Sunday, September 27, 2009

Open colector output

Perhaps it's obvious to most electronics experts reading this, but I thought I'd pass along something I learned today.

As you all know, I'm trying to build a digital ATV repeater. One of the things I needed was a system to key the amp on and off so that it doesn't have to stay biased 24/7 (a waste of power, unnecessary heat and shortens the amp's service life). Fortunately, the amp has a short-to-ground PTT line.

Well, it turns out that the modulator has a TTL "PTT" pin on one of its headers. How do you turn this into a short-to-ground output?

The answer is open collector. I installed an RCA jack in the chassis and simply dead-bug soldered an NPN transistor to the jack, emitter to shell (and ground) and collector to center. I then soldered a 10K resistor to the base, and the other end to a wire to a "pin clip" to clip on to the PTT pin.

And it works perfectly! When I use the menu item to take the transmitter from stand-by to on-air, it keys the amp! Yay!


Anonymous said...

Yeah, I suppose that would work. Two issues though:

1: Must have a common ground between your modulator and amp.

2: Be sure the 10K resistor is appropriately sized to ensure the transistor is in saturation when it's turned on.

Probably would have been better / safer to use an opto-isolator or relay.

Nick said...

1. Well, the RCA cable transporting the PTT from the modulator and the amp would insure there was a common ground even if they weren't already being driven by the same 12 volt power source and had a piece of coax running between them :)

2. It "probably" is, given that the circuit works. But if you've got a suggestion as to how I can prove it, I'm all ears.

3. The amp already has a relay inside it (you can actually hear it kerchunk on and off), so I don't think doubling-up the relay would help anything. And that relay isn't a T/R relay (for switching coax) - this amp isn't set up that way. It really is just switching on and off the modules' bias voltage.