Using this electronic scheme can be built an electronic timer which can be set to measure a maximum of 60 hours. The electronic timer allows the fixation of some intermediate time. On reaching this point, a buzzer will sound. The circuit is built using ICM7217 integrated circuit, manufactured by Intersil, which contains a CMOS counter up / down, with 4 digits and display properly.
IC3 is the clock generator circuit which provides a square wave with period 1 s clock signal is available on pin 3 (Q13). It can be divided by 60 in IC4, if necessary, measurement of longer than one hour.
When S6 is closed, power is connected and IC1 is reset by R9 and C5. S4's position determine if minutes or seconds are counted: not exceeding 59 h and 59 min (position 2) or 59 min and 59 s (position 1).
Miniature switches S10 S7 adjusts the display to indicate 20.00. In a short press of S3, this choice is stored by IC1. After that, S7-S10 is adjusted so that the display to indicate 35.00. Meanwhile, S1 must be open. Pressing S2 cause of integrative ICM7217C start counting backwards,starting from 35.00. When the display shows 20.00, buzzer sounds shorter. Now the clock can be stopped by closing S1 and the subsequent opening of S1, the clock restarts to count backwards to 00.00.
The clock can be reset with the S5, in this case, the buzzer sounds briefly and display shows 00.00.