What is the most versatile, widely used, and longest-living IC you’ve ever seen? Many of you would say it’s the 555 timer IC. In fact, it’s still the electronic hobbyist’s best friend, and it’s widely taught in universities. Chances are you’ve used a 555, maybe even recently. Yet, I wonder why people still use it — after all, it is the 21st century.
Signetics first released the 555 in 1971. In the 36 years since then, Signetics was bought by Philips Semiconductors, which is now known as NXP. We continue to use the 555, especially in hobby projects and school labs, although I haven’t found many new products that use it. Admittedly, I haven’t looked that hard, especially outside of the U.S. But I would bet few of you could show me a brand new product that uses it.
I’ve only found one new product that uses the 555. I was testing Agilent’s new U3000A Instrumentation Training Kit, which works with Agilent test equipment to teach students how to use scopes, function generators/AWGs, and other instruments to make basic measurements. It targets colleges and universities, which rarely seem to teach instrumentation and measurement. In addition to the PIC micro, some analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), and some sensors, the board includes a 555 that acts as a basic clock generator — how appropriate.
I read somewhere recently that millions of new 555s are made each year and that the total sold to date is easily greater than 1 billion. I’m curious to know where these chips are used today. There aren’t that many hobbyists and schools using them, so what is the application? What is its appeal? Why has it survived while most other ICs of that same era have all but disappeared?
What It Is, How It Works
The 555 is a versatile device used primarily for timing and pulse generation. Made with bipolar transistors, it includes two op-amp-type differential comparators, a fixed internal voltage divider that provides a reference voltage to each comparator, a flip flop whose state is determined by the comparator outputs, and a high-current (200 mA) sink/source transistor output. It works with external resistors and a capacitor. Figure 1 shows the 555 connected as an astable that generates rectangular pulses. Capacitor C1 charges and discharges through resistors RA and RB while the comparators change the flip flop state.
The primary package option is an 8-pin mini-DIP (dual-inline package) that will work with a dc supply between 4.5 and 15 V. A dual 555 in a 14-pin DIP called the 556 is also available. According to the latest Jameco and Digi-Key catalogs, you can get the 555 in an SOIC-8 package and the 556 in an SOIC-14 package. And, don’t forget that a CMOS version of the 555 has been available for years.
The original designer of the 555, Hans Camenzind of Signetics, said in the September 1997 issue of IEEE Spectrum that the 555 is dated mainly because it is no longer compatible with the mostly low-voltage (less than 5 V) circuits in use today. Furthermore, it consumes excessive power compared to today’s circuits. He discussed how he would redesign the 555 if he could. (For more, you can read an interview with him at the Transistor Museum at www.transistormuseum.com that gets into the original design background.)
The 555 is most commonly used as a clock oscillator. It’s great for creating low-frequency (less than 1 MHz) rectangular waves for switching lights off and on or producing audio tones. It also can be used as a voltage-controlled oscillator (VCO) with an external dc control input. Another common use is as a one-shot that produces a fixed-output pulse duration when triggered. I’ve used the 555 as a low-power dc-dc converter to generate –5 V from a +5-V supply.
A Google or Yahoo search for "555 timer" on Google or Yahoo will turn up a stock of good info, including some excellent tutorials. The online resources are excellent if you are hell-bent on continuing to use this obsolete device. See my favorite tutorial here.
Replacing The 555
I don’t have anything against the 555, but we generally don’t use it in modern electronics, just as we no longer use the discrete component circuits we once did. (Remember unijunction transistor oscillators?). Chips and products got too fast and started using lower voltages, leaving the 555 in the dust. Sure it’s cheap, and there are numerous second sources, but there also are lots of good alternatives.
One of those alternatives is a more versatile timer made by Exar. Known as the XR-2240, it uses a frequency generator like the 555 but comes with a binary counter/divider for more flexible timing selection and lower-frequency operation. Another choice, Exar’s XR-2206 function generator IC, does pretty much what the 555 does but also generates sine, triangular, and sawtooth waves as well as square waves. Furthermore, it can be amplitude- or frequency-modulated.
I love the XR-2206 and probably have used it far more than the 555. The XR-2240 and XR-2206 are both still available. Another generator IC with similar functions, the 8038, resembles the XR-2206 in features and specs, but I can no longer find it.
Another substitute for the 555 is a CMOS logic gate or inverter connected as an astable (Fig. 2a). Its upper frequency limit is about 1 MHz. Also, don’t overlook the simple astable you can make with a TTL or CMOS Schmitt trigger (Fig. 2b). Only one capacitor is required, and it works well over a very wide frequency range—well beyond the 555.
Just remember that with these circuits, the duty cycle is not 50%. If you require 50% duty cycle, generate the signal at twice the desired frequency. Then, follow the clock oscillator with a flip-flop that divides by 2 and gives you the 50% duty cycle. The Schmitt trigger-style astable also can be implemented with an op amp used as a comparator (Fig. 3). If you need a wider or bipolar output swing at a lower frequency, this circuit is a good choice.
How We Do It Today
What if you have a timing or clock requirement for a new design? How do you implement that? Most circuits require not only higher frequencies but also greater frequency precision than what’s available with RC networks. Today, we tend to use crystals or ceramic resonators to get greater frequency precision as well as improved temperature stability. Yet there are still some good RC network choices.
One solution is to use the 555 but add a programmable digital potentiometer for the external timing resistors (Fig. 4). Xicor’s X9315 is a good fit with the 555. Its serial data input programs the frequency of operation. Also, these digital pots can be incremented and decremented, providing some interesting options for changing frequency and duty cycle. This has got to make the 555 far more acceptable today.