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.
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.
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