A DIY square-wave source doesn’t need to be complicated. This compact 1992 design delivers 57 stable, TTL-compatible frequencies from a single crystal-based IC and a handful of support components.
Originally published by K. Schoenhoff in Elektor in February 1992, this square-wave generator is a great example of mini, IC-driven design. Instead of building an oscillator and divider chain from scratch, the circuit centers on a Seiko-Epson programmable frequency-generator IC (SPG8640BN in the prototype), which integrates a crystal oscillator and two programmable divider stages.
The Design
With the 1-MHz version of the IC, the generator delivers 57 stable output frequencies ranging from 1 Hz up to 1.33 MHz, which is well-suited for testing digital circuits across multiple decades. The first divider offers ratios of 1, 2, 3, 4, 5, 6, 10, or 12, while the second provides either decade or binary division. Six control inputs (CTL1-CTL6) define the settings, selected via two BCD-coded rotary switches. The output is TTL compatible and, in most configurations, a true 50% duty-cycle square wave.
Mini square-wave generator
The surrounding circuitry is minimal:
Six 22-kΩ pull-down resistors for stable logic levels
A 78L05 regulator providing a 5-V rail from a 9-V battery
Proper decoupling capacitors for regulator stability
Reverse-polarity protection diodes
A high-efficiency LED power indicator
Portable, crystal-stable, and hardware-configurable, the design is an interesting example of ’90s bench instrumentation: practical, efficient, and focused entirely on delivering reliable digital test signals.
The front panel foil for the generator (left) and the actual project (right).
The Mini Square-Wave Generator Project
The original article, “Mini Square-Wave Generator,” appeared in Elektor February 1992. Check out the article.
Editor's Note: This article originally appeared in a 1992 issue of Elektor. Due to the project’s age, some parts and components might not be available. Still, the design should inspire you to start a project of your own.
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