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Automation FAQ Sponsored by Creative Control Concepts |
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 Home Automation FAQ : General I/O & Sensors :
How can I interface a lighted doorbell switch to my HA system? |
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Typical doorbell switches have a light wired across the switch. When the switch is open, the light draws current through the door bell solenoid. The current is small enough to keep the solenoid from energizing. However, it makes it difficult to monitor the doorbell switch with a home automation system input since voltage is always present on the circuit because of the light.
You can always use a switch without a light, but many people want their doorbell switch lit at night. Another issue is that doorbells are driven with 24VAC which is not compatible with unbuffered digital inputs.
So how can monitor a doorbell switch with a light?
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There may be a simpler way, but...
To use a normal doorbell switch, use an LM324 comparitor or similar. Use a diode or diode bridge to turn it into DC. The voltage should change (increase) when the button is pressed and the light is shorted. Then have the comparator output drive a digital input.
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I usually attach a relay to the bell's coil inputs and use it to drive a digital input.
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I simply wired a 20 ohm, 10 watt resistor in series with one of the wires
running to the button. Then I tapped off both sides of the resistor to a
digital input.
When the doorbell button is pushed I get around 8VAC across the resistor. More than enough to trigger the DI. I had to do it this way since it was the only wire I could get to since my garage is finished.
Wiring it this way still allows the regular doorbell to chime.
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