Door Chime with ATtiny12

Posted on Mar 1, 2013

The device described here does just that. The circuit connects to the doorbell circuit - taking power from the 18 VAC from the doorbell transformer and switching power to the doorbell circuit most of the day. The user interface is a pushbutton, a toggle switch, and two LEDs. One LED - the green one -blinks at 1 Hz, just to give me some comfort that the circuit is working. The amber LED glows whenever the doorbell is disabled. To set the timer, you just push the button momentarily, and when its released, the amber LED will come on and the doorbell circuit is disabled (the door bell button doesn't work) for a 10 hour period. At the end of the 10 hour period, the amber LED goes off and the doorbell is enabled.

Door Chime with ATtiny12
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24 Hours after the button was last pushed, the amber LED comes back on and the door chime circuit is disabled again. this cycle repeats every 24 hours, and because its timing is set by a crystal oscillator, it should be able to go for months or years without needing to be reset. The circuit consists of a power supply, a capacitor back-up circuit, a Atmel ATtiny12 microcontroller as a timer, and solid state realy circuit to switch the AC voltage in the doorbell circuit. The simple user interface is comprised of a resync button to synchronize the timer, a bypass switch to bypass the timer if desired, and two status indicator LEDs. It is designed to be used by people who are not technically inclined. The bell transformer, the doorbell button, and the doorbell are not in the enclosure - they are built into the house. The bypass switch and resync (or reset) button are mounted on the enclosure. Power for the circuit is derived from a half wave rectifier. One end of the bell transformer (the one with the orange wire, marked "ORN" connected to it) connects to circuit common each half cycle through one of the diodes in the diode bridge. During the same half cycle, the 1N4002 conducts, charging the 330 uf capacitor up to a maximum of 11 volts. The filtered voltage powers the 78L05 5 volt regulator. The current is limited with the 728 Ohm dropping resistor to a maximum of 30 milliamps in order to minimize the maximum load...

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