I am very active on other hobby forums, but this one is new to me. Forgive me for my ignorance, but I am working on a project here and need your help. If you have a normally "closed" switch that has 24ac volts going to it, and when the switch becomes "open" it jumps to 27ac volts, what would I need to make that 3 volt change trip a normally "open" momentary switch to activate something else? A friend told me I would need like a solid-state resistor. Is this right? It is for a circuit board so it can't be big and mechanical. Anyone that replies would be doing me a huge favor if you could possibly answer my question along with a URL to go to for this item. Thanks in advance for any help you can offer!

Aaron

 

Hi Aaron,

where do you get the 27VAC?

A small schematic showing where you measure 27VAC would be helpful.

Boncuk

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Don't have a schematic. The 27VAC is measured at the same point the 24VAC is read at on the board, when the circuit is opened. I need something that can read that 3VAC change and send current to close a momentary switch. Thanks for the reply.

Aaron

 

Sounds like the measurement is between a loaded supply and a unloaded supply. If you just want to detect when switch is off, use the switch contacts.

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Hi Aaron,

Can you at least draw a diagram of the problem? If you don't know schematic symbols then just draw it out as you understand it. It might help us picture what's going on a little better.


Torben

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hi,
The 27Vac state, I suspect is because the load has been disconnected by the switch, its an 'off load' state.?

As soon as you start loading the 27Vac with a detector circuit that 3Vac difference could be much smaller.

This means the detector must very lightly load the 27Vac line.

Quote:

send current to close a momentary switch

Do you mean supply current thru a switch contact or just close a momentary contact.?


Hi Torben, Mike .

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Bill's: www.blueroomelectronics.com/

 

Eric, it sounds like he is using a switch to control a relay or other inductor, and is seeing the back emf as a result of the switch opening.

If that is the case, what he is pseudo-measuring might be the meter needle jump.

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It sounds like you need to rectify the 24vac, filter it, and feed it into an IC comparator that senses the increase in voltage, and the comp drives a relay.
1.4 x 24vac = ~34 vdc, 27vac = ~38 vdc, so the comparator reference voltage should be about 36 vdc.
How fast do you want this circuit to respond?
If the comp output needs to be a pulse, you'll need to add a one-shot.

However, it sounds easier to just sense the voltage across the switch contacts, which goes from zero to 27 vac, and work with that.

 

I own an automotive shop. What I am trying to do is take a standard Genie garage door opener that opens the bay door and wire a wireless chime "into" the already there, IR Safe-T-Beams across the bottom of the door so I know when customers walk through. On the back of the Genie opener are three screws: Top one = wall button, Middle one = common, Bottom one = IR wires. The IR beams of course come in from each side of the door and the red wires go to the bottom screw and the white wires go to the common screw. The voltage to the bottom screw is 24.2vac until the IR beam is broken "or open" which then jumps the voltage to 27.2vac. I want that 3vac rise, to trigger the wireless momentary switch to chime the bell. Guys, I appreciate the responses.

Aaron

 

Yes.

It'd be good if you knew how much current flows through the wires when the voltage is at the low value.

I assume the voltage rise is due to the poor regulation of a 24vac xformer.

Without getting into the guts of the opener, for ~ $20 in parts from www.Hosfelt.com or other suppliers you can build/assemble/test a custom-designed circuit by me or others that will sense this voltage change and provide you with relay contacts that close in order to turn on an audible or visual indicator of some kind.

Depending on your skill level you can build/assemble/test this in a day, so if you pay yourself 10 tax-free dollars per hour for these kinds of projects this will cost you ~$100.

If the output of this comparator circuit needs to interface to an existing chime or wireless momentary switch, please provide info on these items.

You may also want to consider buying a motion detector; that way there is no design, build & test time involved.

 

I considered a motion detector, but I'm worried about a motion detector because the farther the span is, the wider the area it detects. So if I have 15 foot doors by the time it reaches the other side it will detect 15 feet wide or more and be going off all the time with guys working in the shop. I need more of a line or a beam that it breaks to go off.

Aaron

 

Which model do you have?
http://www.geniecompany.com/GenieCompany.aspx?cid=322

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PMX500 ProMax

 

...I'm thinking now that the 24v to 27v change is too small, that normal line voltage variations will swamp out the "signal".

...There might be some way to rectify and peak-detect the 24vac, giving ~34vdc, then feed this signal through a series capacitor into a comparator so that when the beam is interrupted you get a 3vdc drop or rise.
...Slow line voltage variations wouldn't register but an abrupt drop in voltage would trigger a comparator.
...This seems messy and I'm not sure the response time would be fast enough.

...Monitoring the current change that causes the xformer voltage to drop and rise is probably better.

 

So what is the best way to do this Willbe? I really appreciate your involvement!

Aaron