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# Very simple 2sec timing circuit help needed

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#### Meza

##### New Member
Hi there,

I have an very basic circuit consisting of a reed-switch, a 9v battery and an electromagnet. Told you it was basic

This circuit is the contents of my £40 electric cat door! I was a bit gutted when I took it appart to find it so basic, but that makes the next job easier I s'pose.

The problem is my cat doesn't hang its head around long enough for the electromagnet to unlock the door and thus can't get in.

I am looking for a circuit to add a 1 or 2 second delay in holding the electromagnet open. Can anyone let me know how to build something which would do the job? Thanks very much!

Meza

a simple 555 in monostable way is the best thing

*remembers back to school!*

What I want is a high output for two second after a brief trigger input, and it needs to reset itself each time... is that correct?

And will a 555 monostable give me that? The circuits I've found so far seem to be low for an X time, then go high and stay high.

Perfect!

go to the one that says
LM555 Monostable Oscillator Circuit Diagram
you need an ative low trigger

ahh good stuff thanks

Hi Meza,

I'm thinking that your cat has a collar with a magnet?
and that this operates a reed-switch?
and that the reed-switch operates some kind of latch?
Hopefully to let only your cat through the cat-flap.

You said it is simple,
and you want something simple to try to hold the latch
a bit longer, cos your cat doesn't come straight in.

How about trying an electrolytic across the latch-coil?

Maybe 100 MFD just to see how long it will hold the
latch.
If you find that it wants more than 5000 MFD, then
maybe this method might not be good for the reed switch.
Because it would take quite a surge, but then as its a
battery, maybe the reed switch will be ok.

See how it goes, let us know either way,
Cheers, John

John, you got it

I will give it a go, thanks for the suggestion. Will the fraction of a second the reed is closed for at the moment be enough to charge the cap do you think?

Hi Meza,

well, cos its a battery i dont think it would damage
the reed switch,
As to the short time the reed switch is closed,
it wont be so short as all that,
even a brief blip would do.
Let us know how it works out,

Regards, John

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