The time interval of this circuit can be varied digitally through the DIP switches. The time code however must be set in BCD form. A 120 Hz signal generated by doubling the
line frequency is used as the time reference. This is then divided to 1 pulse per minute by the ICs 1 and 2. The counter IC3 counts backwards- that means it starts to count
from the value set by the DIP switches going back to zero.
Pressing the start switch starts the counter and at the same time the triac is triggered into conduction. Once the counter reaches zero, the triac is switched off and a tone
is generated. The stop button offers the possibility of forcibly stopping the counter in the middle of a counting process.
Electronic Circuits volume 1.0 - Circuit Nr. 30
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.