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Thread: 5v relay wiring help!

  1. #31
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    I went ahead and ordered a new version of the device that is supposed to have 12v output it looks the same to me so we will see when i get it.

  2. #32
    Newbie TheGemInIMan's Avatar
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    Quote Originally Posted by Rob Withey View Post
    It will NOT work. With an emitter follower wired with the emitter to the relay, the highest the emitter can go is about one diode drop below the base. With a max base voltage of 3.5V you can expect approx 3V max at the emitter. Go back and read your books about how bipolar transistors work...!
    I stand corrected on his circuit ... only in the fact that there is not enough voltage (or current) to switch on the transistor coming from the FOB ...

    You CAN wire transistors to switch devices with the emitter follower ... but it will need the actual supply voltage to the base to do this ...

    TESTING:
    I done a quick test, just to satisfy my own mind ... a common emitter circuit using a 12v source and a 24v relay (didn't have a 12v one) and a 2N2222A transistor ... put a 1.5v battery on the base with a 1k resistor in series and the relay switched ok ... so the voltage from the FOB should do the same ...

    I also tested in the common collector mode and it didn't work ... it does if you use the actual supply voltage to the base (via a 10k resistor) ... So NO it won't work from the FOB ...

    I am right in what I said about using the transistors EITHER way ... it would've worked if he had a 12v switching source from the FOB ... just wrong on the FOBs 3v being able to switch it, that's all ...

    I have done enough electronics to know how transistors work ... I have used common collector modes quite a lot ... I am more of a practical person than a theorist ...

    The common Emitter mode setup will work fine as is ... it should switch the relay with the FOB supply of 3v ...

    Transistor Switching (the books):
    http://en.wikipedia.org/wiki/Common_collector
    http://en.wikipedia.org/wiki/Common_emitter
    http://hyperphysics.phy-astr.gsu.edu...switch.html#c5

    cheers,
    Gem

  3. #33
    Raw Wave Rob Withey's Avatar
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    Quote Originally Posted by TheGemInIMan View Post
    I stand corrected on his circuit
    You actually stand corrected in your incorrect assertion that it will work. It was already stated that the emitter follower wouldn't work by JimmyFitz, which you chose to disagree with.
    Quote Originally Posted by TheGemInIMan View Post
    I done a quick test, just to satisfy my own mind ... a common emitter circuit using a 12v source and a 24v relay (didn't have a 12v one) and a 2N2222A transistor ... put a 1.5v battery on the base with a 1k resistor in series and the relay switched ok ... so the voltage from the FOB should do the same ...
    Yep, you validated a circuit that was already proposed as working.
    Quote Originally Posted by TheGemInIMan View Post
    I also tested in the common collector mode and it didn't work ... it does if you use the actual supply voltage to the base (via a 10k resistor) ... So NO it won't work from the FOB ...
    Correct.
    Quote Originally Posted by TheGemInIMan View Post
    I am right in what I said about using the transistors EITHER way ... it would've worked if he had a 12v switching source from the FOB ... just wrong on the FOBs 3v being able to switch it, that's all ...
    That's a big "if", and a big "would've", both of which were invalid for this application.
    Quote Originally Posted by TheGemInIMan View Post
    I have done enough electronics to know how transistors work ... I have used common collector modes quite a lot ... I am more of a practical person than a theorist ...
    It shows. Voltage doesn't flow, current flows. Transistors cannot simply be thought of as switches, they are much more complex. The fact that an emitter follower has a unity voltage gain in the common collector link you posted should have been a big clue.
    Quote Originally Posted by TheGemInIMan View Post
    The common Emitter mode setup will work fine as is ... it should switch the relay with the FOB supply of 3v ...
    Yep, I think we established that already.
    Thanks. Now may I recommend the "Art of Electronics" by Horowitz and Hill. The first few chapters give a good grounding in transistors. Following that, "Introduction to semiconductor materials and devices" by Tyagi is a good read to know how semiconductors really behave. This was a uni course book for me many many years ago, and a reasonably good one.
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  4. #34
    Newbie TheGemInIMan's Avatar
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    Crikey ... we can discuss this all day long ... we're taking over this thread ... disregard everything I said ... let's get aalouie's circuit working ...

    cheers
    Gem

  5. #35
    The Last Good Gremlin GizmoQ's Avatar
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    Quote Originally Posted by aalouie View Post
    I went ahead and ordered a new version of the device that is supposed to have 12v output it looks the same to me so we will see when i get it.
    So what ever happened with this? I find myself in the same situation at the moment and would prefer an all-in-one solution ala Rayex LE-3 than having to solder the common emitter setup and mount in a vehicle engine compartment.
    HARDWARE: Fujitsu Stylistic ST5111w/WiFi and dock, internal Hitachi 500G HD, external 1TB HD, Sierra Wireless Aircard 550, DVD-RW, BoomzBox HD radio, XM Commander, Delorme GPS, Saitek X-52 Pro joystick, BluSoleil Bluetooth, TPMS, FB, Elm327

  6. #36
    Constant Bitrate Hessian's Avatar
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    I have a similar problem trying to get my amp to power on with a windows service that raises DTR. I only get 5.4v out when the service turns on and I need 12v to power on the amp.

    I tried a 2222, but I must have wired it wrong, I still only get 5v out the emitter.

    DTR Thread:
    http://www.mp3car.com/vbulletin/soft...r-powerup.html

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