I'm looking for a basic circuit snipping which can either turn on an LED or give a positive voltage when input signal from a guitar is detected. Basically, I have an existing circuit where an effect (volume swell) is activated with a momentary switch. As long as you hold down the switch, the effect is on. When you release, it is gone.

I'd like to be able to accomplish this without the use of a switch and instead use the presence of a signal to operate this switch.

The switch simply connects a transistor emitter to ground, giving me several options of how to complete this circuit. My first thought was some sort of op amp based comparator which could be connected to the base of another transistor, with the collector connected to the emitter of the previously mentioned transistor, and the emitter to ground, so that when an input signal is detected, the comparator goes high and feeds the base, completing the circuit. But I don't know how accurate an idea this is.

I also thought about using a logic chip to feed the transistor, I have a lot of CMOS 4xxx chips so one of those might come in useful.

I wasn't sure exactly what to google to get this to function though, the closest thing I could think of is a noise gate, which is used to only allow the signal to pass if it is above a certain amplitude, such as this one. Is there any way to implement this?

The original schematic I'm working from is here - the switch labelled S1 is the one in question.

enter image description here

  • 1
    \$\begingroup\$ You need to edit your question to credit the original publication as recommended in site rules Referencing. It looks like Electronics and Music Maker, if I remember correctly. Late 1980s? \$\endgroup\$
    – Transistor
    Jan 20, 2019 at 18:41
  • \$\begingroup\$ Do you want a peak detector? \$\endgroup\$
    – Hearth
    Jan 20, 2019 at 18:54

1 Answer 1


I would think an envelope follower would do what you want. Basically, rectify the signal so you are only dealing with positive voltage, then low pass filter that a little to smooth it out, and feed that into comparator to select the level you want the comparator to switch at. Drive an LED from the comparator if it has enough current capability to do so, otherwise drive an open drain MOSFET to sink current thru an LED to turn it on.


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