Car beacon

This is my first project after I started experimenting with the ATtinys. Maybe I should call this the poor mans car alarm?

How it works

The device transmit a signal to the RFXCOM RFXtrx433E USB 433.92MHz Transceiver/DomotiGa to tell that the car is where it supposed be (at home). If the car moves out of range (moved away from home), the signal is no longer present and DomotiGa will will push a message to our cell phones.

I have had the idea for quite some time. Initially I thought about having one of my multi purpose moteino sensors in the car, but that's kind of an overkill. I mean if it's -10┬░ celcius outside, the car is also cold...

The circuit is very simple. One ATtiny85 and one STX88 433MHz transmitter running of a 3V coin cell battery. A resistor and LED might be added for a visual indication. The car beacon sends a signal every 48 seconds to announce its presence. If DomotiGa don't receive the expected signal within a minute, the car is out of range.

Low power

Power saving was important for this project, as always. After some experimentation and measuring I ended up with a sleeping ATtiny that will (theoretically) last for 1.5 years with a 600mAh CR2450 coin cell 3V lithium battery. The circuit use only 4.7uA in standby and wakes up every 48 seconds to send the signal.

I use the X10rf library ( to encode the transmission, as the RFXCOM can receive this protocol. I also use this library for my PIR sensors.

1 x ATtiny85 (I use a V-PU10, the low power version that can run down to 1.8V) 
1 x STX88 433MHz transmitter w/antenna
1 x CR2450 coin cell 3V lithium battery (or any other 3V source)
1 x 3mm LED (optional)
1 x 270R resistor (optional)
1 x box


The code (may change without notice):