# Electrical – Voltage gated switch issues for audio application (guitar pedal)

guitar-pedalmosfetswitchesswitchingvoltage

So i'm a newbie, and with the assistance of my A-Level in electronics step brother we're trying to make a tremolo pedal for guitar. The concept is that an opamp (powered by ±9v) is used as a square wave generator, which will open and close a voltage gated switch which in turn disconnects and reconnects the signal (turns it on and off), at a rate determined by the frequency of the square wave. The circuit below is what i've been using so far.

simulate this circuit – Schematic created using CircuitLab

The issue is that every time the signal cuts out as it's supposed to, there's a loud crack which is what I assume is the shorting of the circuit (the guitar signal, due to 'infinite' current going to ground).

I'm hoping someone has a suggestion how I could resolve this, or alternative suggestions for a voltage gated switch that won't be shorting the circuit. I tried putting the mosfet in series with the guitar signal but this didn't work. If there's a simple way of just getting rid of this crack (which could possibly be solved by softening the edges of the square wave generated resulting in a gradual increase/decrease of voltage above/below threshold potential), this would be hugely appreciated!

Many Thanks,
Zach

• EDIT: Representation with output scope of the relaxation oscillator seen here (please copy and paste as doesn't display link nicely):
http://www.falstad.com/circuit/circuitjs.html?cct=\$+1+0.000005+159.55918304990493+60+10+50%0Aa+304+208+400+208+0+9+-9+1000000%0Aw+304+192+304+160+0%0Ar+336+128+384+128+0+100000%0Aw+400+128+400+208+0%0Aw+400+208+400+272+0%0Ar+400+272+304+272+0+100000%0Aw+304+224+304+272+0%0Ar+304+272+240+272+0+100000%0Ag+240+272+240+288+0%0Ac+304+160+240+160+0+0.0000022+0.25657397239391444%0Ag+240+160+240+176+0%0AO+464+208+464+176+0%0A174+304+160+368+128+0+500000+0.005+Resistance%0Aw+384+128+400+128+0%0Af+464+208+512+208+0+1.5+0.02%0Aw+512+224+512+240+0%0Ag+512+240+512+256+0%0Aw+512+192+512+176+0%0AR+512+176+512+144+0+1+450+1.5+0+0+0.5%0Aw+400+208+464+208+0%0Ao+9+64+0+35+0.3125+0.00009765625+0+-1%0Ao+11+64+0+42+0.625+0.00009765625+1+-1%0A

#### Best Answer

I have used the H11F1 opto-isolator as a volume control. The output section is a JFET, the input is an LED driven by just a few milliamps. Use it to replace the mosfet to 'short' the signal or in series with the signal. If used to short the signal insert a 10K to 100K resistor to limit the signal current, and a 100 ohm to 1K resistor in series with the JFET drain pin (pin 6) to limit the short current. Use a 1K resistor on the output of the op-amp to limit drive current to the LED side (pins 1 and 2) of the H11F1.

I found this link to DIY projects using the H11F1 as a gate or gain control.