555 RFI problem: reprise

"Cause" is a little strong here. It's a mix of the brand of 555, the circuit you designed, the construction technique, and the environment. Some of those are under your control, others aren't!

Presumably the EMI is being conducted in through either ground, power, or load lines.

Or possibly more than one of the above. (Although common-mode EMI for such a simple circuit may not be a big deal.)

The cap on pin 5 is very important in any but the most gentle environments. Bypassing and maybe regulation on the power will help too (while not really keeping out any sort of truly harsh RFI).

Possibly RF chokes or ferrite beads (I'd lean towards the RF chokes at first) would help too.

Certainly not all 555's are equal. Just as not all 7805's etc. In the perfect world we'd engineer our designs so that it didn't matter which brand the purchasing department bought this week :-).

Tim.

are you sure you wasn't working with a CMOS version?

I don't think so. Here's the data:

Jordan

Jamie wrote:

No - it's RFI, or other "wireless" effect. It happens with no wire connection, with separate power supply etc.

I had several suggestions, and tried most of them. Only changing the brand of 555 worked.

J.