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Author Topic: Radio PTT with TIP120 unreliable  (Read 899 times)
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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 smiley-sad

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)??
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If the transmitter draws less than 40 mA it would be fine if it works right. Your diagram looks a little funny... Black is usually ground and brown a control or output lead, you appear to have those 2 leads reversed where black is the control and brown ground. That doesn't look right, re check it, please. and try to post a svchematic in the future I have to make many assumptions when II try to figure out what is going on. The quality of my assumptions and therefore your answeres accuracy depend on the information you present here.

Doc
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Brown is ground.

I don't know what else to say about the circuit that would help. I said it draws 60ma and you say it should be good if it's under 40ma...it's not under 40ma so...
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use a 2N2222 or a 2N3904 both will handle 60 ma easily use a 2K2 base resistor and it should work fine. Verify that the control lead does go high and low when the PIR device is activated, if it doesn't then put a 10K Pull-down from control to ground.

Doc
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OK, I'll try that, thanks for the recommendation.

Is the TIP120 too "big" for the circuit?

I'm at a point where I need a lot more education in electronics to go beyond the theoretical...software is so much easier smiley-wink
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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)??

You might be better off with a MOSFET rather than a BJT. That acts like a switch rather than an amplifier.
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A BJT wired that way is a switch. Because he only needed to switch 60 mA I suggested he use a smaller transistor that doesn't require as much base current, wired the same way as his original diagram and a pull down for good engineering.

Doc
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Thanks for the comments guys.

@Doc, I just reread your comment regarding the 10k pull down and it sunk in. That makes a lot of sense. Thanks again.
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You're Sure Welcome and Thank You.

Doc
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