Various Circuits

There is practically no house without an overhead tank (OHT). People who use electrically-operated water pumps for filling the OHT find it very inconvenient to switch off the pump when their overhead tank starts overflowing, specially when they are busy. So there is plenty of water wastage as well as wastage of power (consumed by the pump). However, there is a solution to get rid of this headache...

This simple circuit is sure to have the police beating a path to your door - however, it has the added advantage of alerting you to their presence even before their footsteps fall on the doormat. The circuit transmits on Medium Wave (this is the small problem with the police). IC1a, together with a sensor (try a 20cm x 20cm sheet of tin foil) oscillates at just over 1MHz. This is modulated by an audio frequency (a continuous beep) produced by IC1b...

A common need in biological experimentation is the ability to unobtrusively monitor the activity of small caged animals, such as mice, rats, and rabbits, as they move about their enclosures. This monitoring must be over long periods and around the clock. The circuit in Figure 1 allows you to effect long-term, unattended collection of animal-behavior data using any standard PC with an RS-232C port...