Radio Shack Safe House Infrared Photorelay Alarm System (catalog #49-201)

The Radio Shack Safe House Infrared Photo relay/Alarm sounds a tone any time something blocks its invisible infrared beam.  You can use the alarm across a doorway or any opening up to fifty feet wide. The invisible bean has to be setup to bounce into a reflector that can be setup up to 50 feet away. There is an alignment indicator (red light on top) that helps you install and test your alarm and a sensitivity control (dial on back) that lets you adjust the alarm for best operation. The unit sensor and reflector have to be aligned to work. Most automatic garage door openers have a safety beam, an invisible light beam across the garage door opening that will automatically reverse the door if anything interrupts the beam while the door is going down. Garage door beams come in pairs while the alarm uses the reflector to bounce the beam back into itself.

The alarm is virtually unaffected by room lights, indirect sunlight, or other light sources, so false alarms from these sources are not likely.

There are 3 Alarm Modes that let you set the alarm so that when someone blocks the beam, the alarm can sound for 5 minutes, sound only while something blocks the beam, or trigger relay contacts - 12-Volt DC output terminals let you connect and power and external device.

What can I use this for? A Halloween project?

What is nice about this unit is the fact that it has 12 VDC output terminals which could be used to power a prop or to switch a relay to power something requiring 120 VAC power.

I’ve used this unit to effectively trigger a fog machine when the invisible light beam is broken.

Beware; the following is intended only for people with electrical and electronic project skills and the knowledge of the involved hazards to property and to life forms including people, and the necessary safety procedures. Use the information here only at your own risk.

Like most people, I own a fog machine that came with a manual switch that connects to the back of the fog machine with a long cord. The connection to the fog machine is a standard IEC male connection. You could buy an IEC extension cord and could use the female end of the cord (see picture below) and could try to figure out the proper wire connections to trigger the fog machine without the store bought switch.

I decided to use the cord that came with the fog machine for the modification that would allow the infrared photorelay to trigger the fog machine’s switch to output fog.

By using a 12 VDC coil relay I hacked the manual fog trigger that came with the fog machine. The switch that came with the fog machine is a momentary switch (one that rocks on and then returns to an off position after you let go).

I opened housing to the switch and determined which two wires, “connect” (complete the electrical circuit) when the switch is pressed and connected the relay; common pin and normally open pin (by wires) to these two wires. I drilled a small hole in the end of the switch housing and pushed a small set (positive/negative) pair of wires into the hole and soldered to the coil contacts of the relay. I secured the relay to the inside of the switch housing by hot glue. The wires that protrude out of the hole I drilled get connected to the output terminals (proper negative/positive connections may be required).

The end product is a simple way to “trigger” a fog machine to spit fog once the beam is broken. Your fog machine should have some internal protection to shut off the fogger when the fogger starts to overheat or can’t not pump fog juice. If your fog machine doesn’t have these protections, you could jeopardize the operation of your machine. I’ve not had a problem with this setup with my machines. Every machine is different.

You can find a used “infrared photorelay alarm system” on Ebay. I’ve seen them sell for $5 + shipping and handling. The unit is somewhat heavy and shipping is usually $10-$15 dollars.