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A small 12v to 9v project; help needed!

  1. May 19, 2010 #1
    Hi guys. I have researched this topic A LOT. In fact, I found these forums as a result. Evidently, it has been asked here before (Google knows all!). Mine has a slight twist to it. All the research I have done so far has been about people wanting to run a continuous 9v load off a 12v source. Mine will be for a fraction of a second at a time.

    Project: I want to hardwire a 9v garage door remote into my motorcycle's low-beam circuit. The jumpers under the activation button on the remote will be soldered together so as to keep it permanently primed to send the pulse if power is provided. I would then solder wires into the remote's battery terminals and splice the positive wire into the power wire of the low-beam circuit while -ve will naturally be grounded. I always ride the bike with my high beam on so the idea is to get close to the garage, switch from high to low for a fraction of a second and switch back. That should open/close the door.


    - Do I even need to drop the voltage if I will be powering the remote for such short bursts? If for any reason, I am forced to have the low-beam on for an extended period (i.e. the high-beam coil burns out while on an unlit road at night), would it immediately fry the remote's circuitry?
    - I was thinking of using a single 1K Ohm resistor and wiring it in series with the low-beam wire and the remote. I came across http://www.bcae1.com/resistrs.htm which suggests that per Ohm's Law a resistor in a circuit takes all the load, BUT they don't say what would happen in my proposed scenario if the voltage is measure forwards of a single resistor. I am sure it would drop somewhat.
    - My research has pointed me to zener diodes as well. But I am thoroughly perplexed by how they work. Are they the answer?
  2. jcsd
  3. May 19, 2010 #2

    http://mipsa.p.lodz.pl/download/BazaWiedzy/zasilania/LM7805.pdf" [Broken]
    Last edited by a moderator: May 4, 2017
  4. May 19, 2010 #3
    That was fast! Thanks. I will look into it. Seems to be hard to find online but I will call a few places to check.
  5. May 20, 2010 #4
    Why not just put a 3.3V Zener in series with the 12V line going to your remote?
  6. May 20, 2010 #5
    Zener option was one of my 3 questions in the original post. But it does appear that it only drops the voltage and cannot regulate it. The bike does not put out a constant 12v, it can vary from 12-14v depending on the load on the battery. So the voltage regulator option seems best because even if I do have the low beam on for an extended period, it will not harm the remote.

    I used one of the stickies in this forum to find LM7809 for sale on several websites, one of which was of my own employer (Arrow Electronics)!. I am in a completely different division and had no idea we even sold such components. So I now have 3 x LM7809 on their way and hope to complete the project by Monday. Thanks to everyone who viewed this thread!
  7. May 20, 2010 #6

  8. May 21, 2010 #7
    Using a 9 volt Zener in conjunction with a load resistor will provide a constant 9 volts as long as the supply is greater than 9 volts. Using the diagram above.
  9. May 21, 2010 #8
    I don't know which brand of garage door opener you have but generally they are not very sensitive to battery voltage. The receivers are super-regenerative with a receive bandwidth of about 1.5 MHz. A little frequency shift won't hurt you and I think the main effect of varying voltage would be a variation in range. I suspect that the components would survive operation on 12 V instead of nine.

    Thus my suggestion to use a 3.3 V zener or perhaps just a dropping resistor would be sufficient. By the way most Chamberlain garage door openers do work on 12 volts though if you don't have Chamberlain system that won't help.
  10. May 21, 2010 #9
    skeptic2, I wish you had posted that sooner! I had read that many 9v devices can run on 12v but wasn't sure if garage door remotes do as well.

    It is a Wayne-Dalton system. They used to make a 12v remote for my model but recently discontinued it and even calling their corporate office directly didn't bring about any success. It was the first thing I tried.

    Chamberlain makes a popular universal remote that would work with my system (303MHz) but it operates on 6v so an even larger drop to contend with. Genie is one brand that almost exclusively has 12v-only transmitters but they don't make a universal model.

    Since I have the LM7809 on the way already, I will go with that option. It seems to be the simplest of them all as well since (based on Xitami's diagram) wiring it alone in series is all that's needed. Thanks for everyone's continued input!
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