On-Air Lamp Controller
By Chad Hauris: Updated 8/3/05

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KJBC radio recently installed a new audio board, an Arrakis Nova 10-C. They wanted to be able to use their classic RCA "On Air" lamps when the microphone was on. The console provided a contact closure that connected a point to ground and could carry 50 milliamps. We could not just connect a relay directly to this point as it would draw too much current through the console.
Click on the pictures for larger views.
Building the on-air controller

The circuit uses a relay to switch the on-air lamp. We installed a 12-volt lamp inside the On-Air fixture for safety. The relay is controlled by a 2N3055 transistor. A 4001 NOR Gate I.C. is used to invert the console logic to provide a positive pulse to turn on the transistor. A 1-megohm resistor feeds 12 volt DC to the inputs of the gate to keep the output in the "off" state when no connection is made to ground. When the console mic switch is turned on, the input to the NOR gate connects to ground, and this "low" status provides a "high" pulse output, which connects to the transistor base with a 4.7 K Ohm resistor. All unused inputs on the NOR gate are connected to ground (this circuit uses only one of the four NOR gates in the I.C.)

Picture of the on-air lamp with audio board

Here is the completed on-air lamp controller at the left near the clock.

Closeup of on-air lamp controller

Closeup of the on-air lamp controller.

On-air lamp in hallway

On-air lamp in hallway outside studio.

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