KC8AON's QRP PROJECTS

PL TONE ENCODER FOR OLDER RIGS

AN EASY TO BUILD PL TONE ENCODER

If you have an older 2 meter or 440 FM rig that doesn't have PL tone capability, here's a simple little circuit you can build to add to the rig that will give you a subaudible tone so you can access your favorite repeater. It is designed around a basic NE555 timer IC and a few capacitors and resistors. It uses a 10 turn 100K ohm PC mount pot for adjusting the tone frequency to the desired tone and another 22K ohm pot to adjust the output level. On some radios, you may need to install a .1 uF cap in series with the output lead to keep the tone from being heard on your transmitted audio. The circuit will operate on 8 to 12 volts, so It is probably best to find a regulated 8 volt source in the radio to connect it to. Just make sure it is 8 volts during transmit, on recieve it could be zero volts with no problems. The picture below shows how small this circuit can be made. I installed one just like the one shown in a old 2 meter HT with plenty of room to spare. As you can see, it is built on a scrap of regular perf board from Radio Shack. Another thing that could be done for multiple tones is to add multiple 100K ohm pots connected to a multiposition switch with each one set to a different tone to access different repeaters.