This article is part of the Ideas for Design Series: Vol. 3, No. 3
Unijunction transistors (UJTs) were common circuit elements several decades ago. A simple ramp generator could be built from a single UJT and a few other components (Fig. 1).
The operating principle is simple. The base-emitter junction is initially in a high-impedance state, and the current source linearly charges the capacitor until a breakdown voltage is reached. At that point, the capacitor discharges through the UJT base until a lower threshold voltage is reached. Then, the base-emitter connection returns to a high-impedance state, and the capacitor can recharge.
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This approach still has a few advantages over digital circuitry, as there are tradeoffs when using a digital-to-analog converter (DAC) to create a ramp. Inexpensive microcontrollers include high-resolution analog-to-digital converters (ADCs), but high-resolution DACs are expensive peripheral components. While such DACs have limited resolution, an analog ramp like one produced by a UJT has infinite resolution.
This file type includes high resolution graphics and schematics when applicable.
It’s possible to emulate the action of a UJT using just three I/O pins of a microcontroller and a few other components. Most microcontrollers allow their I/O pins to be dynamically reconfigured to function as either a high-impedance input or as an output. This feature is the basis for a simple UJT-emulated ramp generator.
In the microcontroller emulation, the current source is implemented by the rail-to-rail operational amplifier IC1b of a dual device and Q1, while IC1a buffers the voltage on capacitor C and provides the ramp output (Fig. 2). This capacitor is connected to both analog/digital converter pin AN0 of the microcontroller (IC2) and to its I/O pin GP1, which is initially configured as an input. A 22-Ω current-limiting resistor allows high-value capacitors to be used without damage to the microcontroller.
The pseudocode listing for the microcontroller continually monitors the capacitor voltage (see the code).
When it reaches a threshold value, it switches the I/O pin from its high-impedance input state to an output state at ground potential. When the capacitor voltage goes below another threshold value, the I/O pin is switched back to a high-impedance state, and the cycle repeats (Fig. 3). The circuit also provides a pulse synchronized with the ramp cycles, which can be used for timing and triggering.
Dev Gualtieri received his PhD in solid-state science from Syracuse University in 1974. After many years doing research for a major aerospace company, he now does computer, electronic, and embedded-systems projects at his consulting company, Tikalon LLC (www.tikalon.com) in Ledgewood, N.J. He is the author of several books, available at Amazon.
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