Precise FM Tuning Indicator

Here is an add on circuit to your FM radio for precise tuning of stations. Usually an LED indicator is provided in FM radio to see whether the station is tuned or not. But it is difficult to see the precise tuning points since the variation in the LED brightness cannot be detected easily. This circuit solves the problem.

Precise  FM Tuning Indicator Circuit diagram :

The circuit uses the Op Amp IC CA3140 as a differential amplifier to sense the voltage level between the terminals of the tuning LED. The output of IC1 drives two LEDs one Red and one Green to indicate whether the station is precisely tuned or not. If both LEDs remain off, it indicates precise tuning. If anyone LED is on, it indicates that tuning is not precise. If both LEDs remain on, it indicates that there is no signal.

First tune the FM receiver in a station having strong signal. The tuning LED will light brilliantly. Then connect point A and B to the soldering points of the tuning LED observing polarity. Adjust VR till both the Red and Green LEDs turn off. Slowly change the tuning knob position. Any one LED will light up. So the circuit is working. Now glue the wiper of VR using adhesive to prevent its position change. Disconnect the tuning LED from the board. Now the circuit is ready to use. Power for the circuit can be obtained from the power supply of FM radio.

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1 Zener Precise Limiter Wiring diagram Schematic

A limiter 1 Zener Precise Limiter Circuit Diagram that requires matched zener diodes can instead use one zener with a full-wave diode bridge. The schema`s two limits are nearly equal when determined by the same zener—only two pairs of forward diodes need to be matched. For best results, an integrated quad of diodes can be used. But, after testing the schema, four single controlled-drop diodes and four ordinary diodes gave about the same accuracy (better than 0.5%). 

 1 Zener Precise Limiter Circuit Diagram

Because the limiting level can be adjusted, zener tolerance can be adjusted out. Gain stability can be optimized by connecting the inverting input to the first op amp to the output of the second to make the schema inherently unity-gain. 

The zener voltage must be increased to 8.2 V to compensate for the two diode drops. Placing small capacitors across the resistors in the loop stabilized the schema adequately and response is orders of magnitude faster than conventional diagram. Moreover, it`s limited primarily by the op amp`s slew rate.

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