versatile timer - programmable from seconds to years

. . . program m able from seconds to years The timer described uses few com- ponents but yet has a relatively wide timing range which may be pro- grammed by BCD thumb-wheel switches. The circuit may be modified to cater for even longer periods than shown, up to years if required. 4-98 1N 4004 30 mA IC8 78L05 5 V —s—C ) Tam O 1711: O The circuit consists basically of a clock generator and a presettable N BCD down counter. The master clock is derived from the output of the bridge rectifier. This voltage is stabilized at 5 V by a zener diode and then applied to schmitt-trigger N9 to produce a pulse train of 100 Hz. The pulse train is first div- ided by 100 in IC1 and then by 60 in IC2 to give a final clock of 1 pulse/min. Counter IC3 counts down from a number set by the BCD switches at a rate of one step per clock pulse. Ordinary SPST switches may be used instead of the BCD ones, but the binary code for the required time IC4 4 4„Y—* TR N2 interval must then be remembered. W h...