Sensing 16VAC-24VAC Switch Closures

KatyBri

Hello Everyone,
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I have a project which has a 16 VAC circuit powering a·several standard doorbell buttons. I am using the 16 VAC to power the light bulbs in each button, which I want to work.
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To interface these button to sense their closure, typically requires the use of standard relays - one relay for each button. The coil of the relay would be energized by the switch closure, and the subsequent relay's contact points closure would tell the BS2 which relay (button) closed.
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1. Is there a simpler way to do this using the 16 VAC loop?
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2. Would it be possible to replace the bulb in each doorbell button with a clear 5 VDC LED, and power the loop with 5 VDC? This would eliminate the need for all of the relays, allowing me to connect the buttons directly to the BS2 and keep the doorbell buttons lighted.

I'm not familiar with the newer types of LEDs used in flashlights, etc. I don't know if I need one of this type or a standard type to properly "light" the doorbell button.·Furthermore, I·do not know if they are offered in sizes small enough to fit into a doorbell button.
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Thanks.

Sparks-R-Fun

KatyBri said...
1. Is there a simpler way to do this using the 16 VAC loop?

There is probably a simpler way, depending on what you mean by simple. What you have is pretty simple right now. You could use resistors, diodes and capacitors to condition the 16VAC signal into something that can easily be sensed by the BS2. This eliminates the relays and possibly costs less. It also increases the parts count. Does that make it simpler? You decide.

KatyBri said...
2. Would it be possible to replace the bulb in each doorbell button with a clear 5 VDC LED, and power the loop with 5 VDC?

I am sure there is a way to do this as well. Do you really need clear LEDs? I think the more common JUMBO LEDs will light a doorbell button just fine if the one of the standard colors is to your liking.

I think option 2 is a good choice, especially if these doorbell buttons are not actually housed outside. (In a harsh outdoor environment the 16VAC signal has some advantages over 5VD is terms of prolonging contact life and ensuring reliable switching.)

The lighted doorbell buttons I have seen power the light and ring the bell using the same two-wire pair. This makes detecting a button press a little more complicated than sensing the simple switch closure of a non-lighted button. Do you need the light and button to share the same two-wire pair or are separate light and button circuits acceptable?

- Sparks

JonnyMac

I've used an AC opto detector call the H11AA1 to sense AC for an SX project -- these are cheap, easy, and provide optical isolation. You could use back-to-back LEDs to replace you lamps, just insert proper current limiters for each.

KatyBri

"I've used an AC opto detector call the H11AA1 to sense AC for an SX project ..."

I looked at the datasheets for the H11AA1 and did not see any examples. Could you post a schematic showing how to connect the AC input and its output to the STAMP?

JonnyMac

It's pretty easy, really, a couple 680-ohm resistors on the input, a 10K pull-up on the output. The signal to the BASIC Stamp is active-low.

KatyBri

JonnyMac,

Thanks very much!

mskelect

I need to do a similar thing with respect to sensing the presents of a AC voltage but in my case, 220 volt AC to a BS input pin. I would suppose that the circuit shown could be used but with an additional voltage divider circuit. Anyone care to provide a circuit that would work?

JonnyMac

I use that circuit at with 15K resistors into the opto in 120 VAC systems; it stands to reason that you could bump those to 30K for a 220 system. Use 5w resistors, and be very careful as each resistor will be dropping over 100 volts across it.