I have a number of scanners all operating at the same time.  It allows me to catch various conversations, rather than having a single scanner go through the entire list before returning to a channel again.  The downside is that it's often difficult to determine which scanner is talking.  Bob Parnass had an article years ago regarding a Carrier Operated Relay (COR).  From there, he came up with the Carrier Operated Light.  This is a very simple circuit using an NPN transistor, a couple of resistors, and an LED.  The power is taken from the switched 12V source on the volume potentiometer/switch.

Bob's article was written for the PRO-2004 scanner.  However, the PRO-2005, PRO-2006, and PRO-2042 scanners (and probably many other GRE scanners of this era), all use the TK10420 chip.



When the scanner detects a signal, a small voltage appears on pin 13 (Scan Control).  That's not enough to drive an LED directly, but it's enough to drive a transistor, which is acting as a switch to operate the LED.

It takes a steady hand, but it's easy to solder a wire directly to pin 13 of the chip.

Sometimes finding the chip is a challenge.  Only on my PRO-2004 is the chip hidden inside a metal RF shield.  In this case, it's the box on the top side of the scanner, right behind the front panel.  It's the one with all the holes in the top.

I could have tried to sneak the wire out of one of the existing holes, but there's not much room between the lid, and the components below.  It was easier to drill a hole in the lid right where I needed it.

A Unibit does a nice job, and chamfers the hole when done.  I drilled the other side too.  No sharp edge for the wire to chafe upon.  If this was for a mobile radio, I would have put a grommet on the hole too.

The RF lid was put back in place.  The white wire is now ready to be connected.

This is a very simple circuit.  You could mount it to a small circuit board, but that's not necessary.  I soldered everything directly to the transistor.

A piece of heatshrink was put on each leg, then shrunk.

A larger piece of shrink was put over the entire thing, including the transistor body itself.  Since this is a low current circuit, heat disspation isn't a factor.

The black wire is connected to the negative battery terminal near the 9V battery compartment.  This is a good grounding point.  The white wire from the chip is connected to the white wire on the circuit we just built.

I used the earphone jack on my radios.  I never use headphones, and didn't want to drill an extra hole in the faceplate.  There is a tool for removing this type of nut, but I've found that using a sharp tipped set of wire cutters effectively grabs the grooves in the nut.

I did have to enlarge the hole slightly to fit the 1/4" LED.  On the PRO-2005/6/2042 scanners, the existing hole fits the 1/4" LED without modification. 

If you simply unplug the wire harness going to the headphone jack, your internal speaker will also quit working.  This connector has a switch inside.  You can short the two wires going to the switch, and remove the connector completely, but this method is easy...  Just put a piece of heatshrink onto the jack.

While the shrink is still hot, clamp the end with needle nosed pliers.  This will fuse the ends.

With a single tie-wrap, it's attached to a nearby wire harness.  Safely out of the way, and can be easily put back in place someday if needed.

The LED positive wireis connected to the brown wire on the back of the volume potentiometer/switch.  The brown wire is 12V+ only when the radio is on.  The negative wire from the LED is connected to the green wire of the COL circuit.

The COL circuit is attached to an existing wiring harness with another tie-wrap.

Each of my scanners now has a Carrier Operated Light.  All were fairly easy, but all had the guts slightly differently arranged than the other, except the PRO-2005/6 are about identical.  The PRO-2005/6 earphone jack is removed from the back of the display.  A tiny screw holds it in place.  The PRO-2042 is very similar, but I had to trim a plastic screw post away to make room for the LED.




Last updated 06/23/14    All rights reserved.