To do this you’ll want to use triple-pole double-throw footswitches (3PDT) instead of the double-pole double-throw footswitches specified in the original article. The third pole of each footswitch will be used to switch on an LED when that particular loop is activated. Any time you are using an LED, a resistor must be added in series to limit the current from the battery to prevent the LED from burning up. In this case we’ll use a 1k½ w-watt resistor, which is small enough to make the LED burn brightly, but will limit the current to protect the LED and make the battery last much longer.
To switch the battery off and on, use a Switchcraft stereo 1/4" phone jack (also called a tip-ring-sleeve or TRS jack) for the input instead of the mono jack as originally specified. Hook up the tip and sleeve terminals on the jack exactly as indicated in the original article. The difference is you will connect the minus lead (black wire) from the battery snap connector to the ring of the stereo jack. This way, when a 1/4" mono plug is inserted into the jack, the ring will short to the sleeve (remember the sleeve is grounded) and the battery will be switched into the circuit. Always unplug the 1/4" male plug from the input jack when you are not using the unit as this will conserve battery power by disconnecting the minus lead of the battery from the circuit.
You will need some additional parts for this project, and except for the 3PDT footswitches, all the other parts can be found at Radio Shack.
1 – 9-volt battery snap connector. (Radio Shack part # 270-325)
1 – Battery holder (Radio Shack part # 270-326)
1 – 9-volt alkaline battery
1 – Stereo 1/4" female jack (Radio Shack part # 274-312)
3 – LEDs. Use three different colors. (Radio Shack part # 276-1622)
3 – 1k½ w-watt resistors (Radio Shack part # 271-1118)
3 – 3PDT footswitches (part # P-H501 at tubesandmore.com)
Let’s start building. Look at the diagram (left). The resistors can be mounted with one lead on the switch and the other leads flying. You may want to put insulation around each resistor’s leads to keep them from shorting against other components. Sometimes I use the insulation from hookup wire to insulate the resistor leads, but shrink tubing will also work. If you don’t have shrink tubing, a soda straw could be cut into short lengths and slipped over the each resistor. It is important to get the polarity of the LEDs correct or they will not light. You want to ground the cathode end of the LED. How do you know which is the cathode end? Look at the leads coming out of the LED. One lead is shorter than the other. The shorter one is the cathode and must be grounded. The longer lead goes to the switch.
As with any project, you should spend time planning and reviewing how all the parts will fit together before you start building. Take your time and make sure about parts placement before any holes are drilled. I would suggest installing the jacks first, then you can see where the switches and other components should go. The battery holder can be glued into the enclosure with silicone glue (I use E-6000, which is available at Wal-Mart). —Gerald Weber, Kendrick Amplifiers.