I've got a little Morse Code beacon project that utilizes a TIP120 to pull the PTT line LOW and send some audio (via PWM), then release the PTT.
I've gotten the whole thing working almost flawlessly (audio is quiet) on my solder-less breadboard, then once it's all soldered up in a nice package, it's really unreliable...basically the PTT is never released, or sometimes if I'm lucky, it releases 30 seconds later.
Here's my schematic:
It's setup so that when the PIR detects motion and goes LOW, pin 8 goes HIGH powering the transistor which pulls PTT LOW. It then outputs Morse code via PWM on pin 9. Again, all of this works correctly on the solderless breadboard, but when soldered in the above configuration, pin 8 goes LOW (turning off the transistor) but the PTT in the radio stays on
I've done what I can to detect bad solders (ie. shorts, etc.) and it all seems fine. Also, in the name of full disclosure, the TIP120 has been modified slightly so that it sits closer to the board, and its heatsink has been trimmed to clear a Pro Mini on top of this board via stackable headers.
Lastly, a multimeter tells me that the radio's PTT is 2.95V (@about 60ma) and my Pro Mini is the 5V version. I was fed up and tried connecting PTT straight to pin 8; setting it HIGH shuts the radio off (could be worse!)
To the point: do I even need a transistor here? Is there a better way to pull the PTT LOW without having to give it 5V on HIGH (in the "off" state)??