80-Bus News


July–August 1984 · Volume 3 · Issue 4

Page 10 of 43


For the absent minded, some sort of timer is needed, and for simplicity, one with a fixed period of 10 or 20 minutes before it switches off the supply will be described. Some sort of audible indication is a good idea, particularly for those enthusiasts who are deeply into meditation (or assembly language).



The timer and audible warning can be made up from a couple of 555 timer i.c.s, one connected as a long period monostable pulse generator and the other as an astable multivibrator. The pulse generator operates a relay with two pairs of SPCO contacts (coil resistance greater than about 60 ohms) and allows one set to supply current to the lamp for the predetermined period (10 or 20 minutes). The 10k preset resistor may require some slight adjustment to compensate for the tolerances of the timing capacitor and resistor. For a period of 10 minutes, Ct is 47 uF and Rt is 11.5 megohms while for 20 minutes, Ct is 100 uF and Rt is 11 megohms; these values are probably accurate enough for most purposes. At the conclusion of the timing period, the relay de-energises and the other set of contacts supply current to the astable. A tone of about 1kHz is produced by the high impedance loudspeaker or a ceramic resonator which will continue until the power supply is switched off or the timer reset. The 555 ic is very suitable for these applications since it can operate a load of up to 200 mA directly and works well from a 12 volt supply line.

A more sophisticated version which enables the user to check on the state of erasure was featured recently in Elektor (no. 108 – April 1984) – the magazine whose projects usually work first time!

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