Project of the Month

Icom IC-400-PRO External Signal Strength Meter.

This project adds a signal strength meter to the popular Icom UHF CB radio, the IC-400PRO.

The IF IC, a TA31136, has a dynamic range of 80dB, which would make an outstanding signal strength meter, but unfortunately, Icom failed to exploit it. I guess they just don't know the CB market. You will not have to remove the circuit board from the radio to do this mod, but you will have to build a small circuit.

You will need:

A meter. A 'VU' meter out of an old tape deck or similar will be the best choice, because they are marked in dB, and are logarithmic.

An LM358 op-amp IC.

A 1N4148 diode.

A 100K trimpot.

A small piece of veroboard.

Step 1

Get the RSSI output from pin 12 of the IF IC, IC1.

Turn the radio over and remove the bottom lid.

The Pin 12 output is connected to a track that travels a short distance and terminates to a 'via', a plate-thru hole, as indicated on the partial component overlay below. Solder a fine wire to the 'via'. You will need a fine tipped soldering iron, a steady hand, and a magnifying glass for that part. Tape or glue this wire down so that if you pull on the wire, it doesn't pull the track off the board.

Step 2

Build the small circuit below.

Build this circuit on a small piece of veroboard.

You can't just connect a meter to the RSSI output of the IF IC. It would load it down too much. We need a 'buffer'. The circuit below is a 'buffer', also known as a 'voltage follower'.

The output of the LM358 is the same as it's input, but the LM358 has a beefier output to drive a meter with. The RSSI output of the IF IC has about 0.7 of a volt output even when not receiving a signal, so we put a diode in series with the LM358 output to drop it by about 0.6 of a volt. This voltage the goes into a trimpot, to set the full-scale reading of the meter. You might want to use a ten-turn trimpot here to make it easy to set.

You can either put this circuit inside the radio and run the wires out to the meter, or run the RSSI wire out of the radio and put the circuit in a box with the meter. If you put the circuit inside the radio, make sure it is insulated and/or glued down.

CAUTION:

Be careful not to short the RSSI output from the radio to ground while the radio is turned on. Doing this will blow the RSSI part of the IF IC. The radio will still work OK, but you will no longer have a signal strength output signal.

Step 3

Setting it up.

Transmit on a nearby radio so that the Icom is receiving a strong signal. Adjust the trimpot VR1 so that the needle on the meter is full-scale.

That's it. Build it into a pretty box with illumination.