Add On S-Meter Circuit for Motorola® Micor® Receivers

Editors note by Mike WA6ILQ: I received the three image files below via email with no accompanying text.   The artwork style of the opamp style and the meter face both suggest that is is an ACC design and is from an old issue of ACC Notes.   I have credited it as such.   I wrote the text below from scratch, if anybody can locate the original text I will add it.

Concept:
Provide an analog s-meter output proportional to signal input.   This DC voltage can be fed to a analog input of a repeater controller.   The controller can be programmed to give an indication of signal strength, perhaps by varying the frequency of the courtesy beep.

Description:
Looking at the schematic below you will see that the circuit point after IC101 and just before the IF Filter Y103 is identified as "Point H".   This is the point of interest.

The circuit below provides a dc voltage from near 0 to just over 5 volts dc depending on signal input to allow a local meter or the ACC controllers s-meter input to be driven.   A calibration pot is provided to allow the user to set the full-scale s-meter calibration as desired.

The 68pf capacitor provides DC isolation from the IF alplifier output, the two diodes and the 100pf capacitor produce the varying DC level, and the LM324 opamp acts as an variable gain buffer amplifier and prevents whatever load there is (a meter or the ACC analog input) from dragging down the DC level.

Construction:
Construction can be simple breadboard wiring as the whole circuit will fit on Radio Shacks 1 7/8" by 2 7/8" breadboard. The IC should be mounted in a socket to allow easy replacement.   Use a 7808 voltage regulator or a local 8-9 vdc if available to power the circuit.   The circuit does not load the receiver in any way and will not degrade the receivers performance.   Try to keep the lead from the if amp/limiter/detector mixer output as short as possible, and if it is going to be any real length I'd use shielded cable like RG-174. The detector diodes should be germanium for best performance.   The whole unit could be mounted inside the reciever chassis, and perhaps use one of the unused frequency select lines to bring the DC voltage out to the controller.

Text by Mike Morris   WA6ILQ        Up one level        Back to Home