Radio Shack Scanner EL Backlight Control

Overview

Early Radio Shack scanners used Electroluminescent (EL) panels as the backlight for their front panel Liquid Crystal Displays (LCD).  Unfortunately, these electroluminescent panels have a fairly short lifespan, and once they burn out, you can't easily replace them.  This quick little modification allows the backlight to only light during the scanner's detection of a signal.  This not only extends the lifespan of the electroluminescent panel, but also acts has a handy "carrier on" indicator.  You can monitor any activity with just a short glimpse at the scanner's front panel.  This can be very handy for mobile installations.

Construction

This how-to will cover an old Radio Shack PRO-2022 scanner.  The FM detection and demodulation IC for this particular scanner is based around a Motorola MC3361.  This IC has a "scan control" pin, which is used to signal the scanner's microprocessor that a signal has been received, and to stop scanning and display the frequency.  The scan control line goes to around +4 VDC when a signal is detected (or the squelch breaks).  If we tap this control line, we can also use it to activate a relay which will turn the electroluminescent backlight on and off.  You'll need to use a relay to control the electroluminescent panel's voltage as it is around 100 volts or so.  An ordinary pass transistor won't handle this high of voltage.

Block Diagram & Schematic

Pictures

Front panel overview of a Radio Shack PRO-2022 scanner.

Close up picture of the scanner's LCD display.  The electroluminescent panel is behind the LCD display to act as a backlight.  The two wires coming out the left side are the electroluminescent panel's + and - wires.

Top view.  Better view of the electroluminescent panel's + and - wires.  They connect to a high-voltage inverter.

Close up picture of the electroluminescent panel's + and - connections on the scanner's PC board.

Top view of the Motorola MC3361.  Pin 13 is circled.  The piece of mini-coax and blue capacitor are for the discriminator tap.

Modified wiring.  You may want to solder a new, longer wire to the electroluminescent panel instead of splicing into the old one.

The backlight relay is completed and installed.  It is soldered to a shield on the underside of the scanner's main PC board.  The white wire going up the top is a +12 VDC tap from the incoming power line (this scanner is DC powered).  The orange and white electroluminescent panel wires go out the side.

Tapping pin 13 of the Motorola MC3361 on the underside of the scanner's main PC board.  Ignore the coax on pin 16, it's a 10.7 MHz IF output tap.

No signal received, the backlight is off.

Signal received, the backlight is on.

The relay can "chatter" if the signal is borderline breaking the squelch.  The addition of the anti-chatter capacitor can fix this.  Experiment with different values.

Here is an example schematic of a backlight controller using a 555-timer.  This should help to elminate any relay chatter.  The relay will be activated for about six seconds each time the squelch breaks.