December 4, 1997

Edge detector runs off of single supply

Joe Meloche, Ingersoll-Rand Fluid Products Division, Bryan, OH

You can use a single-supply circuit to generate a pulse on both the rising and the falling edges of a signal for use with counters or similar devices that require only a rising edge to trigger (Figure 1a). R₁ and C₁ form a differentiator that converts rising edges to positive spikes and falling edges to negative spikes. This output drives the LM358, which the circuit configures as a comparator with a reference input equal to 1/2V+. As expected, the LM358 converts the positive spike to a positive square pulse of approximately 2.5 lowercase greek tau in duration, where lowercase greek tau=R₁×C₁. The LM358 also converts the negative spikes to similar positive pulses because of the LM358's characteristic ability to operate as a fully functional single-supply op amp (Figure 1b). Tests using LM358s from National Semiconductor (Santa Clara, CA) and Motorola (Austin, TX) confirm proper operation. (DI #2116)

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