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  The Audio S-Meter
The Squeeker

By Clive Mott-Gotobed G4ODM
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Web page recreated and HTML tweaked by Mike Morris WA6ILQ (see text)

The use of a 555 timer chip as an audio oscillator is not new, it was in the original 555 data sheet. An early Popular Electronics article on the then-new 555 chip showed how it could be used as an audio VCO. Locally here in Los Angeles a local T-Hunting group had audio-s-meters on their T-Hunt receivers as early as 1978. I distinctly remember T-hunting with a friend and twisting the 2-inch PVC pipe that went from a bearing mounted to the truck floor up through the truck roof to a 4-element quad antenna, and hearing the audio tone go from a low growl to a 2 kilohertz tone and back down again as I swung through the peak signal. Years later when a friend wanted to build something similar I did a web search for something to point him to and found the Basingstoke web site and the "Squeeker".

The web page below (originallly by David Deane G3ZOI) was originally at the "Basingstoke Amateur Radio ARDF Fox Hunting" web site (at www.barc.uklinux.net), now defunct. I saved a copy in the 2004-2005 era, and regenerated this page (from my saved copy) in late 2007. Except for (a) adding the Repeater-Builder header, (b) cleaning up some of the HTML, and (c) removing links to other pages at the site (since the site is gone the links would be dead) the material between the black lines below is from the saved web page.

a) The UK Icom IC02E is the same radio as the USA Icom IC02AT except that it is limited to 144-146 (the UK 2m band).
b) The input impedance of the Squeeker circuit is high, and not RF bypassed. Use short leads, shield them if necessary.
c) The audio output waveshape is very square (i.e. has high harmonic content). Do not run it through a transmitter without some additional shaping to round off the corners.
d) In this design pin 4 of the 555 chip (the RESET input) is floating, as is pin 5. The design notes state that pin 4 should be pulled high or connected to pin 8 (the power pin) if it is not used, so add a pin-4-to-pin-8 jumper. Pin 5 needs a 0.01uf capacitor to ground.
e) Here's a copy of the 555 data sheet from Phillips, Fairchild, and ST Microelectronics.





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 this 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 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 Smeter)
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 the Motorola family of 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 etc.

by G4ODM July 2002

© 2002 BARC

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Text between the black lines by G4ODM and the copyright is held by the Basingstoke Amateur Radio Club and by David Deane G3ZOI.
Everything else on this page, including the HTML polishing, is by Mike Morris WA6ILQ.

This page originally posted on 24-Oct-2007