Electronics > Projects, Designs, and Technical Stuff
Need some help with a 555 timer circuit
Ian.M:
Have you considered bounce on switch opening triggering it? Assuming the operator holds the button down till the feed stops, its undesirable for it to immediately resume feeding when the button is released!
I'm sure it *COULD* be worked around by careful choice of components in the negative edge pulse shaping circuit, but developing that while retaining good EMI immunity and >10mA, >10V switch wetting current & voltage was more design work than I was willing to do as a freebie. The extra 555 gives me a clean squared up button pulse so I don't have to deal with rising edge bounce.
Sometimes for a one-off or very small hand-built production run, throwing extra parts at the problem to save time is more cost-effective.
Zero999:
--- Quote from: Ian.M on August 12, 2020, 09:30:19 pm ---Have you considered bounce on switch opening triggering it? Assuming the operator holds the button down till the feed stops, its undesirable for it to immediately resume feeding when the button is released!
I'm sure it *COULD* be worked around by careful choice of components in the negative edge pulse shaping circuit, but developing that while retaining good EMI immunity and >10mA, >10V switch wetting current & voltage was more design work than I was willing to do as a freebie. The extra 555 gives me a clean squared up button pulse so I don't have to deal with rising edge bounce.
Sometimes for a one-off or very small hand-built production run, throwing extra parts at the problem to save time is more cost-effective.
--- End quote ---
No, I've not experienced switch bounce, when the contacts open. I suppose it's a possibility. There's no need for two 555 timers, just put a capacitor (try 100nF) across the switch. It will also help to close the contacts, by providing a surge to make the contacts wet.
Ian.M:
You are probably right, and that would certainly help with EMI immunity, but I'd like to see a fully worked up design of the single 555 approach. Also see my comments earlier about possible contact damage from shorting a charged cap. Without a detailed spec sheet for the switch, one doesn't know which side of the line one is operating on between an adequate surge current for wetting, and progressively damaging the contacts by ablating the contact surfaces as they micro-bounce on closure. 'Belt and Braces' would be a small series resistor for the cap to limit the peak wetting current.
N.B. The switch bounce generator in the sim I posted earlier was half-borked due to undocumented limitations of the LTspice PWL TRIGGER modifier. It produced a bounce pattern but did so very inefficiently. If you want to have a hack at simplifying my design, rather than starting over from scratch, and don't have a good switch bounce generator of your own, please use the switch bounce generator I posted in the topic I previously linked: "... my comments on simulating switches [here]."
ozcar:
--- Quote from: Zero999 on August 12, 2020, 09:48:50 pm ---No, I've not experienced switch bounce, when the contacts open. I suppose it's a possibility. There's no need for two 555 timers, just put a capacitor (try 100nF) across the switch. It will also help to close the contacts, by providing a surge to make the contacts wet.
--- End quote ---
With the circuit as it was originally proposed, it would not trigger on switch release until the resistor across the capacitor discharged it. But I've no idea how it evolved - one of the first suggestions was to rip that resistor out.
Pizzashape23:
Yeah, one of the first design changes I made was to change the 4.7k to 10k, remove the 100k across the capacitor and pull up the switch to 12v through a 10k resistor. Thanks to Ian.M we are well past that design, making it simpler would be nice but his circuit contains a lot of safety and stability features that I think are worth the additional complexity. My original design probably would have worked after making the change mentioned above, but at the moment I am running with Ian.M's design.
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