Byonics TinyTrak3+ GPS Power Control

GPS power from MOSFET relay
GPS power from MOSFET relay

The Byonics TinyTrak3+ GPS encoder has a “Power Control” output that can switch the power to a radio or GPS interface. J6 provides the interface: pin 1 = common, pin 2 = high active.

With the “Power Switch” option enabled in the config program, you can set the number of seconds to allow the GPS unit to get up to speed before the next scheduled transmission.

I glued a surface-mount MOSFET relay to the back of the PCB with urethane adhesive; it fits neatly between the DIP microcontroller’s pins with one output lead soldered to the 5V pad of J7. The other lead goes to the center +V pad; because the relay uses back-to-back MOSFETs, the polarity doesn’t matter.

That replaces the normal solder bridge across J7 that provides power (on pin 4) to the GPS2 plugged into the DB9 connector. When the relay’s on, it connects the GPS to the power supply. When it’s off, the GPS goes dark.

The relay input is an LED with a forward drop of 1.3 V max and requires 4 mA to turn on: figure 3.7 V / 4 mA = 925 Ω max. I kludged an 890 Ω resistor by paralleling (stacking!) 1.5 k and 2.2 k resistors; you could probably use anything near that and it’d work fine.

The relay is an OMRON G3VM-21GR1, part number A11171 from Electronics Goldmine, but I suspect any teeny little solid-state relay would work. The max on resistance is about 1 Ω and the receiver draws about 65 mA. I measured about 20 mV of drop, so the actual resistance is a lot lower than the spec.

I initially set the power-on delay to 10 seconds, which seemed to be OK: the GPS (green) LED would blink a few times, then go solid. Alas, the warm-start spec for the Byonics GPS2 (see the GPS3 for details) receiver is really 38 seconds, average, and it was definitely producing bogus position data. So I set the delay to 60 seconds and we’ll see how that works; early reports indicate the coordinates still have plenty of jitter.

[Update: 60 seconds is iffy. 90 seconds seems to work pretty well. A bit of rummaging says that the satellites broadcast their ephemeris data every 30 seconds, so 90 seconds allows for two complete update cycles. Maybe 100 seconds would be even better. Some old background info for Garmin hand-held receivers is there.]

It’s obviously a tradeoff between accuracy and battery life. This is for use on a bicycle and, believe me, I don’t want to tote a huge battery!

If the control signal was low-active, then you could use a cheap PNP transistor as a high-side power switch.

The white/orange wire routes regulated 5 V through an otherwise unused pin to the homebrew interface that combines the GPS data with helmet mic audio. The tiny rectangle is a 1 µF cap that helps cut down digital noise. There’s no need for a connector on that end, as it’s wired directly to the interface circuit board inside a small enclosure.