Adding Bluetooth to a
Garmin ForeTrex 101

Overview

The Garmin ForeTrex 101 is a basic GPS released circa 2005 that runs off of two AAA batteries and has a serial port for input and output including standard NMEA. I've since replaced mine with a more capable GPS but kept it thinking I might think of something interesting to use it for. I recently found out about the HC-05 Bluetooth to TTL serial interface boards. These boards are tiny (2.8 cm x 1.4 cm x 2 mm), low power (10 mA paired), can run at 3V, and are inexpensive (~ 5 USD). I decided to try adding one to my old ForeTrex 101 so that I could use it to provide location data to my Android tablet that doesn't have a built-in GPS. The modification is very simple and works quite well. It's also a great way to get new life out of otherwise obsolete and unused hardware.
Note that the information provided here is for educational purposes only. Performing these modifications will void your warranty (are these things even still under warranty?) and may lead to the GPS and/or HC-05 being irreparably damaged. Proceed at your own risk.
Later in this document I recommend several specific products. I have no affiliation with their creators or sellers other than being a satisfied user.

The ForeTrex 101 hardware

The ForeTrex 101 is held together using a rubbery glue. To open the case one can simply use a small screwdriver to pry the two halves apart. It's easiest to start adjacent to the buttons and go from there. Though I didn't try it, it may be easier to first place the GPS in an oven for about 10 minutes at around 150F (65C) to soften the adhesive. The modification shown here might also work with the similar ForeTrex 201. If you try this with the 201 be very careful not to damage the battery (i.e. by heating, puncturing, or short-circuiting it) as I believe battery damage could lead to a nasty fire.
Click for larger image
Once you've gotten the case open you'll see the circuit board with the GPS receiver (under the metal shielding, interface circuitry (in the middle), and computer (shielded with a trace on the circuit board). The serial port (2.5 mm jack) and speaker are connected to the main board via a 5-pin header. There's just enough room on top of the GPS receiver shielding to fit the HC-05 module. Ideally we wouldn't place the Bluetooth transmitter right on top of the sensitive receiver circuitry but in this case there's no-where else it will fit. To interface the HC-05 with the GPS we need to find a 3V power source and we need to get the transmitted serial data in TTL form.
Near the center of the board is a boost converter (LTC3400B, click for datasheet) that raises the voltage of the two AAA batteries to 3V. The output appears to drop to 0V when the GPS is turned off; this is perfect for us since it means the HC-05 won't draw power while the unit isn't in use. The circuit used is very similar to the one shown on the first page of the datasheet (reproduced below) with the addition of the optional output Schottky diode between pins 1 and 5 (the purpose of the diode is improve efficiency). The values of R1 and R2 appear to be close to 100k Ω and 70k Ω which gives a VOUT of 3V. The easiest place to tap in to this is on either side of capacitor C2 (see photo).
Interestingly, the computer appears to run at 1.8V, not 3V so take care if you decide to interface directly to it.
To enable NMEA output on the GPS, go to Main menu → Setup → Set interface → I/O format and select NMEA.
The GPS's serial output appears to correctly follow the RS232 standard with logic 1 being -3V to -25V and logic 0 being +3V to +25V. The voltage swing appears to be at least -4V to +1V but I don't have the tools to do a proper measurement. Since the HC-05 expects TTL signalling with logic 1 being VCC (i.e. +3V) and logic 0 being 0V we'll need to either find the TTL signal on the board or do the conversion ourselves.
D3P dual transistor IC
The IC adjacent to the serial header seems like an obvious candidate for an RS232 driver. The chip (labelled D3P1) is an NPN/PNP transistor pair plus resistors (see schematic to right and the datasheet). I was unable to determine how it connects to the serial port (if indeed it connects to it at all) so I decided instead to convert the RS232 signal back to TTL. The conversion circuit is shown later in this document. The enterprising reader might try examining the D3P chip with an oscilloscope and logic analyser to determine whether TTL serial signals are present on this chip. I suspect that pin 6 is TTL RxD and pin 5 is TTL TxD.

The HC-05 Bluetooth serial module

The serial to Bluetooth adapter used is the HC-05. These modules are readily available from many sources (just search for them) for about 5 USD. Make sure that you get the HC-05 and not the similar, but far less capable, HC-06.
To program the module, you'll need an RS232 or USB to 3.3V TTL serial adapter. I used a Prop Plug for this but any adapter capable of 3.3V TTL serial at 38400 bps will suffice. You will also need a 3.3V power supply. Hook up the circuit shown at the bottom of this section. The LEDs are optional but can be useful for debugging.
Once you've built the circuit, hook the adapter (Prop Plug in this example) up to the computer and connect to it using a terminal emulator. On *nix I recommend using picocom; if you're using Windows take a look under Start → Accessories → Communications → Terminal emulator (IIRC; also, I'm not sure if recent versions of Windows ship with a terminal emulator). You must set your terminal emulator to send CRLF as the line-ending characters. Change /dev/ttyUSB0 to the actual location of your serial adapter. The HC-05 supports a large number of commands beyond those mentioned below; see one of the many datasheets for details (the HC-05 is refered to as the EGBT-045MS in the linked-to datasheet).
HC-05 programming circuit

RS232 to TTL adapter

The RS232 to TTL circuit is shown at right. It comprises three 10k Ω resistors and one general-purpose NPN transistor (I chose a 2N5551 that I had on hand). This circuit only supports data output; supporting input over Bluetooth would greatly complicate the circuit. Since I have no need for input (nor a desire to draw more power than is needed) I decided to make the circuit output-only. Two similar circuits (which do support input) can be found here and here.
The physical circuit is shown below. (Note that different transistors have different pin assignments; if in doubt, check your transistor's datasheet) The circuit needs to fit into the gap between the battery compartment and the outer wall of GPS. This space is less than about 1 cm wide (0.4") so the circuit needs to be long and thin. Solder the components together and, before trimming them, check that the circuit fits properly. For RS232 TxD in, the resistor's lead is soldered directly to the 5-pin connector's TxD pin. This lead therefore has to be carefully cut to length and bent into shape (see photos below). If everything looks good then trim the remaining leads short.
Next cut wires for TTL out (the blue wire), 3V (the red one), and Ground (the green one). An additional pair of wires is needed for the HC-05's VCC and Ground lines. I recommend using wire-wrapping wire for the connections. This wire is semi-rigid and the insulation resists heat very well while still being easy to scrape off using an X-Acto knife. Solder three of the wires onto the converter circuit as shown in the photo below. One red wire goes to 3V, one green wire goes to Ground, and the blue one goes to TTL out.
The completed adapter, ready to solder onto the board.
Cut some heatshrink tubing to use as insulation (sizes are approximate): Note that despite its intended purpose, the tubing is not shrunk onto the components; this should make any future rework easier. If you don't have appropriate tubing, electrical tape would make an OK (but less than ideal) substitute.
The wires can now be soldered into place (see photo at right for locations). The completed circuit is shown below.
The case can now be re-assembled. Make sure that none of the wires are blocking the power contacts. In the photo above one of the red VCC lines needs to be moved out of the way. Also verify that no metal on the added circuitry is touching the existing circuitry. With the screen facing down, carefully push the two halves back together.
Put in a couple of AAA batteries in, press the power button and see whether it powers up. If it doesn't then re-open the case and check whether anything is preventing the case from seating. In particular, make sure that the HC-05 isn't getting in the way and that there are no wires over the two square power contacts on the circuit board. You might also try (gently!) bending the two contacts on the battery compartment towards the inside of the GPS. If you're still not able to get it to power on you should double-check that all your wiring is correct.
To reseal the case some kind of contact adhesive (think heavy-duty rubber cement) would probably work best. Carefully remove any existing adhesive from the case. Next, apply a thin layer to the mating groove on each side of the case, let it dry, and then firmly press the two sides together.

A few notes on soldering

Be very careful when you're doing the surface mount soldering. If you get a blob of solder in the wrong place it will be very difficult to remove. Likewise, if you de-solder a surface mount component you'll have a rather fun time getting it soldered back into place. If you've never done surface mount work before I recommend practicing on e.g. an old PCI card before doing it for real. I recommend using fine silver solder; the stuff I use is 0.22" (0.5 mm) 62/32/6 though thinner would be better. A fine tip, low wattage (15W is fine) soldering iron is essential. If you can't find a fine tip for your iron you can modify a larger tip by filing it to a thin point (the last 3 mm or so should be no more than about 1 mm in diameter). For the surface-mount connections it's probably easiest to do the soldering by first tinning each side of the connection then placing the two (or three) sides together and reflowing the solder to make the final connection.

Android software

Once you've completed the modifications to the ForeTrex 101, you can set up the Android side of things.
Now that the ForeTrex 101 has been set up as a GPS provider under Android you can start using it for real. On the ForeTrex 101 go to Main menu → Setup → Set system → GPS mode and select Normal, WAAS, or Battery save. This will provide real data to Android as opposed to demo data. One annoyance is that Bluetooth GPS will drop the connection if it doesn't see a valid position in the NMEA stream. In practical terms this means that if the ForeTrex 101 loses a fix on the satellites (which it does a lot) then the connection will also drop. This is why Bluetooth GPS needs to have a very short reconnect interval set.

Final notes

All in all this worked out better than I expected. Despite my concerns, the Bluetooth radio doesn't seem to be adversely affecting the GPS receiver's reception. If you want you could also connect the status LEDs in the final circuit. In my opinion though this would just reduce battery life with very little gain (and a loss in terms of weather resistance). The same circuit should also be adaptable to any other GPS that has serial NMEA output. Get creative and enjoy!
Last modified 2019-03-31
© 2024 Alexander Hajnal