Alarm Exit Delay Circuit

The small box above houses a circuit that I designed and programed to allow a seven minute delay before the alarm is set. The device consist of a pic 16F877A microcontroller, a transistor amplifier, a relay and a piezoelectric speaker. When the button is depressed the unit comes out of stanby mode and begins a series of sharp tones, the LED also begins to flash. after 5 minutes the off cycle of the tones shortens this signals time is running out. In the last minute the LED is on constant and the tone is constant. After the cycle is complete the tones, and LED cease and a relay drops out which closes the contacts for the alarm. During the timing sequence the user can return the alarm to standby mode by depressing the button for aprox. 5 seconds until the tone goes to constant then releasing the button.

Top view of the wire wrap vector board assembly

Bottom view of the wire wrap vector board assembly