Tuesday, November 25, 2008 Zulu
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.

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 application.

My Project

My Picaxe-Piezo Sensor

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 first.

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 highlight.

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 another thread.

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.

APRS Package

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:

- posted by Leigh @ 00:11 z 0 comments
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Saturday, November 15, 2008 Zulu
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.
- posted by Leigh @ 03:24 z 0 comments
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Monday, November 10, 2008 Zulu
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:
  1. Enter the android Maps app
  2. Press the search button (magnifying glass)
  3. 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.
- posted by Leigh @ 04:43 z 2 comments
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Tuesday, November 04, 2008 Zulu
Election Day PICAXE

- posted by Leigh @ 18:54 z 0 comments
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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.

Here's an application for the tricolor LED and Picaxe 08m, from Inkless Press.

- posted by Leigh @ 05:31 z 0 comments
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