How can I add LED to this solid state switching scheme?

[Michael Allen]

I'm using CD4093 NOT gates to control a CD4066 to switch my Trem and Reverb on/off.

The trem is the MOSFET trem adaptation from Geofex and the reverb is one tube. Both are turned on/off with an SPST to ground. This is where the 4066 comes in. I'm triggering the control pin with a single 4093 gate wired as a NOT gate. There is a 220k/0.1uF RC network on the gate input along with an SPST latching footswitch. This is basically the Wicked Switch setup from The Tone God. V+ is connected to the 220k.

I'm using this switching instead of just using the footswitch because I want to add LED indicators. Also using just the footswitches means both the trem and reverb use the same ground point in the footswitch box and I was getting some Trem ticking.

I've tried just attaching an LED (green, 2.1V forward voltage at 20mA) from the switch/RC to ground but it lowers the voltage at the gate input to the point that the gate is in low state no matter what. The supply voltage comes from the heater tap using a voltage multiplier to get about 9V. I connected the V+ output to three parallel 1N4148. One supplies the V+ for both chips, and the other two go to the separate 220K resistors. I did this just to make sure grounding one switch wouldn't affect the supply for the other switch.

I also tried using a simple transistor inverter in the footswitch to control the LED, but this also pulls the supply so low that the chip is in low state always.

How can I attach LEDs here?

[Michael Allen]

I decided to ditch the CD4093 and just actuate the 4066 directly. I used the 4066 switches similar to logic gates, and wired them in series. Now I can directly attach an LED to the switch control pin without brining it low. Here's the schem. Works great, the switch, LED and LED resistor are in the footswitch box. TRS switched jack to connect both channels and shut off the verb and trem if the pedal is not present.

[R.G.]

The trick to this is to think in currents, not voltages.

The remote footswitch should contain a resistor which switches the cathode of an LED to ground. The Anode of the LED is connected to a wire going back to the amp.

Inside the amp, there is a circuit which limits and also senses the *current* pulled by the LED and switch. No current, no action.

The simplest thing to use for this is a PNP transistor. From some modest power supply voltage, like perhaps 5Vdc to 12Vdc, you connect the PNP's emitter to the power supply, the base through a current limiting resistor to the connector/cable to the footswitch unit. Making the switch lets current flow in the LED, turning it on. It also turns on the PNP.

The PNP's collector pulls up to close to the power supply. This can be tied to a resistor to ground and provide a high/low voltage signal to local logic.