Nuts and Volts - February 2011

OPENCOMMUNICATION

THE LATEST IN NETWORKING AND WIRELESS TECHNOLOGIES

■ BY LOUIS E. FRENZEL W5LEF

555 TIMER IC REINVENTED — NEW VERSIONS OF THIS LEGACY CHIP EXTEND ITS LIFE

Who hasn’t used a 555 timer IC? If you are an engineer, technician, teacher, hobbyist, or an electronic whatever, chances are you have encountered the ubiquitous 555. This “ancient” chip was introduced by Signetics (then Philips, then NXP) in 1971. It was an instant hit because of its incredible versatility. That chip spawned thousands of applications circuits from simple audio oscillators to sophisticated timers, and dozens of other unique circuits. Literally, billions have been sold. Despite the widespread use of cheap single-chip microcontrollers, the 555 timer still seems to find a place in school labs and even some new products.

While the legacy 555 continues its popularity, that basic design just may garner some renewed interest thanks to two new and improved versions of this chip recently by Advanced Linear Devices and Semtech. Here is a look at this incredible chip and the new models that may further its longevity.

555 BASICS

signals. The voltage divider sets the threshold and trigger levels at two thirds and one third, respectively, of the +V supply voltage. When the trigger input is less than the trigger level, the output is high. When the threshold input is higher than the threshold simultaneously with the trigger input being higher than the trigger level, the flip-flop is reset and the output goes low. The trigger and threshold inputs usually come from an external capacitor. The discharge output is used to switch the capacitor. A reset input overrides any other input condition when it goes The architecture of the 555 is simplicity itself (see Figure 1). It consists of a simple set-reset flip-flop driven by two comparators. The comparators get their inputs from an internal three-resistor voltage divider and some outside