555 timer design consumes zero current in quiescent mode

Article By : T A Babu

In this design, a classic 555 timer IC consumes zero current in its quiescent mode and wakes up quickly in a known state.

The growth of battery-operated applications has put steady pressure on designers to steeply reduce their products’ operational power consumption and bring standby power as close to zero as possible by carefully managing the devices’ quiescent currents. In this Design Idea, we’ve taught the classic 555 timer IC to consume zero current in its quiescent mode and wake up quickly in a known state. It is a pleasure to find an innovative way to use this classic device because we have a long history together. In fact, I still remember the thrill of playing with 555 timer ICs during a long summer break from school at the age of 15. By way of thanks for those memories, I dedicate this article to Hans Camenzind for his invention of this great little chip.

The highlights of this design are that it is versatile and easily adaptable, uses off-the-shelf components, has a low component count, is activated by a positive going trigger input, and draws zero off current.

555 block diagramFigure 1 This simplified block diagram represents the internal circuitry of the 555 timer.

Monostable operation

In this mode of operation, the timer functions as a one-shot. The circuit operates in the following manner. A short pulse (> 1.2V) applied to point I/P energizes transistor Q1, pulling the 9V battery’s negative terminal down very close to the circuit’s GND value. The resulting pulse triggers an immediate output from pin 3 of the 555 IC, which holds the transistor Q1 ON for the duration of the time delay determined by the values of R3 and C1.

555 timer circuit with zero quiescent currentFigure 2 This diagram shows a 555 timer circuit with zero quiescent current.

When the voltage across the capacitor C1 equals 2⁄3 VCC, the comparator resets the timer’s internal flip-flop, driving its output (pin 3) to a low state, thereby turning off the transistor Q1. Once Q1 is turned off, it is no longer in pull-down mode, so the whole circuit is now drawing zero quiescent current.

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Timing adjustments

The 555’s output pulse width can be lengthened or shortened to the meet your application’s requirements by adjusting the values of R3 and C1. The output pulse width of time T is given by

T=1.1 x R3 x C1

read more design ideasUsually a 555 timer IC is limited because it requires a negative going trigger input but in this design, it is triggered on a positive going pulse which takes the IC out of its zero-current quiescent state for the duration of its programmed time cycle.

This article was originally published on EDN.

Editor’s note: We are very sad to report that long-time EDN contributor T.A. Babu has passed away. It was a pleasure to work with Mr. Babu and we are grateful that he shared his passion for electronics with our community. Learn more about our friend and read all of his Design Ideas here.

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