Post by Electronics Notes by Ian Poole

Back to Basics: Creating Short Pulses with Simple Transistor Circuits Sometimes the best solution to a design challenge isn't a complex IC or a microcontroller—it’s a handful of discrete components and a fundamental understanding of circuit behaviour. If you’ve ever needed to trigger a logic circuit or generate a timing pulse from the edge of a square wave, a Transistor Pulse Generator is a tool every engineer should have in their back pocket. In my latest summary, I explore how to design these using simple Bipolar Junction Transistors (BJTs). It’s also similarly possible to use FETs, but here’s the bipolar version. How it works: The key combination is a CR (Capacitor-Resistor) network acting as a differentiator. When a rising edge hits the network, the capacitor passes a brief "spike" as it charges. By pairing this with a transistor, we can: ✅ Clean up the pulse into a sharp, usable square wave. ✅ Remove unwanted negative-going spikes. ✅ Invert or maintain the polarity of the input signal. Two configurations to consider: 1. The One-Transistor Design: Ideal for simplicity, providing a negative-going pulse from a positive-going input. 2. The Two-Transistor Non-Inverting Design: Uses two capacitively coupled NPN switches to create a positive pulse from a rising edge—a common requirement for TTL logic. A Quick Tip for the Workbench: When designing the two-transistor version, be mindful of your rail voltage! If it exceeds ~7V, the negative voltage spike at the base-emitter junction could cause a breakdown. Keeping it around 5V (standard logic level) is the "sweet spot" for reliability. Discrete designs like these remind us that even in an age of SOCs, there is still immense value in straightforward, cost-effective analog building blocks. Want more information? Check the link in the comments. #ElectronicsEngineering #CircuitDesign #HardwareDesign #AnalogElectronics #ElectricalEngineering #EngineeringTips #electronicsnotes