If you’re not familiar with the 555 timer, suffice it to say that this versatile integrated circuit is probably the most successful ever designed, and has been used in countless designs, many of which fall very far afield from the original intent. From its introduction, the legendary 555 has found favor both with professional designers and hobbyists, and continues to be used in designs from both camps. New versions of the IC are still being cranked out, and discrete versions are built for fun, a temptation I just couldn’t resist after starting this article.
If you think all 555s are the same, think again. Today, a number of manufacturers continue to produce the 555 in the original bipolar formulation as well as lower-power CMOS. While the metal can version is no longer available, the DIP-8 is still around, as are new surface-mount packages all the way down to the chip-scale. Some vendors have also started making simplified variants to reduce the pinout. Finally, you can assemble your own version from a few parts if you need something the commercial offerings won’t do, or just want a fun weekend project. In my case, I came up with what is probably the fastest 555-alike around, although I spared little expense in doing so.
Follow along for a tour of the current state of the 555, and maybe get inspired to design something entirely new with this most versatile of parts.
What’s In the Box?
The first 555 timers, the NE555V/SE555T from Signetics, were produced more than 47 years ago. Designed by Hans Camenzind, this part eventually sold over a billion units per year, and has been used in everything from children’s toys to spacecraft; the original bipolar-technology 555 didn’t need radiation hardening, only very rigorous testing, for space qualification. Hans sadly passed away in 2012, but you can hear some audio clips of him discussing the 555 at the Transistor Museum site. In that interview, he relates how Signetics chose not to patent the design. At that time, Silicon Valley enjoyed a culture of mutually assured destruction with respect to intellectual property: companies freely stole ideas from each other, secure in the knowledge that the first to file a patent suit would be hit with an overwhelming response to all of their own infringements. This environment allowed versions of the 555 to be sourced by multiple vendors, keeping prices low and increasing the part’s appeal to designers.
The Art of Electronics called the 555 a kit of parts, and that’s probably the best overall description you can come up with. Inside, you’ll find a voltage divider producing references at 1/3 and 2/3 of the supply voltage. One of two comparators fires when the voltage on the TRIGger pin is less than the lower reference or that on the THREShold pin is greater than the upper one. The outputs of the comparators lead into a digital latch, which can also be reset with an external active-low signal. Finally, the output from the latch drives the OUTput …read more
Source:: Hackaday