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APRS Engineering MIM/Mic-Lite Module

http://www.toad.net/~wclement/bruninga/mic-lite.html

I'm on a quest to build the smallest stand alone GPS tracker I can. I can't stand the mass of wires and cables that drags across the car seat, and gets tangled at the slightest movement. For use in events, or someone else's car, the smaller and cleaner the better. The Tripmate mod off the hardware page is a start, it eliminates the external power converter. It also allowed me to make up one wiring harness to supply power and interconnect the GPS, TNC and radio. But that's still three separate parts.

The MIM is capable of accepting NMEA data from a GPS, and formatting that data for APRS packet transmissions. It also has analog and digital input lines to operate as a radio telemetry unit (want to launch a balloon?). It can also be set up to do Mic-E APRS, and/or send a message or ID in CW. What it will not do is decode packet, so you can not use it to hold a packet QSO, or as a RELAY or WIDE node. It is capable of "listening" to the audio on frequency, and waiting for other transmissions to stop before it transmits. This assures that it does not double, or collide with another packet transmission. This is refered to as "HOLD OFF". You will need to make a simple circuit that is nothing more than a electronic switch, it toggles ON or OFF biased on the presence or absence of audio coming into the circuit.

HOLD OFF CIRCUIT PARTS LISTING: R1 - 100 ohm R2 - 100K ohm C1 - 10uF C2 - 0.68uF A 2N3904 transistor (or any similar type)

I used RJ45 telephone plugs and jacks to make the connections, so I can easily remove the TNC, or connect it to the laptop for programming. One connector passes power, PTT, audio out, audio in and ground (all the radio functions). The other connector passes the serial data RX and TX and a ground (all the computer functions). This allows me to keep power to the unit, and connect the serial port to a laptop for programing. The black shrink wrapped item is a small board that contains the audio sense "HOLD OFF" circuit.

I run the Tripmate off externally supplied 12 volts (see the Tripmate 12V mod), so I removed the AA battery holder from the top of the yellow Tripmate case. This provides plenty of room to add the MIM, jacks and etc. All wiring connections are also made inside the case. I placed the MIM on a piece of foam to shock mount it, and keep it from moving around inside the case. The placement of the MIM, connectors, and the audio sensing transistor circuit are a little fun to lay out, but it will all fit.

One cable to the Tripmate supplies power to both the GPS and TNC, and the same cable brings data and PTT to the radio. Very clean, my current Tracker of choice!

Here is a more in depth page covering the wiring of the Tripmate / MIM tracker.

For more info on The MIM module

Jan. 21, 2002