Friday, March 02, 2012

FreeTrak progress


FreeTrak configuration



FreeTrak schematic




zl3ame said...

Regarding the level shifting: as built, it's for RS232 voltage levels. Probably for 3V3 you should ditch the transistors and put a voltage divider on the Tx line from the PIC to the GPS. You don't need one on the line from the GPS to the PIC as the logic 1 level (3V3) is above the logic 1 level for TTL. You should also check the source code and look for the logic sense for the serial interface (true or inverted logic). If you need to invert it then you can do it in software, or with a transistor inverter.

Richard G3CWI said...


The 16F628A is specified to run down to 3V so just run it on 3.3V - the same as the GPS. 2 x 1.5V batteries will be just fine.

Incidentally, I have some APRS code running on the Propellor.



Kenneth Finnegan said...

For the record, 3.3V logic into 5V controllers *usually* works, but is usually outside the spec (look for a "minimum logic high = 0.7 * Vcc" value in the datasheet or something), so you'd want to use a 74HCT125, or any other kind of HCT logic, to be "proper," which isn't important until you start doing production runs, really...

(Compare the first line of page 4 of this datasheet vs your typical 74HC125 buffer to see what I mean.


Richard G3CWI said...

APRS for the Propellor!

zl3ame said...

Hmm, the first part of my comment was chopped. I said I built one of these and I replaced the OP196 with a Microchip MCP601 (which I was able to get easily).

HTH. 73!