I have a driveway that is over 300 feet long and it is nice to have some advance notice that someone is driving or walking in. Previously, I have used a very expensive IR beam-break detector. It gave a lot of false alarms and eventually failed due to a lightning strike. It also required that I run a very long cable that could survive outdoors, which added more expense. It’s time to switch to a wireless sensor.
I found this inexpensive one at Harbor Freight ($17 with coupon.) It is a simple, stand-alone alarm with a receiver that just flashes some LEDs and emits a tone. It claims to work up to 400 feet and I verified that it works to at least 350 feet, which is good enough for my application. I created the circuit described below to interface this to my Raspberry Pi based home alarm system.
The receiver can use 3 C-batteries or a 6V adapter (not included) and I found that it would work fine on 5V. The simplest way to get a wired signal out of the receiver is to connect to one of the LEDs. This picture shows how I soldered a wire to the positive side of the LED.
This will provide a very brief pulse of 5V, but I need to simulate a normally open switch that connects to ground when triggered. Also, that signal needs to lasts for at least a second to guarantee that the Raspberry Pi will see it when polling the GPIO states.
For more detail: Interface to Wireless Driveway Sensor
