555 timer help

[just-a-learner]

Hi all,

I have found the timer circuit below, and i want to change it so that the timer cycle does not start until i release the switch. I am totally stumped. . . Can't think of a way to do this.

I have changed the 22uF cap for a 100uF cap to alter the delay.

please, please help

 

[audioguru]

The 555 timer starts when pin 2 goes low. The 100k resistor holds it high and releasing the switch will cause the charge stored in the capacitor at the switch to go higher, so the 555 does not start.

Change the switch type to one that closes its contact when you release it, or add an inverter to the existing switch.

 

[just-a-learner]

I can't change the switch, because it's an existing switch on the equipment I'm adding this circuit to. How could I make an inverter? maybe use a transistor?
I understand what the components do, but I can't get my head round how to get them to do what i want. . .

 

[Hero999]

If it's just a normally open switch then a transistor inverter is the best solution.

 

[just-a-learner]

Just a NPN transistor then? I thought about that, but wasn't sure if it would work. . . i thought that was just the same as the switch. . . doesn't it just make the circuit when the switch is pressed?
I'm probably wrong, as the name suggests I'm just learning!

 

[audioguru]

The closed switch shorts the base-emitter of an NPN transistor which turns it off. When the switch is pressed and its contact opens then a resistor from the base to the positive supply turns on the transistor causing its collector to go low and it triggers the timer.

Note: The 555 timer needs the capacitor at its pin 2 because it will never finish the time period with its pin 2 still low.

 

[just-a-learner]

Thanks a lot, that's great.

I thin I understand better how it works now. You've both beena really great help.

 

[Hero999]

Just a NPN transistor then? I thought about that, but wasn't sure if it would work. . . i thought that was just the same as the switch. . .

I'd made a mistake, I'd got the switch and resistor the wrong way round! I've corrected it for you.

 

[just-a-learner]

That's great. Thanks again. Maybe if I bang my head against the desk hard enough, I'b be able to work this out myself.