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# simple timer circuit

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Hi there

i need help with a new project of mine.

i need to design a timing circuit using a 555 timer. it requires that the timer gives a signal out for a specified time. once this time has elapsed, the signal must go to 0V.

all i really want to know is, how do i control the timing period of a 555 using voltage?

for example: i want to give a voltage of 3V to the timer and i want the signal to stay on for 10 seconds. then, if i increase the voltage to say 5V, the signal must stay on for 20 seconds.(this is just an example).

it would be greatly appreciated.
thanx
mphotis

NE-555.pdf

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EnigmaOne,
mphotis, knows how to make a monostable 555, what he really wants to know is how to control the timing period of a 555 using voltage

Hi mphotis, try this circuit.

When the trigger goes low the output will go high until C1 charges to a level set by the Control Voltage.

C1 is an AC coupling or DC blocking capacitor it prevents the 555 from remaining on if the trigger pulse lasts for too long, this is optional you don't need it if you know the trigger pulse is only going to be short.

R1 & R2 are just pull up resistors they are optional too, but R2 is a must if you choose to include C1.

If you want posative edge triggering you can connect the trigger to 0V via a transistor, if you want both positive and negative edge triggering then just ask me.

The time delay depends on C2 and R3, and of course the control voltage, I've forgotten the formula but I will look if up for you if no one else knows it.

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My apologies.

I had intended to go with a Lab curiculum sheet that I had open on one desktop, and got myself mixed up when I was referencing the Fairchild DS file on another desktop.

At any rate, the attached schematic calls for the addition of a voltage divider (R4 and R5) to select the potential at Vcont (5). Multiple voltage dividers can be added and selected with a switch.

The formulas are fast-n-loose derivations from manufacturer's data (done while I was cooking dinner and taking care of a sick six year-old), so sanity-check things before you put anything on copper-clad or perf board. With 5 kids left in the house, it's a wonder I get anything done. :P

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