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555 delay circuit help

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Smegory

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Hi, kinda new here but keen to learn!

Im building a little project that requires a momentary switch to be pushed by my finger, then after a delay of about 1.5 seconds it is automatically momentarily closed electronically and then resets itself until I push it again.

Ive been reading a fair bit about the 555 timer - I think monostable is what im looking for but I cant work out how to make it delay first. Everything ive read turns it on as soon as it is pushed.

I think I need 2 555 chips - or a 556 - connected back to back so the first one goes low for 1.5 seconds. When this goes high it would trigger the second to go high instantly across the monetary switch then reset itself after .5 of a second.

I just want to know if im on the right track and have my highs and lows around the correct way. Also, how do the chips reset? Or maybe there is a more simple way to do this?

Thanks heaps for any help.
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi, kinda new here but keen to learn!

Im building a little project that requires a momentary switch to be pushed by my finger, then after a delay of about 1.5 seconds it is automatically momentarily closed electronically and then resets itself until I push it again.
hi,
Its the 'pushed by my finger bit thats causing the problem',,, no joke!

You mean you want the 1.5sec time to start when you release the push button or do you mean, you must press the push button for 1.5sec for it to auto set.???
 
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Smegory

New Member
Ha - ok, perhaps i should have been more specific!

When I RELEASE the momentary switch ( which will only be tapped breifly for a second - not held down) I would like it to reactivate or short out the same switch 1.5 seconds later.
 

MikeMl

Well-Known Member
Most Helpful Member
What is the operating voltage?
What is the load?
Load Current?
Active high or active low?
 

Smegory

New Member
It will operate on 3 volts - 2 1.5V batteries. The load is tiny, very minimal as is the load current.

Not sure what you mean by active high or low, but the end result is the switch needs to be high when closed to activate it. If you could clarify that a bit more I could work it out and go into more detail?
 

MikeMl

Well-Known Member
Most Helpful Member
A TLC555 (CMOS) chip will work on 3V. The Bipolar LM or NE555 will not operate at <=3V.

Active high or low has to with how you connect the load. Normally, while triggered (while it is timing) the output of the 555 is high. Since you want your load to be turned on after the timing period, then it would have to be wired between pin 3 and +3V. Is that ok?

To paraphrase your requirement: The load is ON initially. You tap a button. The load turns OFF, and then turns back on 1.5sec later. The load is always ON as long at the battery is connected; except for the 1.5sec after the button is released.
 
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Smegory

New Member
Active high or low has to with how you connect the load. Normally, while triggered (while it is timing) the output of the 555 is high. Since you want your load to be turned on after the timing period, then it would have to be wired between pin 3 and +3V. Is that ok?
This makes sense and seems correct to me, but from what I have read I think i need the first 555 to go low for 1.5 seconds then go high to trigger the second 555 , which will then go high for a slit second turning on my load for a split second. This theory of mine could be completely wrong tho....Actually, when I re-read this, I'm not sure why I need the second 555. I think it was because I didnt know how to make them reset themselves after going high. But I dont know how this will do it either...kinda confused now.

To paraphrase your requirement: The load is ON initially. You tap a button. The load turns OFF, and then turns back on 1.5sec later. The load is always ON as long at the battery is connected; except for the 1.5sec after the button is released.
However this isnt quite right.

Initially the load is OFF. I momentarily press the button - the load goes ON then OFF when I release the button. 1.5 seconds later the timer momentarily turns the load back ON then OFF and resets itself until i push the button again. Each time the load is turned on its only for about 0.5 second.

Im sorry for any confusion and I hope that clears it up :)
 

MikeMl

Well-Known Member
Most Helpful Member
Stare at this:
 

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Smegory

New Member
That looks awesome - thanks heaps!

But - I may be wrong about this - I think the red V(load) graph is actually completely the opposite of what I would like. My interpretation of the graph is the load is on and goes OFF when the switch is activated.

I want the load to be off initially, on for a microsecond as I push and release the momentary switch, off for 1.5 seconds, then on for a microsecond as the timer circuit turns on the load then it should reset until I push it again.

I really appreciate your help tho - and is that a 172 in your profile? im into avionics.....
 
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MikeMl

Well-Known Member
Most Helpful Member
Here is a modified circuit. Note how "Your Load" is connected. I asked you about an active high vs active low output. By connecting the positive end of you load directly to the positive pole of the battery, and driving the other end low, your load is only on for the short duration shown.

It is a 182. 3400 hours in 182s and counting... Going flying today.
 

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