The timer is intended to actuate an apparatus for a predetermined period of time, for instance, an ultraviolet exposure unit or a photographic enlarger. Periods may be set between 0.1 s and 999.9 s.
The periods are set with the due aid of thumbwheel switches and stored in four Type 74HCT190 counters, IC4-IC7. The setting is facilitated by the set times being displayed instantly on the LED display. After the start button has been pressed, the inputs of the counters are disabled and the countdown starts. At the same time, relay Re1 is energized.
In quiescent operation, the contacts of K2 are linked to those of K1, Since K1 is connected to the mains, the load connected across k2 is supplied with mains voltage; there is no voltage supplied to the load connected across K3. When Re1 is energized, that is, during the switch-on period, the mains voltage is switched from K2 to K3. At the end of the switch-on period, a buzzer in the collector circuit of T1 sounds. In an emergency, the switch-on period may be prematurely terminated with the aid of the stop button.
When the switch-on period is over, oscillator IC2 is disabled and the relay is de-energized. The preset period is then shown on the LED display again.
Power for the clock may be provided by a simple 300 mA mains adapter combined with a Type 7805 regulator.