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Old 5th February 2009, 09:13 PM   #1
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Default 555 Timer Trigger Input?

I am figuring out how to activate the 555 timer trigger in an automotive enrironment (12volt).

Does the ground to the trigger of the timer turn it on?

What I am wanting to do is turn a short pulse (ground connection from a remote module in my car. when i press "lock" twice on the remote, it makes a very brief ground connection for the horn) into a longer durration signal. I Get the feeling that i can use that short ground connection pulse to turn on the 555 timer. Am I correct?

Thank you for your time!
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Old 6th February 2009, 06:33 AM   #2
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Hello silentblackhat,

It sounds like you are looking for a "one shot" configuration for the 555 timer. Look at this page for details:


Using the circuit on the far left under the one shot section, if you pull pin 2 low, that triggers one pulse out of the 555, which will last for 1.1*R*C seconds.
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Old 7th February 2009, 01:11 AM   #3
CBS240 is offline CBS240  United States
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Here is a free program you can download that is a 555 timer calculator. Could be useful.
All the trouble I've ever been in started out as fun......
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Old 8th February 2009, 07:22 PM   #4
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Here is the schematic I drew up for the timer. Does the variable resistor need to be tied to ground? or can one end of the full resistance and the wiper just be used(as shown in this schematic). One other thing is, is 100uF too big for the discharge pin? I was thinking that the sudden short to ground that the discharge pin makes might burn out that tiny transistor inside.

Also, does everything else look ok?

Click the image to open in full size.
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