[Reprinted with permission from Bob Heil)
In this day and age of the silicon devices, we can perform some neat remote or local switching of various low voltage circuits such as antennas, crystals, keying lines, rotors, etc.
One of the great plus factors in switching devices is that we can use a single control wire, which could be a plus factor where we have existing lines run and need to control several devices from that one line. By using various voltage level Zener diodes, it is easy to feed a matching voltage level down the single line to cause the proper diode to conduct, therefore turning on one of the transistor switches. When using an NPN device, a positive voltage on the base resistor, will cause the transistor to conduct, and the collector will produce a low, or ground. Turning off the control voltage to that base resistor will cause the collector to come back to a high state. This one action is the basis of all solid state happenings and if you haven’t built up such a circuit and become familiar with it, it would be a good test to do so and become acquainted with this great world of solid state switching.
Once you have that collector going high and low (+12 volts and ground), you can connect a relay, an LED, a small buzzer, etc. The important thing to understand is that the current draw of this device you connect to that collector cannot exceed the current handling capacity of the switching transistor.
This simple switching network can be applied to a number of uses and requires a minimum of parts and knowledge to get working. One helpful hint to any transistor switching circuit that works in high RF environment, is to by-pass the bases of the devices to ground, using 100 to 200 picofarad disc ceramic capacitors. The base bias resistor and this by-pass cap looks like a short at high RF frequencies. This is a fact that you also may want to know when trying to remove RF from all types of audio and switching circuits.