Magnetic switch connection problem.

Jul 24, 2007 5 Replies
S

Hi to all. I have a simple(I think) problem that I'm hoping someone can help me with. I want to connect a remote transmitter / receiver pair ( garage door opener type thing) to an alarm system magnetic switch.When the door/ window is opened it sends a signal to the receiver and activates the relay. Hope I'm getting the idea across. I only have normally open switches. If the door/window is closed and the magnet is near the reed switch it is CLOSED. Does anyone have any idea how I can connect the xmitter to this arrangement so that it is turned on when the switch becomes open. It must draw no power while closed.(battery operation) The obvious answer is use a switch that is open when near the magnet and closed when moved away(N/O) , and connect this to the button of the xmitter , But I don't have this type of sensor. My initial idea was to connect the xmitter from Batt+ to gnd through a PNP transistor(emitter) , collector to GND. The base connected to Batt+ via mag switch and the base to GND via a resistor.The switch keeps the pnp off until it opens and the resistor turns the transistor on.Problem it the constant current draw via the switch and resistor.



Any clever ideas how to get this arrangement to work? Hope I'm not being to vague in my description.



Cheers Rob



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J

Use a bias magnet to keep the switch closed when the window is open and place it so that when the window approaches the switch the magnet on the window distorts the part of the bias magnet\'s field affecting the switch ,allowing the switch to open.
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S

Thanks for the tip John , but it still does not help me. I probably did not explain myself well. How can I connect the switch so that when it opens the xmitter gets powered. I'll short the button on the xmitter so that when power is applied it transmits.

Cheers Robin

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E

The real issue lies in exactly what you said: "The obvious answer is use a switch that is open when near the magnet and closed when moved away(N/O) , and connect this to the button of the xmitter , But I don't have this type of sensor."

John's suggestion converts the sensor you do have into the sensor you don't have.

It provides the function you want by modifying the switch, not the circuit.

Ed

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S

Ahh , got it now. Thanks for the help. Cheers Robin

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J

Run a solar cell on the roof to keep batteries charged. Make sure it's hard to get to. Use a MOS Type switch which requires very little current to operate the gate and you then simply short the bias source with your switches in series, one end of the string to common and the other end to the input sensor.. Look for Logic FETS, HexFets ring a bell. they have many. you need to look at the turn off/ON resistance etc..

"I\'m never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5
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