I'm curious as to how long this mod works. When I last added electronics to a car, it was a 1989 Civic and the CMOS chip I used lasted two weeks. I had to protect the circuit from noise on the power line; then it worked for the remaining 8 years or so I had the car. Do these garage door openers have protection? They wouldn't really need it to run from a battery. Are the new Civics better about delivering a more constant unwavering voltage? Firing spark plugs needs lots of current, so it makes noise, but I don't know how much it's isolated from the rest of the car's power supply.
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Originally Posted by J_Bone
If I remember right, you can add a 390 ohm resister in line to reduce the Voltage from 12volts to 9 volts.
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Might be a little late, but it is my understanding that will work only if the circuit draws about 31mA. (V=IR) If it needs more current, the voltage will drop further. If it needs less, the voltage will rise. It'll probably be OK if the voltage doesn't rise above 9.5v, but expect the voltage to drop a little under the load of transmitting so measure when not transmitting. The voltage from a battery will drop, too, especially as the battery wears out. The transmitters are made to deal with this, so I think using a resistor will work, but you'll need to measure your results and change resistors accordingly. A more robust solution is to use a 78L09 voltage regulator, but a garage door transmitter probably doesn't need the extra help.