BELLS AND WHISTLES
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!