Timer

This timer can be set to count a maxi- mum of 60 hours. It also allows an inter- val to be set. When this interval is reached, a buzzer sounds. The larger part of the circuit is con- tained in an Intersil Type ICM7217 four- digit CMOS up/down counter and dis- play. Circuit IC3 is the clock that generates a square wave with a period of 1 s. The clock signal is available at pin 3 (Q13). The clock signal may be divided by 60 in IC4 if it is required to time more than 1 hour. When S6 is closed, the supply is switched on and IC, is reset via Its and CS. The position of S4 determines whether minutes or seconds are counted: maximum 59 h 59 min (pos 2) or 59 min 59 sec (pos 1). If, for instance, a total time of 35 min with an interval at 20 min is to be counted, S 4 is set to position 1. Thumb wheel switchesS7 to S1 0 arethensetforadis- play reading of 20.00. Briefly pressing S 3 stores this setting in the memory of ICi. Then S7 to S1,3 are set for a display reading of 35.00. During these...