latching/logic for an automotive audio application

Thread Starter


Joined Nov 18, 2007
The core of my project is this: I have two control signals coming from my stereo ("Combi Control" and "Aux Control Out"), and one control signal going in to the stereo ("Aux Control In"). These were originally set up to control an audio cassette player, but I'm using them to enable and disable the line input.

I want to create an auxiliary input adapter with a 3.5mm jack that replicates the logic of the cassette player. Pressing a pushbutton would act like powering on the cassette player, and it could respond to Combi Control and Aux Control Out correctly to switch the line input on and off seamlessly.

I need to have my circuit respond to a rising edge from Combi Control to set Aux Control In to 0v, and a rising edge from the pushbutton or Aux Control Out to set Aux Control In to +5v.

If anything is unclear, you can read another description here along with someone else's solution, based on an op-amp used as a comparator, which some people on that forum have reported problems using.

I came up with this circuit that ignores the Aux Control Out, though that could be added with some simple logic. I'm still adding to it, I know some things are missing, which is why I'm asking here. :)

1. Aux Control In in the stereo is the base of an NPN transistor. I know I need nonzero current to drive this high... do I just need, say, a 10k resistor between Q and Aux Control In?

2. Likewise, do I need anything to drive CLK from Combi Control? I don't know what's going on inside the stereo other than that it drives Combi Control to +5v when active and, according to the other guy's project, he saw 0v on a voltmeter when it was inactive. Should I add a diode here to protect the stereo?

3. What flip-flop and schmitt triggers should I use? 74HC74 and 74HC14?

I liberally took some ideas from this thread for the switch debouncing and the power-on reset. This circuit will ultimately be powered by +ACC using a 78L05 for a +5v supply.