When hunting mobile, especially mobile-in-motion, a large S meter on the dashboard is very handy. It is more convenient. It can be read from farther away. It can be nicely lighted.
Here is a suggested circuit to add an external S meter to an existing radio. It assumes that the radio's S meter is grounded.
The first step is to look at its circuit diagram to see if its meter circuit is like the one shown in the our diagram. (The meter and calibration resister positions could be reversed in their series circuit.)
Next, measure the voltage "V" at the drive point when the meter is at full scale. You will need this to calculate a resistor value in the new circuit. The goal of that calculation is to produce +5 volts output from the Op-Amp when the internal S meter is at full scale. Using a +/- 10% resistor should be OK.
Next, find a meter for your dashboard box. Any movement from 50 micro amps to 10 ma is OK. 100 ma movements are usually best because they usually have the best dynamic characteristics for this application ... not too fast and not too slow.
Using a single ground lead between the meter unit and the radio could produce some effect in the meter reading due to current flowing through the resistance of a common ground. Usually connector contacts are unreliable, and their resistance can become significant. Using two ground leads prevents this.
Do not neglect the wiring shown for the unused Op-Amps. This prevents the unused units from breaking into oscillation. If that happens, the IC can overheat, and also, the harmonics may be heard in the radio.
Notes:
George R. Andrews, President
Display the figure as a
separate page.
Down load as a GIF
Image. Its name is
RemMeter.GIF.
Send E-mail to grandrews@aol.com.
(A message window will open.)
Return to Home Brew Articles,
Contents.
Return to TOP
PAGE.
