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Automotive timer ignition disable help needed.

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Imagewerx

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Using this basic 555 timer circuit in my car to give a 10 second or so pulse.......

**broken link removed**

It's triggered by a short positive pulse but has to output a much longer negative pulse,which I can do.
What I also need which I can't figure out is to disable it when the car's ignition is turned on.I can quite easily open circuit the supply to the timer with a relay,but I really want to keep it all solid state if possible.
So is there anywhere in the above circuit I can apply + 12 volts to that will inhibit it from working?

Chris.
 
hi Chris,
You need a pull up resistor on pin 2 to +V.
You could use a transistor circuit on the 555 's RESET pin 4 to +V and pull it low with the extra transistor.

Do you follow OK.?

E

Rough Sketch:D

Drawing CORRECTED!
 

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Hi Eric,
I thought I understood the concept of pull up and pull down resistors,but last time I tried it there was a burning smell and then nothing worked after wards.

So do I need to use all the components you've added to my circuit diagram?

Chris.
 
Hi Eric,
I thought I understood the concept of pull up and pull down resistors,but last time I tried it there was a burning smell and then nothing worked after wards.

So do I need to use all the components you've added to my circuit diagram?

Chris.

Hi,

I have corrected a small error in my previously posted drawing.

Using pull ups of the correct resistive value should not cause smoke.

You do need the components I have added, you had pin 2 floating in your circuit.

E
EDIT:
This simulation shows the action of the 555 using the Reset pin.

If you pull and Hold the R6 input High the 555 will not respond to the +ve trig, also if the the 555 output High when R6 is pulled High the 555 output will go low.

Note the decoupling capacitors on the 12V rail.
Eric
 

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Ok I think I understand.S1 is the short positive trigger pulse,S2 is the ignition disable input and "1" is my timed positive pulse output?
 
Ok I've redrawn it in a format my brain understands (I need to see the earth connection actually as a line for it to make sense in my head),is this all correct please?

**broken link removed**
hi,
If the 'Ign Sense In' is say being pulled up to +12V when the Ignition switch is ON, you could connect Pin #4 of the 555 via a 4k7 to the Ign Switch.

Give me few minutes, it will be easier to edit your drawing..

Eric
 
Hi,
Look at this option, please ask if you dont follow.

The 555 is Reset while the Ignition is ON, so the +ve trigger will have no effect.

The 'Extended pulse output will be o/c [ ie: NOT pulled to 0V while the 555 is Reset].


E
 

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Ok thanks,so like this then......

Lets know how it goes.
I did show a 100nF on pin#4, you may need that if the IGN connection is electrically noisy.
E
 
hi,
That should give you the control over the disabler.
Lets know how it goes
E
 
Just to let you know Eric,I got this working today on the bench,but it could only be triggered by a -ve pulse.I had to remove the 2k2 resistor so it could be triggered by a +ve pulse as I need it to,is it ok to do this?

Chris.
 
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