Vehicle Tracking Beacon


Overview

Build a simple RF tracking beacon ("bumper beeper") using a slightly modified Family Radio Service (FRS) radio.  This will use a 555-timer to pulse the radio's Push-to-Talk (PTT) key (to transmit for about 1.5 seconds) every 20 seconds or so.  The transmitting beacon can then be tracked using normal RF direction finding techniques (doppler, rotating Yagis, body-beam, etc.).  The pulsing control circuitry is small enough to fit inside the FRS radio, and can be powered from the radio's internal batteries.  The pulsing control circuit is based around the 555-timer, and is the same circuit which was used for the Mosque Time-Lapse Surveillance Camera project in GBPPR 'Zine, Issue #12.  Review that article for a more detailed explanation of the 555-timer operation and the equations needed for tweaking the transmit/wait times.

Attach powerful magnets to the radio so you can easily (and covertly) mount it to the underside of a vehicle.  Hiding it inside the vehicle's dashboard is actually the best, as nobody will think to look in there and the RF can escape through the windshield.  This does require one to physically enter the vehicle though.

Uses

This device will be useful for tracking human rights abusers:

... or even for tracking terrorists:

Pulse Control Schematic

Internal view of the Cobra PR3500DX FRS/GMRS radio used.  Just about any radio will probably work, but this particular radio has the "high-power" 2 Watt setting, which will be very useful for tracking beacon purposes.  Note the large open space just below the battery compartment on the lower left.

The addition of an external antenna jack may also be useful.  When mounted on the underside of a vehicle, vertical antenna polarization will be more effective.

Picture showing the pulsing circuitry added.  The circuit uses mostly surface mount components to keep it physically small.  Only three wires are needed for the pulsing circuit: a wire running to the +6 VDC POSITIVE terminal on the battery compartment, a wire running to the NEGATIVE or GROUND terminal (which is common throughout the radio), and a wire running to the PTT switch.  When the PTT switch is grounded, the radio will transmit.

Overview of the wiring connections.

Note that the vibrator motor and the speaker are removed to reduce the weight of the radio slightly.  Leave the microphone in, as it will receive any ambient noise.  If you hear road noise, the vehicle is moving.  If you hear crickets chirping, the vehicle is parked somewhere.  Pretty fucking clever, eh?

Another overview of the wiring connections.  The RED wire is POSITIVE, the BLACK wire is GROUND.  The wire for the PTT is connected to the top solder terminal on the little circuit board sticking up.

Closeup picture of the PTT connection.  There are three solder terminals, solder to the top one (closest to the antenna).