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555 monostable sensitivity? Answered

Has anyone built a 555 monostable circuit that was extremely sensitive for no apparent reason?
I'm trying to use one in a rube-goldburg machine, and for some reason it's extremely sensitive to any sort of touch! The circuit sometimes comes on for no apparent reason, other times it will stay on for longer than the expected duration after the button is pushed. I'm using non-CMOS 555 on about 5 volts to run a small LED. The capacitor I used was a 0.01 uF on pin 1 to 5, and a 1 uF electrolytic on pin 1 to 6. I used a 1M resistor on pin 7 to 8. 
I'm simply baffled by it. I've tried using different breadboards, different PSUs, brand new components, and correct gauge wire, but despite everything being seemingly fine, it's still shows the sensitivity. Anyone have any advice?

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iceng

5 years ago

See the ground symbol for the touch switch
When only pin 2 reacts you become the ground
through body antenna to surrounding fields.

If R1 is increased >> just a wave will trip the IC..

A

555 timer touch switch .png
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tylervitalesteveastrouk

Answer 5 years ago

You mean just connect pin 1 to electrical ground? (the green wire in wall sockets)
That actually makes a lot of sense, because I noticed a huge difference when I held onto the negative lead.

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frollardtylervitale

Answer 5 years ago

With a 1M resistor in your design it will take VERY little to trigger -- even waving a hand overhead will be enough to cause it to fire. Having it with a stable ground means stray signals go where they're supposed to.

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tylervitalefrollard

Answer 5 years ago

So, I grounded it, and it did sorta help. It seems to function OK, but it does still sometimes trigger for no reason, and it also sometimes stays on for multiple cycles. (longer than expected)

What you said about the resistor intrigues me though...
Are you saying that a different resistor value could change the circuit to be less sensitive? If so, which way should I go? Higher or lower?

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steveastrouktylervitale

Answer 5 years ago

If its noisy, make sure that you've added capacitors across the part, to stop inteference

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frollardtylervitale

Answer 5 years ago

A 555 basically compares the charge/discharge rate of a capacitor to another value -- when they intersect it flip/flops. Having extreme values in there can make for funny behaviour.

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steveastrouktylervitale

Answer 5 years ago

The problem is the input resistance of the circuitry which partially set by the 1Meg resistor. Grounding should help set the circuits idea of zero volts.