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Newbie Question about resistors,transistors?

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AaronOregon

New Member
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
 

Boncuk

New Member
Hi Aaron,

where do you get the 27VAC?

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

Boncuk
 
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AaronOregon

New Member
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
 

Mikebits

Well-Known Member
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.
 

Torben

Well-Known Member
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
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
 

ericgibbs

Well-Known Member
Most Helpful Member
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
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.

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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AllVol

New Member
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.



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


Hi Torben, Mike .:)
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.
 

Willbe

New Member
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?
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.
 
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AaronOregon

New Member
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
 

Willbe

New Member
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.
I want that 3vac rise, to trigger the wireless momentary switch to chime the bell.
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.
 
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AaronOregon

New Member
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
 

Willbe

New Member
...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.
 
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Willbe

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

Aaron
...The best would be for the PMX500 people to cough up a system schematic, since you're a "paying customer". Maybe there is one inside the opener housing. I'm sure there are signals inside the unit that could be used without modification, if we only knew where they were.
...Lacking that I'd need to know what current flows through the sensing box when the voltage is at the high level and at the low level. You'd need an ammeter to measure this, clamp-on or otherwise. Or, you could use a low value sensing resistor in series with the box and measure the voltage change across it.
...I'm still not sure how to interface to the wireless chime thing you mentioned; my gadget would provide relay contacts that could be used to set off an audible or visual indicator.
...Photos may help, closeups of terminal labelling and distance shots.
 

Muhammad89

New Member
hi everyone
I am new to this chat and would like a helping hand from the experienced to help me out understand the what X CUT TRACK means in the circuit attached. I tried to connect the circuit exactly as it is wired but it didnt seem to work properly. All lights were on at the same time instead of functioning each one on its own to fulfill the purpose it is built for , as I traffic lights controlling circuit.
any help would be appreciated.
 

Muhammad89

New Member
hi everyone
I am new to this chat and would like a helping hand from the experienced to help me out understand the what X CUT TRACK means in the circuit attached. I tried to connect the circuit exactly as it is wired but it didnt seem to work properly. All lights were on at the same time instead of functioning each one on its own to fulfill the purpose it is built for , as I traffic lights controlling circuit.
any help would be appreciated.

 

AaronOregon

New Member
hi everyone
I am new to this chat and would like a helping hand from the experienced to help me out understand the what X CUT TRACK means in the circuit attached. I tried to connect the circuit exactly as it is wired but it didnt seem to work properly. All lights were on at the same time instead of functioning each one on its own to fulfill the purpose it is built for , as I traffic lights controlling circuit.
any help would be appreciated.

Huh????? You might want to start a new thread Muhammad!

Aaron
 

kchriste

New Member
Forum Supporter
PMX500 ProMax
Looking at the manual, there are only two wires going to both the "source" and "sensor" which leads me to believe that the "AC" you are measuring is a square wave. They must pass both data and power on these two wires to the "source" and "sensor". The "jump" in voltage that you measure may just be a data burst on the line. A little experiment for you. Measure the "AC" voltage going to the STBs while using the remote opener to open/close the door and see what happens then.
 
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