I thought it was time to add a bit of security to the gear I take camping, so this GPS tracker unit was sourced from eBay. This is a Rewire Security 103RS, a slightly customised version of the common Chinese TK103 GPS tracker.
The small module has all it’s power connections on one end of the unit, on a Molex multi-way block. The white connector is for a piezo-shock sensor – this interfaces with the alarm functionality of the unit. There’s an indicator LED for both the GPS & GSM status, and a switch for the backup battery.
The other end has the antenna connections, microphone connection for the monitor function, along with the SIM & SD card slots.
Once the end panel is removed, the PCB just slides out of the aluminium extruded casing. It’s pretty heavily packed with components in here. A switching regulator deals with the 12v input from the vehicle battery, and is protected by a polyfuse on the right. The GSM module is hiding under the Li-Po backup cell, unfortunately the sticky pad used to secure this wouldn’t come off without damaging something. The pigtails for both the GPS & GSM antennas are permanently soldered to the board here.
The bottom of the PCB has the GPS module, and mainly input protection & bypassing components. There is a FNK4421 Dual P-Channel MOSFET here as well, probably used for switching the external relay or alarm siren. The SIM socket for the GSM modem is located here in the corner.
These units are used to broadcast local audio, such as from a public address system or local microphone. They accomplish this by producing a modulated magnetic field that a hearing aid is capable of picking up.
Not many controls on this bit of equipment. A bi-colour LED for status indications, a microphone, external audio input, charging input & a power switch.
Popping the cover off reveals a small lead-acid battery, 2.1Ah at 12v. This is used when the loop is unplugged.
Here’s the main PCB, which takes care of the audio & battery charging. The inductive loop itself is just visible as the tape-covered wire bundle around the edge of the casing.
Here’s the input section of the main PCB. The microphone input is handled by a SSM2166 front-end preamplifier from Analog Devices.
This audio is then fed into a TDA2003 10W Mono Power Amplifier IC, which directly drives the induction coil as if it were a speaker. Any suitable receiving coil & amplifier can then receive the signal & change it back into audio.