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From the Q&A

With TJ Byers

555 Shows Its Ugly Side


 I stumbled across a neat timer circuit that uses a 555 chip, which I decided to build. It works, but only somewhat. It works for a cycle or two, then becomes erratic, or mostly, locks up. If I disconnect the battery and let the circuit sit for a few minutes, it will work for a few cycles and screw up again. I even bought a new 555 from RadioShack, and it does the same thing. How come it works sometimes and not others?

Sal Marino
via Internet


You've discovered one of the best kept secrets of the 555. That is, they aren't all the same. In fact, I know which one you have. It's the NE555 from Texas Instrument (silver top, right?), because that's what RadioShack stocks even though the catalog says LM555. The difference is that the LM555 is a bipolar chip and the NE555 is a CMOS chip. And I bet that pin 2 of this circuit is waving in the air — picking up static electricity. When the first CMOS 555s showed up on my workbench, I had the same problem until somebody told me they were CMOS, then the light went on. When working with CMOS technology, every pin has to go somewhere, used or not. What you need to do is tie pin 2 high through a 10k resistor, that's pretty close to its bipolar equivalent, and the circuit will work. Here's an excellent tutorial web site on the 555 that's a must for any serious experimenter.