I've got a guitar signal that I want to mute with a transistor by sending the signal to ground. I'm using a 2N3904 transistor.
I have a 1k resistor connected to an Arduino pin, and that connected to the transistor Base. I have the Emitter connected to Ground, and the Collector connected to the guitar signal. The plan was to apply current (set pin HIGH) to the Base, and it would ground out the signal. When I do this the signal does disappear, but is replaced by a loud higher pitched buzzing sound. If I just take a wire and touch the signal to the ground, it mutes just fine.
Any suggestions on how to better wire things? Thanks!
Have you tried connecting the resistor directly to 5V? If you still get noise that way, there may be noise in your power supply, so try a battery (directly to the resistor without the Arduino).
You might also try a capacitor across the base-emitter junction. Maybe 0.1uF, or a 1uF electrolytic, or something larger if that's what you have.
If all else fails, you may need to use a relay. The relay contacts will be completely isolated from the Arduino, and the grounds can be isolated too.
P.S.
There's no harm in shorting-out the guitar pick-up coil, but you shouldn't short the output of an effects pedal or an active pick-up. (It's OK to grounding/shorting the input of a pedal.)
A MOSFET acts more like a resistor (use a logic level MOSFET), although the intrinsic body diode may cause clipping if/when the signal goes higher than 0.7V peak. But a transistor may experience similar effects due to the CB diode when the Base is pulled Low.
Here's what I've done: Guitar, 1k resistor, photocell from the 1k resistor to ground. Junction of photocell and 1k resistor to the amp.
Glue a big LED, as close to the size of the photocell as possible, to the photocell. Paint it all black or enclose it in black heat shrink.
Turn the LED on, photocell goes low resistance and mutes. Some signal may still get through, but it should be very quiet.
In general, the larger the photocell, the lower the resistance when well-lit. And it is optoisolated, and responds slowly enough that only rather low frequency noise on the LED current will get through.
Try a 74HC4066. You have four gates, arrange one in series with the signal and one parallel to it, switched alternately. Or even two to ground and one in series in a "pi" network for maximum muting.
OTOH, your present problem sounds like either a port switched to PWM, or a very noisy power supply.
