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The Squeeker
By Clive Mott-Gotobed G4ODM
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The Audio S-Meter or The Squeeker By Clive Mott-Gotobed G4ODM |
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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.
Notes:
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.
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THE AUDIO S METERorThe SQUEEKERBy G4ODMThere 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 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 Connection points for the FT23R Mk1 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 |
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© 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
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.
