Convert on/off switch to momentary contact

[aaronml]

For a project I am working on I need to "transform" an on/off switch to instead cause a momentary contact. The current switch just closes and opens in a 3V DC circuit. I want it to cause a momentary contact when it closes, and also when it opens, effectively acting like a momentary contact push button switch.

Any suggestions are appreciated!

 

[Hayato]

A monostable multivibrator should work fine.

 

[aaronml]

Thanks Hayato!

That will cause an output to momentarily close a switch when my trigger switch either closes or opens?

I'm starting from a near zero knowledge of electronics, but have been looking around the web a lot for monostable multivibrator circuits. Based on what I found I think I will experiment with a 555 chip plus the necessary resistors and capacitors.

The trigger switch is actually an electronic house thermostat and the switch I want to close momentarily is a transmitter used for opening and closing a garage door. I'm thinking the first circuit at Finkbuilt Blog Archive 555 One-Shot Timer Project
would do the trick.

 

[KMoffett]

Couple of questions:

1: Will you have any other power source (5v? 12v? 24v? DC? AC?) other than the "3V DC circuit" to power an external circuit?

2: Can you provide a drawing or schematic of the existing circuit?

Ken

 

[aaronml]

Thank you for your response.

I was just planning on using a 9V battery as the power source of the circuit.

On further reflection I believe some of my original assumptions about this whole idea are incorrect, but let me try to explain further. I'm essentially trying to construct an inexpensive wireless thermostat

I have an electronic house thermostat that I want to relocate to the other side of the room, and cannot run wires to that other side without tearing into ceilings and walls. Since the thermostat basically just closes a switch when a call for heat occurs and opens it when the call has been satisfied I thought I should be able to use the closing and opening of that switch to provide an output that would momentarily close the contacts on a transmitter used to open/close a garage door. If I understand correctly, the receiver part of the this simple opener just closes a circuit when the transmitter sends a signal. Closing that receiver when in a circuit with the original thermostat wires would then activate the heating appliance. I was also erroneously thinking that another signal from the transmitter would open the receiver circuit, but I now realize that is most likely not true. Rather, I would need a way when the thermostat stopped calling for heat to cause the transmitted signal to open the receiver circuit rather than to close it.

I appreciate any thoughts you might have.

 

[Hayato]

I didn't get it. Make it more clear, if possible.

 

[KMoffett]

arronml,

A problem with the pulse-on/pulse-off concept is that if there is a glitch in the system, and an unintended pulse gets sent or seemingly received, the furnace can actually be working opposite what the thermostat is requesting. With a garage door, if the door is moving up when you thought it was commanded down, you just push the button again. In an unattended system like your thermostat, this could have dangerous consequences.

Can you power your transmitter from something like a wall-wart, instead of a battery? Then you could have the transmitter continuously sending when heat is demanded, and silent when not.

Ken

 

[aaronml]

Hi Hayato and Ken,

First to answer Ken's question:

I suppose I could power the transmitter from a wall-wart though the transmitter's battery is 12V and I do not think I have any wall-wart that size. However, I just tried a little experiment and it does not seem that the circuit on the receiver side remains in the closed state even when the transmitter pushbutton is held closed. I put an ohm meter across the receiver leads and held the transmitter button down, but after initially changing to show the receiver side circuit closing it quickly reverts to being open. So maybe the garage door opener idea is a non-starter.

As for the problem with signal glitches, I realize this could be a problem and agree that a more positive mechanism would be desirable, but it seems to me it would be kind of self-correcting. If the temperature set point had been reached causing the receiver circuit to be opened and a stray signal occurred to close it again the heat condition should be quickly satisfied and another signal sent to open the receiver circuit. Similarly, if a stray signal opened the receiver circuit before the set point had been reached, the thermostat should quickly call for heat again sending a new signal and closing the receiver side circuit. If the sending circuitry got out of whack and was spuriously sending signals all over the place, that would be a problem.

To try to clarify for Hayato:

I'm trying to make an inexpensive wireless thermostat that will turn a heating appliance on when the thermostat calls for heat and off when the temperature set point has been reached. I have an electronic thermostat that is currently mounted close to the appliance and is hard-wired. All this thermostat basically does is to close a circuit when there is a call for heat and open the circuit when the temperature has been reached. Because it is so close to the appliance it does not control the temperature very well so I want to move it across the room, but I would have to tear into a ceiling and wall, or run the wires on the ceiling and wall surfaces, to extend the current wiring and I prefer not to do either one.

I wanted to use a transmitter and receiver where the closing of the thermostat (calling for heat) would cause a signal to be sent to close the appliance circuit, and the opening of the thermostat (temperature satisfied) would cause a signal to be sent to open the appliance circuit. My idea was to use a transmitter/receiver that controls the opening and closing of a garage door to send/receive the signals. I chose this type of control because I had a spare one lying around. The transmitter part of the control is just a momentary contact switch, and I had thought that the circuit on the receiver side stayed closed once activated. But, from the experiment I did (see my answer to Ken above) it seems the receiver is just making momentary contact too, so that the receiver side circuit would not remain closed even if the transmitter side switch is held in contact.

Maybe I need to look into an IR transmitter/receiver.

Thanks to both of you for all of your input!

Mark

 

[nineball]

I know this is and old thread...but an Arduino can do anything you want it to do. It's open source and relatively cheap.

I'm looking to do something very similar it it looks like the answer...plus opening to door to your imagination