Further 555 wierdness

OK, I’m obviously doing something daft here… SOS?

Please look at the attached schematic. I’ve built that circuit on some stripboard (also I’ve added a decoupling cap to the 555 which isn’t shown).

If I leave the middle connection (between 555-out and 4022-clock) disconnected, the 555 runs fine, with slightly different-sized pulses, as I’ve got different-sized resistors going on.

As soon as I plug the output into the 4022’s clock, the waveform goes to a “dit dit dah dit” form; so out of the 4022 I get 3 short pulses and 1 long one… (sorry, haven’t got a photo of the trace). The 4022 is working exactly as described, i.e. it’s counting from 0 to 3 then resetting back to 0 as per spec. But the 555 keeps throwing this long pulse out… It becomes more pronounced the larger the capacitor C2 is… I tried everything from 10pF through 10nF to 0.1uF - each size increase slowed the clock (as expected) but made the short pulses much shorter in relation to the long pulse…

I’m guessing I’ve a) missed something blindingly obvious that I just can’t see, or b) I’m using the 555 all wrong.

Any assistance gratefully received…


Firstly double check all the connections - and that you don't have any floating inputs.

You have enough decoupling on the 555? (ie LOTS) Have you tried a 7555 CMOS 555?

Hi, place a 0.1uF cap from pin5 of the 555 to gnd, it is usual practice to do this if pin 5 is not used. LM555 uses a 1/3 ,2/3 voltage divider to do its timing and switching, pin 5 is connected to it and a cap will help keep any noise from getting on the divider. Also use a decoupling cap on the 4022. Also a 10uF across the supply on the board might help. Hope this helps. Tom...... :)

Thanks chaps, I'll try these suggestions out.

At the moment, I've only decoupled the chips using 10nF caps, I'll add the 0.1uF to pin 5 as suggested, and another one between the rails. Not sure i've got a 10uF handy, but I can probably salvage one from somewhere.

7555 - I don't have any of those unfortunately, I'll be sure to buy some next time I'm electronics shopping.

I'd test everything tonight, but I just took delivery of a logitech g27 game wheel for my PS3, so everything stops for that tonight XD