I am trying to make a small 12V circuit. I need something that will have a switched on/off output that switches quickly. Like a fuel injector pulse. Maybe not so quickly but along those lines. I was told a LM555 timer would be good for this but I dont really know what resistors and capacitors to use to make it work. I googled a calculator for it many times and I get LED calculators or circuit diagrams but nothing I really 100% understood. I can solder and everything I just dont know what components to use to make it work. Can anyone help?? Any ideas on how to make the timer work or any other ideas to make this timer circuit?

[This message has been edited by turboguy327 (edited 09-09-2008).]

What are you trying to do? a transistor is a very fast switch. A 555 timer can pulse very fast like a pwm signal from a sensor or ecm.

What are you switching, how much power does it draw, and how fast do you need it to switch?

I am basically trying to make a PWM. The electronics guy at my work told me to use a LM555 timer to do that. I am trying to make a fuel injector fire in pulses but not in time with anything. Just for bench testing and that.

I wonder how many amps those draw. Are you firing one or several? Measure resistance across one.

Then take 12 (system volts) / Ohms = amps

Then you need to make sure your transistor can handle the amps you are trying to switch.

Is there any particular speed you want to switch these? To bench test injectors you dont actually need pulses, just wire 12v to it and check the spray pattern.

Your best bet since you dont have a set speed would be to use a pot in place of R1 in the 555 timer circuit, then you can adjust the firing speed.

12.6 ohms so its just about 1 amp.

Well your most common NPN transistor 2N2222, can only handle aboue 800 ma, you might be able to squeak by, but it would be getting pretty hot. You should get something more like a 2N3055, its a larger transistor, looks like a button cell battery sort of. They're only $2 at any radioshack.

They will handle up to 15 amps and disapate the heat much better. And they can switch at up to 2.5mhz, so they will be more than fast enough.

I will go check out radio shack later tonight. Id rather be safe than sorry. I asked some of the guys there and they only know where stuff is on the shelves and thats about it.

Yea radioshack should be called cellphone shack. They dont know the first thing about anything other than selling cell phones, used to be half the store was electronic componets, now its like one rack.

quote Originally posted by 86GT3.4DOHC: Yea radioshack should be called cellphone shack. They dont know the first thing about anything other than selling cell phones, used to be half the store was electronic componets, now its like one rack.

Sad, isn't it.

worked great. Plusses for all who helped me. Now does anybody know how I can make headlights on a ford stay on for like 30 seconds after the key is shut off or the headlight switch is shut off?? My dodge truck does it and my gf wants me to make her car do the same thing. I would like to do it to my winter beater as well. Any ideas??

quote Originally posted by turboguy327: I am trying to make a small 12V circuit. I need something that will have a switched on/off output that switches quickly. Like a fuel injector pulse. Maybe not so quickly but along those lines. I was told a LM555 timer would be good for this but I dont really know what resistors and capacitors to use to make it work. I googled a calculator for it many times and I get LED calculators or circuit diagrams but nothing I really 100% understood. I can solder and everything I just dont know what components to use to make it work. Can anyone help?? Any ideas on how to make the timer work or any other ideas to make this timer circuit?

You need a adjustable "mark to space generator"
there is one on the net using a LM555 and 4 FET's

quote Originally posted by turboguy327: worked great. Plusses for all who helped me. Now does anybody know how I can make headlights on a ford stay on for like 30 seconds after the key is shut off or the headlight switch is shut off?? My dodge truck does it and my gf wants me to make her car do the same thing. I would like to do it to my winter beater as well. Any ideas??

Go to the dodge dealer & tell them you want the timer that does that. ;-)