Motion activated APRS Tracker using OT1+ and Picaxe
I built an Argent Data OT1+ open source APRS tracker.
I wanted to make my OT1+ turn on when it's moved and stay on for a while. The "for a while" could be done in software, either in the OT1+ or in a
companion uC, but I had to do some some research on detecting motion.
I had some discussions with Scott N1VG, and Scott suggested using a piezo sensor.
I've read you can just put a weight such as a bolt on the side of a piezo
disk and use a transistor:
Dedicated vibration sensors:
These still require some interface circuit and a weight and may not be
better than a surplus piezo disk:
Rolling ball switches:
These switches generate off-on transitions when in motion.
The simplest app note shows a pullup resistor and some uC code.
I wasn't able to find small quanity pricing on these, but it appears
they're in the $5 range unless you can find them surplus. There are
similar "tilt" devices, but they depend on detecting a particular
orientation (horizontal vs vertical) and aren't suitable, at least for my
I took Scott's suggestion to use a piezo transducer and have built a
small project box with an inertial motion sensor.
The pix of the completed project, a partially-complete board, and the
program are here.
The piezo transducer (Radio Shack 273-073A, removed from case) is
cantilevered atop the project box with double-sided sticky tape.
An alligator clip provides mass, so that motion causes a force on the
transducer, making a voltage which is clamped to 3.3v by a Zener diode
and then clipped to positive by a 1N400x. The transducer is sensitive
enough that it gets triggered when someone walking by touches the bike.
This input drives a Picaxe 08M on its proto board, which is in a polling
loop. (PICAXE 08m interrupts are not very powerful.) When there is
motion, the picaxe triggers an FDP5800 MOSFET (chosen for its low RDSon
at TTL drive levels) and then waits 15 minutes and turns back off. This
causes a glitch 15 minutes after the first turn on, as if the bike is
still in motion it will momentarily turn off. So, it might be nice to
change the code to have the picaxe turn the MOSFET on and then continue
checking, and turn off 15 minutes after the last motion, instead of the
The DC input is a 12V APP connector, and the switched DC output is a
pair of APP connectors, one for the HT and one for the OT1+/GPS.
I'll try to get pix of the fully completed board up sometime. The
trickiest part was getting the LVDO 5V regulator mounted on the Picaxe
proto board, as it wasn't designed for it, and that's what the photos
The current draw of the box is about 330uA, so it's probably less drain
than self-discharge on the HT's NiMH pack. (I'm beaconing voltage every
time; it might be nice to do it less often.) So my goal is to have the
OT1+ stop beaconing when it's on but the bike is parked but the OT is
still on. And I can do that with Profile Switching, I now see from
It might be nice to dispense with the picaxe altogether and use the OT1+
itself, but I understand there's not much code space left in the OT1+
series, so doing this as an external box made sense for me.
I put the OT1+ and sensor in a Pelican case with a Radio Shack HTX-202, an Argent Data OEM GPS, and a WWManufacturing battery.
On the outside I used a 97 cent Radio Shack BNC on-window mount bent straight, and on the outside of the case I pasted this official-looking label:
12v 1.04mW HeNe Laser
I got this "1.04 mW" HeNe laser with a 12v supply. It's cute. It seems to be this one
, though I haven't looked inside to see whose tube is in there.
There's a third wire, currently tied to +12 and I'm hoping it's a "TTL" input. The cathode is easily accessible as well so I may try a modulation transformer there.
Android G1 APRS Map
I was going to write an APRS map app, but it seems almost trivial.
I read this
about the iPhone and the same thing works:
- Enter the android Maps app
- Press the search button (magnifying glass)
- Paste in the following URL: http://aprs.fi/aprsupdate.kml?call=wa5znu-11
It doesn't do tracks, but it shows the current location.
The regular aprs.fi site Google Map works to show tracks, but it's a little fussy to operate the scroll and zoom controls.
PICAXE 08m Tricolor LED flasher
I was in Radio Shack today and saw a three-color 5mm LED. It doesn't say, but it's common anode.
(I guess the two flat sides are a clue.)
Here's a Picaxe 08m
LED flasher program for it.
- Uses Radio Shack 276-0028 5mm common-anode RGB LED
- Use 3 1.5V batteries for supply
- Hook LED long lead to V+
- Hook Other pins to DIP pins 3, 4, and 5 each through a 330 ohm resistor.
Here's an application for the tricolor LED and Picaxe 08m, from Inkless Press.