# Transistor help!

Discussion in 'General Electronics Discussion' started by eveares, Feb 25, 2013.

1. ### eveares

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Feb 25, 2013
Hi, I am new to electronics and thus sorry if this does not make sense.

I want to build a simple circuit with transistors and resistors that will operate a emitter pin on a transistor when the current reaches or exceeds a certain level on a smoke detector.

The collector pin voltage will be 15-24 volts

The base pin voltage will be 5 volts or less

The emitter pin voltage will be 5 volts or less

And the the emitter needs to operate when the collector pins has 1ma or more passing through it.
(smoke detector consumes under 1ma in standby state, over 1ma in alarm state)

Finally can you please tell me how to calculate such a thing and what transistors and resistors i need how they should be connected.

Regards; Elliott V

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
I think you don't understand transistors.

Explain what you're actually trying to do, like"

"I want to operate a relay when the smoke detector goes off"

It sounds like you've determined that there is a current which varies and that is what you want to use as a control signal. However I'd have to know how you've come to this conclusion

3. ### eveares

3
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Feb 25, 2013
Thank you Steve;

Basically I want to control and activate a alarm/sounder when the smoke detector is triggered. The detector consumed 16ma at 22.5v in alarm state and 60 micro amps at 22.5v in normal state. I want to be able to detect when the detector is in alarm state and activate a transistor what will pass 5 volts to a pin on a i/o board.

(How do Conventional fire alarm panels detect detector state?)

A Relay can not be on its own due to the detector does not let enough current through to activate the relay. The detector wired directly to the alarm/sounder is a similar situation.

4. ### Miguel Lopez

252
63
Jan 25, 2012
That's why transistors are used. To amplify signals.

Do you have any draft of the circuit you intend to build?

Take care. Smoke detectors usually contains radioactive materials.

AFAIK, they use the radioactive material to ionize certain piece of space between two electrodes to allow current to flow in the air. When smoke gets into this space it block the flow of current and the detector changes from one logical level to another. Something like that, more or less.

5. ### eveares

3
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Feb 25, 2013
Re: Miguel Lopez

Thank you, however i still have a couple of questions.

1) How do I know that the transistor will amplify the alarm state/16 milli amp current draw signal but not the normal standby state signal/60 micro amps current draw of the optical smoke detector?

2) How do conventional fire alarm panels with 2 wire detectors (i.i non addressable panels) detect whether a smoke detector is in alarm or normal state. (Guessing resistance change)

3) What transistor would be needed (22.5 volts from detectors, 5 volts to i/o board) - so may of them?

Help, much appreciated - Elliott Veares.

6. ### Miguel Lopez

252
63
Jan 25, 2012
Yes, detecting the change in the current consumption can be an option.

Sorry, no idea. Everything depends on the design of your smoke detector and the panel.

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
Detecting the supply current to the alarm itself is an option but it is not a very good one.

You are better off finding a signal output from the alarm and use that to turn the transistor on and off.

Conventional alarm panels often rely on a loop that goes open to indicate an alarm condition (that way a cut or broken wire will indicate an alarm condition)

The radioactive source inside these devices is pretty harmless unless you ingest it (or do something which allows it tho be ingested). As an alpha source, the radiation can't penetrate your skin and only go a few cm in air. However it's not a good idea to remove the covers and expose it. The cover I'm talking about is an obvious shield, not the plastic case of the smoke detector.

Perhaps you can get some ideas from this.