5V logic pulser is battery-powered

W Stephen Woodward, University of North Carolina, Chapel Hill, NC -- 9/16/1999

A battery-powered, pushbutton-triggered TTL/CMOS-compatible source of debounced 5V logic pulses is a simple but handy piece of test equipment to have in any tool kit (Figure 1). The circuit's battery-powered operation complicates what would otherwise be a trivial exercise in switch-bounce and timing-circuit design. The convenient use of battery power simultaneously imposes two requirements: near-zero quiescent-current draw and input-variation-tolerant voltage regulation. A near-zero quiescent-current draw minimizes the likelihood that you will encounter a dead battery when you need to use the logic pulser, and input-variation-tolerant voltage regulation accommodates the inevitable voltage droop as the battery ages and traverses its service-life curve from fresh to flat.

Of course, the unglamorous on/off switch is a traditional and serviceable way to fulfill the first requirement. Unfortunately, the effectiveness of an on/off switch depends directly, and regrettably, on whether you remember to use it. Neglecting proper and timely operation of on/off switches may result in battery damage. Thus, a key feature of the logic pulser in Figure 1 is a satisfactory battery life without dependence upon a separate, manual on/off switch.

The trick to this design feature is the use of a single NO spst momentary-contact pushbutton switch to control both the trigger logic and the voltage regulator. In the quiescent state, S₁ is open, which leaves the ground-reference pin of the 78L05 regulator floating and causes the regulator's output voltage to saturate about 1V less than the input voltage from the battery. The circuit applies the resulting 7 to 8V to the V_DD pin of the 74C04 and, via R₅, to the debounce time-out circuit comprising R₆, R₁, and C₁.

In this state, the 74C04 typically consumes less than 1 ?A and is the only power demand on the battery. The regulator is floating and consumes no power, despite its normally multimilliamp quiescent current. This low power implies a battery-life expectancy that is limited only by the self-discharge characteristics, or shelf life, of the battery. This expectancy is typically 10 years for alkaline and much longer for lithium. Meanwhile, the fact that the circuit applies V_DD to the output gates of IC_1B and IC_1C while the steady state of timing circuit R₂, R₃, R₄, and C₂ applies a logic 1 to the inputs of these gates ensures a solid low-impedance logic low at the pulser's output.

When you push the trigger button, the sequence of events begins with the grounding of the regulator's reference pin and the consequent regulation-independent of battery voltage-of the 74C04's V_DD to 5V. Simultaneously, grounding one end of R₆ starts the discharge of C₂. The resulting approximate 20-msec delay provides adequate time for reliable debounce of the pushbutton, thus preventing the possibility of spurious multiple pulses in response to a single button press.

When IC_1E's input ramps to the approximate 2.5V CMOS logic threshold, the positive feedback around the IC_1E-IC_1D pair via C₁ causes the circuit to present a clean logic transition to the C₂/R₃ differentiator. The R₂, R₃, and C₂ network combines with the IC_1F-IC_1A regenerative pair to convert the resulting edge into a single, pristine, 100-?sec pulse. The parallel IC_1B-IC_1C pair then inverts and buffers this pulse to the output. Following the pulse, the circuit output returns to a stable 0V level; the output remains low until you release the pushbutton, which readies the trigger circuit for another cycle. (DI #2412)

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