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Step 1: What You Need
Step 2: Prepare the Receiving Sensor
I tried to get inside the garage door sensor to access the circuit but after a bit of prying decided that it would be better to leave it intact and keep the waterproof design as it was clear the case was glued together. So, using a photocell over the receiver LED was the best option.
1. Epoxy the photocell to the receiver to prevent any light from getting to the cell. I also added some coats of spray paint as these cells are extremely sensitive to light.
NOTE: To see which is the receiver, you can power up the sensors by using 3-4 "AA" batteries (4.5-6 volts). Connecting a 100ohm resistor connected to one of the sensor positive (+) lines will tell you which is the receiver when they both light up.
Step 3: Build the Circuit
The circuit is designed so the photocell triggers the 6 volt relay which in turn closes the switch on the transmitter, sending the signal to the receiver and ringing the chime. I built it as small as possible because I wanted everything to fit inside the smallest Radio Shack hobby box.
Step 4: Finish It
Install the new circuit along with the transmitter circuit inside the hobby box. I drilled 3 small holes. Two on the side for the wiring that goes to the sensor transmitter/receiver and power and one more hole on the bottom that the leads for the photocell would fit inside. I also used rubber grommets and silicone to seal those holes. I used "5-Minute" epoxy to glue the box to the bracket that came with the garage door sensors.
Step 5: Use It
Install the sensor transmitter and the receiver wherever you want to use it. They come with around 30 feet of wire. Power it up and when the beam is crossed, the doorbell transmitter should signal the receiver and you will hear a chime whenever the beam is broken.
I hope you enjoyed this Instructable!