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Zach1
Premium
join:2006-11-26
NW Minnesota
reply to bemis

Re: using a lighted doorbell button in/output from uC

I think I would start the project by connecting a full-wave bridge with the AC input side in series with the existing doorbell circuit. This would allow current monitoring and control using off-the-shelf DC components and techniques with very minimal disruption to the existing doorbell circuit. For the current monitoring portion, I'm thinking two standard diodes in forward bias between the '+' and '-' of the bridge output. This configuration would give you 1.2~1.4V across the diode array when there is a load on the circuit. A resistor (guessing about 5~10 Ohms) in parallel with the diode array should allow the button light to stay lit while keeping the voltage across the diode below forward bias. An OP amp would work as input buffering for the PIC. A transistor would work to cut power once candy runs out and the tricks begin.

What a cool idea though!
--
Zach



cowboyro
Premium
join:2000-10-11
Shelton, CT

said by Zach1:

For the current monitoring portion, I'm thinking two standard diodes in forward bias between the '+' and '-' of the bridge output. This configuration would give you 1.2~1.4V across the diode array when there is a load on the circuit.

That's a really bad idea when dealing with separate circuits. There should be galvanic separation between the uC circuit and the doorbell circuit. An optocoupler or relay should be used in such an instance. Imagine the transformer fails and you end up with high voltage on the 24V line... Not to mention you are starting with a floating circuit (24V) so you don't have a defined potential in respect to the ground or to the uC circuit.