Gate/Threshold indication circuit suggestions

[knutolai]

Hi
I'm cooking up a delay circuit (at the moment only on paper and in my head) and I thought of adding a simple gate function to the feedback loop. The idea is to put a LDR (coupled with a LED) i the feedback loop as a soft-switch. The LED would be switched on when the guitar-input signal exceed a threshold (variable by a potentiometer), and switched of again when the input signal drops below the threshold. The lowest threshold would leave the gate open at all times.

I know how to make a simple AC to DC converter for using the input as a control-signal, but am uncertain how I would implement a threshold control and turn on and off a switch with the control-signal (was thinking of using a 4066 or 4053 IC).

I've had a look at the expander in this thread: http://www.diystompboxes.com/smfforum/index.php?topic=105397.0 Could I convert the upper part of this circuit to switch a gate on and off? If so I suspect the circuit could be simplified as I don't indent the gate to be dynamic apart from on or off.

[merlinb]

You might like to watch this tutorial on the subject (you would only need one comparator, but the principle still holds)
http://www.eevblog.com/2013/05/18/eevblog-471-overload-detector-circuit-design/

[knutolai]

Thanks that was very informative   Could normal opamps be used as comparators too? Also if I wanted the LED to fade out instead of keeping constant for a set amount of time after the signal goes below the threshold. Could I simply put a cap in parallel with the diode and resistor?
Edit: never mind that. It wont work as the LED wont be connected to ground any longer. Ill figure something else out for that. I think Ill use a dual comparator as its as big as the single comparator chip. Meaning I wont need to make a full wave rectifier. Should a circuit component such as this one be separated from the audio path with a buffer? Also it there any way I could use a basic IC such as a tl072 for this application? I haven't got any comparator chips in stock atm and would like to give this a stab asap.

Also I guess i would be a good idea to give the input signal some lowpass filtering before feeding it to the comparator?

[armdnrdy]

I would imagine that this could be achieved with a LM3914 and a bar graph display or line of single LEDs.

Look up LM3914 circuit.

[knutolai]

Could this work?

-With P1 sets the threshold. When P1 = 0, threshold = input bias. (forgot to label the input resistors, they are supposed to be equal)
-When the threshold is exceeded (or met?) the PNP opens, the LED instantly turns on and the cap charges up.
-When the threshold isn't exceeded the PNP closes and the LED fades out as the cap charges out.