On Indicator

  
Battery-operated equipment can work on one set of batteries for a long time nowadays. However, if it is left on inadvertently,
On Indicator - schematic

that "long time" is over very quickly. Moreover, flat (dead) batteries are always found at the wrong moment. The circuit proposed here is a sort of aide-memoire. Every two minutes, it emits 5 to 10 pips to indicate that the equipment is still switched on. Basically, the circuit consists of three rectangle-wave generators and an inverter. The first of the generators is formed by N1 and provides a signal with a period of about two minutes and a pulse duration of around 10 seconds. During those 10 seconds, the second generator starts operating in a one-second rhythm. Thus, N2 outputs 10 pulses every 2 minutes. That output is inverted so that N4, like N2, can only be enabled during the 10-second pulse train from Nl. The difference is that during those 10 seconds, N4 is enabled and inhibited 10 times; this is what causes the pips. Do not take the times and number of pulses too literally, because wide variances are between ICs from different manufacturers. On the other hand, component values are not critical, so it is fairly easy to adapt the circuit to personal taste or requirements. The buzzer can be a standard ToKOhm type or equivalent. The current drawn by the circuit is negligible.




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