive dozens of speakers directly on a 100V or 70V line system.... [read more]

This dual polarity power supply is easy to build, requires few parts, and is adjustable from 0-15 volts. It is great for powering op amp circuits, as well as other circuits that require a dual supply voltage.... [read more]

This circuit takes an ordinary loudspeaker and allows it to be used in reverse, as a microphone.... [read more]

The circuit below is designed to be used with the bi-directional lamp sequencer shown above on this same page. Two additional transistors are used to increase the current from the 74HCT138 decoder to control 12 volt 25 watt lamps. A 6.8 volt/1 watt zener diode is used in series with the ground connection of all the CMOS ICs (74HC14, CD4516 and 74HC138s) so that the total voltage across the CMOS devices will be about 5.2 volts and the outputs will move from +12 to about +7 when selected.... [read more]

This simple circuit will energize and de-energize a relay at the push of a button. Pushing the button once - will energize the relay. And pushing it a second time - will de-energize the relay. The accompanying Circuit Description offers a good introduction to the workings of the Cmos 4013.... [read more]

This circuit has been very useful in filling a header tank for a reticulated water supply on a farm. Eight troughs are supplied in different paddocks where a lack of water would have serious consequences for the stock. In the past, the tank had been filled daily by a time clock which was not successful. During hot weather, the stock would empty the tank on a regular basis and then be without water for several hours or the tank would overflow and flood the area if the weather was wet and the cattle did not drink much.... [read more]