555 timer

[Argus25]

Originally Posted by kalee20

Interesting!

However, I would also suggest that this is a 'bad design,' as it relies on an unspecified aspect of the component. Another batch of devices, which didn't work in the circuit, would be difficult to reject to the manufacturers as they would still be to the published spec.

It could of course be the case, that the designers negotiated with Signetics to assure supplies of devices for this particular mask set, for the production life of their product, or otherwise tied it down. Manufacturers are generally interested in 'specials' if volume is high enough. I'd be interested to know!

I think in this case, in the year 1972 when it was designed the NE555N was the go to part. I'm not sure if the National LM555 was out yet, might have been.

Every version of the original pcb I have seen had the Signetics NE555N in it, so the designer may never have known that he created a circuit didn't work properly with other variants. I only found out it was like this because I built some clone pcb's and initially thought any 555 variant would be ok and I got a surprise when I found out it was not the case.

[G6Tanuki]

The 555 is indeed versatile: in the past I used one - feeding a four-diode-four-capacitor voltage-doubler - to generate +/-18V bias-supplies from a single-ended 12V supply. I only needed a milliamp or so of current for each bias-rail so a 555 clocking at about 750Hz did the job perfectly, and didn't involve the need for any expensive-in-small-numbers specialist inductors.

The 555 was also my first "one-shot noise-generating IC" when I inadvertently fed 48VDC into one: the bang and cloud of smoke as the top of the package exploded was quite something!