Electronic Design

The 555: Best IC Ever Or Obsolete Anachronism?

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.

Another possibility is the Dallas Semiconductor (now Maxim Integrated Circuits) DS1065. It contains an oscillator, prescalers, and a frequency divider. Housed in a three-lead TO-92 transistor package, it’s a full oscillator and divider that can be set to frequencies between 30 kHz and 100 MHz. Its single I/O pin provides the output. But with a pull-up resistor on this pin, the chip goes into a programmable mode that enables you to enter prescaler and divider selection options to get the frequency you want. And the really cool thing about this chip is that it uses zero external components. The frequency tolerance is ±0.5% initially but can vary ±3% over temperature and voltage variations—pretty good for noncritical clock applications. Maxim’s DS1085 is another such oscillator/divider IC. It’s a pretty good frequency synthesizer that can be set to frequencies between 8.1 kHz and 133 MHz and comes in an eight-pin SOIC package. A two-wire serial interface lets you program the device to your desired frequency. Its frequency tolerance is ± 1%. Also, Maxim’s MAX7387 and MAX7388 specialty clock timer ICs have a fixed-frequency oscillator in the 32-kHz to 32-MHz range. Standard frequencies of 1, 4, 8, and 16 MHz are off the shelf, but you can order any frequency you want in that range for a price. This particular part is used mainly for watchdog and power-fail applications. Another interesting replacement for the 555, Linear Technology’s LTC6906, is a pulse oscillator with a frequency range of 10 kHz to 1 MHz—pretty much the range of the 555. It’s made from CMOS and uses only a single external resistor to set the frequency. No external capacitor is needed. And, the accuracy is 0.5%. The chip operates from a 2.25- to 5.5-V supply and consumes very little power (less than 12 µA at 100 kHz). The package is a six-pin, 3- by 3-mm SOT-23 (ThinSOT) surface-mount type (Fig. 5). Its external resistor (RSET) determines the frequency according to the simple formula:

fOUT = (1 MHz/N)(100k/RSET)

For greatest precision, it is best to use a metal film resistor with a tolerance of 0.5%. The N in the formula above is a frequency divider factor. The LTC6906 has a built-in frequency divider with ratios of 1, 3, and 10. The desired ratio is selected by voltage on the DIV pin: ground for divide by 1, open for divide by 3, and +V (the supply voltage) for divide by 10. You can get frequencies down to about 10 kHz or a little less up to 1 MHz. It’s a pretty cool part and one of the first in a long time that addresses clock and timing applications. Linear Technology also has another similar part that can replace crystal or ceramic clock oscillators in some applications. The LTC6905 operates from 2.7 to 5.5 V and is housed in a SOT-23 package. Its frequency range is 17 to 170 MHz. Again, the frequency is selected with a single external resistor and a divider ratio. The frequency error is only about ±0.5%, and the jitter is less than 50 ps at 170 MHz. This is pretty good and can be used in many digital applications as a clock oscillator without the need for an expensive crystal. Variations include the LTC1799, LTC6900, LTC6902, LTC6903, and LTC6904 with different frequency ranges and the single external resistor programming. The LTC6903 and LTC6904 have a frequency range of 1 kHz to 68 MHz. The LTC6903 is programmable via an I²C bus, while the LTC6904 is programmable via an SPI bus. If you absolutely must have crystal accuracy at low cost, you might be able to use National Semiconductor’s MM5369. It uses a low-cost 3.579545-MHz crystal to make an oscillator followed by a 17-stage divider. The divider is programmable to provide a wide range of frequencies, including 60 Hz. Probably the most popular approach to generating a clock of programmable frequency is to use a cheap embedded controller (Fig. 6). Any 8-bit microprocessor can easily duplicate the operation of a 555 and then some. All you do is write some code that puts the processor into a timing loop or two and pulse an output port. But is it wise to replace the super-cheap and easy-to-use 555 with a more expensive and complex microprocessor that requires us to write software? Absolutely. Most products are digital anyway, and virtually every one of them contains at least one embedded controller. So, it is natural to implement timing functions with a microprocessor. Embedded controllers like the PIC, 8051 derivatives, 68HC05/11, and others are dirt cheap in high quantities. Also, most of you are already steeped in programming, so writing the code is trivial. Besides, a crystal or ceramic resonator usually controls the microcontroller clock, so it produces really accurate timing pulses. This mitigates the problems inherent in attempting to achieve timing accuracy and temperature stability with an RC network. With an embedded controller, you get high accuracy with ultra-reliable repeatability. Add to that the presence of pulse-width modulation (PWM) functions on many microcontollers, and you have added functionality without new hardware—just some more code. By using a long count loop or nested loops or the built-in counters and timers plus precisely counting instruction execution times, you can program the chip to provide even the oddball frequencies your application may need. So What About The 555? I’m not knocking the 555. I’m just trying to understand its ongoing use when so many other solutions are available. And why has it survived when other ICs of the 1970s and later have become obsolete? In most cases, microcontrollers and programmable logic devices like PROMs, PALs, GALs, and FPGAs have replaced the once-ubiquitous DTL and TTL logic chips as well as CMOS logic chips. Yes, you can still get TTL and CMOS logic, but they aren’t often used in new designs. Older linear devices are probably still more common than older digital devices. Almost as old as the 555 timer is the 741 op amp. Like the 555, it probably will never die, especially since good linear devices never seem to fade out. These linears, for example, are still around: the 301 op amp, 78xx linear regulators, 317 regulators, the 1496 balanced modulator, and the 602 RF mixer. I still like the old RCA 3028. My guess is that the 555’s long life comes from a combination of factors like very low cost, multiple sources of availability, extreme versatility, and widespread design knowledge. Its old-style DIP format also makes it easy to use, plus it is rugged and forgiving – that’s hard to beat. While you won’t find the 555 in many new designs, I suspect we will continue to use it in our hobby projects, and schools will continue to teach it. Every now and then, you may find the ideal application for it. Just remember that there are alternatives that will produce smaller circuits with fewer discrete components. For now, congrats to Hans Camenzind and the crew of designers at Signetics, wherever you are. Long live the 555.

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