#1 2016-03-03 20:21:49

Registered: 2016-02-26
Posts: 2

LibAPRS stroller tracker, how I got it working =)

I wanted to share my experience getting a Libaprs micromodem aprs tracker for my stroller working and sending my first packet that got recorded on Aprs.fi under my callsign SA0MIK-5.

First, thank you @markqvist and the community for a great libaprs and micromodem!

This was my test setup:

The setup is a micromodem based libaprs tracker built on an Arduino Nano clone, an ublox neo-6m gps board and a temperature probe. The tracker is connected to a Baofeng bf-f8hp using a 3.5mm and 2.5mm connector.

I built a micromodem on a breadboard around an Arduino Nano according to the github spec. During the build and initial testing I encountered a few problems which I would like to share:

5v 16mhz needed
Without knowing I tried using a Pro Mini 3v3 8MHz and programmed it with the 3v3 reference version of the MicroAPRS binary. After hooking it up on serial on 9600 baud it returned garbage but on 4800 baud (aha, the 8mhz difference) the serial worked but of course no packets could be sent or decoded. Switching to a 5v 16MHz Nano made everything work good. I think it might be good noting in e.g. the quickstart guide that a 5v 16MHz is a requirement for the circuit.

Problems getting ptt to work
I connected the ptt circuit according to the micromodem circuit. The ptt worked fine when not connected to the radio. Then I connected the audio cables with speaker and microphone to the bf-f8hp according to the schematic below:

Now when the ptt opened from time to time it would not close and then I could also not hear packets being sent properly on my other radio. I of course double checked the circuit but found no problem. After putting the cable around a crude ferrite (a screwdriver) these problems stopped. Probably the 2n7000 mosfet would not close due to RF interference.

I also found that my mic 3.5mm and speaker 2.5mm connectors on the radio didn’t quite make a perfect contact with the radio ports. If especially the mic 3.55 connector came lose the ptt would not close either. This was resolved after taping the connectors firmly to the radio. To get a more safe solution I have ordered a new official handsfree cable to use with the radio.

Both of these problems of course seem to be more about my radio then the micromodem setup but maybe of interest to anyone putting a similar setup together.

Another problem related to the ptt not turning off was that a few times the Nano would restart after starting to send a packet. It became clear that this was due to RF interference when the radio was transmitting when this situation was resolved by adding a ferrite to the mic and speaker cables. It might also be a good idea to throw in a 1uF decoupling cap on the Nanos power input since I used an external regulator and not the internal regulator.

Problems with SoftwareSerial
Once I got the Micromodem schematic and LibAPRS working good with sending packets with the radio I hooked up a Ublox neo-6m GPS module using SoftwareSerial at 9600 baud on port 11 and 12. The GPS worked good and sent serial data that was parsed fine with TinyGPS++. When I hooked in the radio however I could hear the packets sending very slow, like a packet being sent in slow motion over a few seconds in stead of less than a second. They weren’t decoded of course on my other radio. My guess is that there was interference between LibAPRS and SoftwareSerial? I then tried different ports on the Nano without success. After that I tried AltSoftSerial, the difference then was that the sound of the packets was a bit faster but still slow and no packets decoded. In the end I used the hardware serial port RX0 on the Nano and removed SoftwareSerial, now everything works good both GPS and sending packets.

Looking ahead
Now I’m looking to build a more sturdy version (smd components on a proper pcb) of this tracker that will include an internal DRA818V radio. Therefore I’d really like to understand how this setup could be improved.

I also hooked up a DRA818V radio chip to the tracker and initial tests are promising. I switched the Nano for a 5v 16Mhz Pro Mini as well which also worked fine. I have tested it for a few days and packets are being sent and received through local IGates to aprs.fi. Great to get rid of the baofeng radio and the connectors!


Looking forward to hear any thoughts and feedback! I hope my experience can be useful for someone wanting to test this out smile


#2 2016-03-10 23:39:30

Registered: 2014-12-01
Posts: 109

Re: LibAPRS stroller tracker, how I got it working =)

Yay, this is so cool! I'm so much going to make an APRS stroller as well smile Thanks for your great notes and explanations, there's some great info here. And yes, I really need to clarify all the stuff about 3V/5V reference, it is kinda confusing!


#3 2016-03-14 08:59:54

Registered: 2016-03-05
Posts: 8

Re: LibAPRS stroller tracker, how I got it working =)

That is way cool!!!


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