Audio Frequency Counter

Audio Frequency Counter Kit assembled

The audio frequency counter kit by Bob Grieb of Tauntek counts frequency for inputs above 1kHz,
and at lower frequencies counts period and calculates the reciprocal. This gives good resolution
over the entire audio band. I bought the kit without the optional comparator because I want to
use my own input signal processing. It runs on 3 volts so I shall need a suitable regulator.

Additional Circuitry

The additional circuitry consists of a 78L33 regulator and an LM393 comparator. The regulator
provides 3.3 volts for the counter board. The comparator responds to small input signals and
triggers to supply a good square wave to the counter board input. The comparator output pull
up resistor is fed from 3.3 volts so the swing is 3 volts peak-to-peak. There is provision for
powering an optional 5V display backlight.

Board Layout, Component Side

The board is laid out to include the comparator and supply regulators. Broken lines indicate the
no-go area beneath the battery box.

Completed Board Assembled in Case

The board lies beneath the battery box with components placed to each side. The three pin sil header to
the left connects with the counter board. The header on the right supplies 5V to the display backlight.

AF Counter mounted on Function Generator

I tried several different displays. The original was a 1601 without backlight. I tried a 1602 with backlight
and this worked but showed the frequency in two lines of eight characters each. Finally I got a 1601 with
backlight and this worked perfectly, single line display, illuminated, with excellent contrast and viewing angle.

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