Squeeker or audio S meter


There are times when an accurate signal strength reading is required and its not always convenient to look at an S meter. Best example is of course during ARDF or Foxhunting.

Imagine running through a forest trying to go in the direction of the strongest signal and look at the S meter at the same time without falling over roots and other objects on the ground. With the Audio S meter, you can (unless you are tone deaf) keep your eyes where you are going and take readings by listening for the highest pitch sound.

The circuit described herewith can be made to work with "mid generation" handy talky sets like the

Yaesu FT23 and the Icom IC02E etc. These and any others where it is possible to access an Analogue

 S meter signal can be used with this "Squeeker circuit. Very modern handsets drive the S meter display directly from the all embracing microprocessor and the required analogue signal is not accessible.

 The circuit uses an operational amplifier as a level

shifter so that the small  S meter signal that starts at

 zero volts and goes positive becomes amplified and

starts from about half the receiver supply voltage.

As the S meter voltage increases the output of the op-amp

will rise adding to the charging current for the timing capacitor of the 555 based oscillator. Hence the 555

oscillator will run faster. When the S meter level is low the op-amp will drain current from the timing capacitor slowing the charging time causing it to run slowly. If very low the oscillator will stop.  This way a wide range of audio frequencies can be achieved. It is a Voltage Controlled Oscillator.

Connection points for the IC02E

Negative to PLL unit IC201 pin 12 (MC3357)

S Meter signal to PLL unit C201 pos. (0 - 2 V at max S meter)

Pos 5v supply to PLL unit IC201 pin 4 (MC3357)

Audio out to slider of rigs volume control potentiometer.




 Connection points for the FT23R Mk1

Negative to Any ground connection.

S Meter signal to  R16 on MIC AMP UNIT. Second pin from left on small PCB. NOTE (One part of the MIC AMP op-amp is used in the S meter circuit)

Pos  supply to  ON/OFF switch in rig.

Audio out to slider of rigs volume control potentiometer.

Most rigs that use FM discriminator ICs have both FM outputs and AM outputs. It is the AM output that is fed to the S meter and this is also suitable for the squeeker circuit input.

e.g. MC3357,  TK1042M etc.



On IC02 Handset an RF gain control can be implemented by using a potentiometer of nominally 1K0 to vary the DC voltage applied to the RF receiver front end.

On the PLL unit board identify resistors R220 (100R) and R223 (100R)
Identify the end of each resistor connected to the supply line that is +5v on receive. Unsolder and lift these ends clear of the board foil. Connect these lifted ends to the slider of a 1K0 potentiometer. Connect the hi side of the pot to IC 201 pin 4. Connect the low side of the pot to negative at IC201 pin12. Use short wires.

When the potiometer is set so that the full 5 volts is applied to the resistors the receiver operates normally. When the voltage is set to zero the rig is very deaf - but NOT totally deaf. In between there is a full range of adjustment.



Return to home page