Interface to Wireless Driveway Sensor

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.

I use a 555 timer IC in monostable configuration to provide the longer pulse.  The 555 is triggered by a low pulse, so I also need an inverter.  I chose a 7404 IC because I have a stock of these reclaimed from salvage many years ago.  The output of the 555 is a high pulse that lasts 2.2 seconds with the capacitor and resistor values in this circuit.  This is used to control an NPN transistor that will provide a connection to ground as the signal output.

This circuit uses three wires to connect to the wireless receiver to provide power and read the LED state.  Put it all into a project box with some screw terminals and it is ready to connect.  I will find out over the next several days how reliable this motion detector is.  I am sure it will trigger when any deer come by it.  I am hoping that it doesn't produce a lot of false alarms.  Otherwise I will probably end up disconnecting it.

Here is the interface circuit connected to the receiver.  The white wire on the receiver is the antenna.

And here it is all put together.