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!