Stereo Sleep Timer

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I wanted to be able to fall asleep listening to my stereo playing the radio, but I did not have a sleep timer fuction on my receiver, so I built this device to add sleep timer functionality to the stereo. The timer can be configured to run in two ranges: 99..1 seconds or 99..1 minutes. This is a countdown type of timer, and when zero is reached, a relay is tripped and remains on until the timer is reset. The timer also remembers the previous count-down time since it is stored in a non-volatile EEPROM on the microcontroller.

Details:
The whole circuit is run on a PIC16F84, which is an 8-bit microcontroller from www.microchip.com. The nice thing about this micro is that it has flash memory, so it is easy to program and it has 64 bytes of EEPROM for storing program configuration, etc. The relay is hooked up to the input of a buffer, and when this is grounded, the stereo is muted. The entire circuit can be powered from a +5 volt supply found on the main board of the stereo.

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