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This circuit is not a novelty, but it proved so useful, simple and cheap that it's worth building.... [read more]

If you’re expecting an important visitor but you just have to step out for a moment, an electronic doorbell memory can come in handy so you can see whether someone rang while you were out. Of course, you can’t tell whether it was the visitor you were expecting who dropped by then, but a call to the mobile phone of the person concerned can quickly answer that question. A doorbell memory can also save you the trouble of going to the front door (if you live upstairs) when you think you heard the bell but aren’t sure. And if you can’t buy one, then of course you can build one yourself! Read on to find out how.... [read more]

This circuit relies upon the extra high input impedance of a FET, and also demonstrates the gate terminals sensitivity to changes in voltage. The gate terminal here is left open circuit, connected only to the "probe" this being just a few inches of bare copper wire.... [read more]

The circuit constitutes a natural extension of Mic-Line balance unit, but can be adapted to any other audio circuit. It is made up of two units in the series : A) A classic unit bass/ Treble and B) a parametric unit of mid frequencies. Both units can also be used autonomously or each one separately, with the required buffer unit, from previous and subsequent circuits.... [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 is intended for alerting purposes after a certain time is elapsed. It is suitable for table games requiring a fixed time to answer a question, or to move a piece etc. In this view it's a modern substitute for the old sandglass. Useful also for time control when children are brushing teeth (at least two minutes!), or in the kitchen, and so on.... [read more]