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  • Old RC toy made into a garage alarm

    Posted on January 4th, 2010 Patrick 3 comments

    My wife wants some sort of indicator to show when the garage door is open. The garage is visible through the back window, but sight lines make it difficult to determine. Also, when the shades are drawn, there is no visiblity. I considered X10 modules at first, but that just seemed like overkill when all I really wanted was a small led to illuminate.

    Then, I remembered an old RC toy car my son had. The controller unit was simple, with just one button. Pushing the button made the car go forward. Releasing it made it go backwards while turning. On the receiver (car) side, the controller signal caused a voltage to toggle between -1.5 volts and +1.5 volts. With a little testing, I found a point that toggled between 0 and +3 Volts. This was just enough to provide drive an LED. A magnetic reed switch from Axman Surplus worked perfectly for the door sensor. This switch is in the off mode when the magnet and reed unit are near each other (door closed). When the door opens, the magnet will be pulled away and complete the circuit. The transmitter and switch are hooked up to a PIC16F684 chip. The reed switch is connected to pin 4, which is configured as a digital input. When the reed switch is open (door closed), the signal on pin 2 goes high. This turns on an NPN2222 transistor, which in turn causes the transmitter to send a signal. The receiving unit goes to +3 volts on the output when there is no signal, and drops to 0 volts when there is a signal. In other words, when the door is down, the LED indicator is off. This may seem overly complicated, but it has one advantage. If the garage-side (transmitting) unit fails, the light will come on. Although this may be a false positive for the door being up, it indicates that something is wrong on the transmitting side (power failure, loose wire?).

    Circuitry from RC car repurposed as a garage door alarm

    Circuitry from RC car repurposed as a garage door alarm


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