555 timer circuit help needed

[jonjonr6]

I need help with a 555 timer circuit design.
I'll try to make this simple.
I have: 12v car ingition input
I need: a micro-relay to close a circuit (laptop power button), triggered by the ignition input, duration of trigger needs to be about 2 seconds.<p>Keep in mind, the car ignition is on till the key is off, but the trigger only needs to be 2 seconds.
I also need a bypass circuit, which I think I can handle. When the device (laptop) is on, the serial output will open a micro-relay to prevent the above circuit from receiving the trigger. The laptop will turn off via Windows sleep timer.<p>If you can help me design the above circuit, give me a link to a website with a useful design, or offer ideas, I'd appreciate it.
Thanks,
Jon

[russlk]

You will find the schematic and charts for choosing the R and C at this URL: http://rocky.digikey.com/WebLib/Fairchi ... _SA555.pdf

[jonjonr6]

Thanks,
I'll read over it. I see it has a monostable diagram. If I understand correclty, the monostable operation is what I'd be using for my application.

[haklesup]

Check out this extensive tutorial
www.uoguelph.ca/~antoon/gadgets/555/555.html

[Ron H]

If you use a 555 monostable triggered by a negative-going pulse on pin 2, you'll need to invert the ignition switch output and run the output through a differentiator. You can use almost any small-signal NPN as the inverter (2N3904, 2N2222, etc.). Connect a 10k resistor from the ignition switch to the base of the transistor and a 1k from the base to ground. Connect the emitter to ground. Connect 10k from the collector to Vbatt (+12v). Connect 0.1uF from the collector to pin 2 of the 555. Connect 100k from pin 2 of the 555 to Vbatt. I think you'll have to power the 555 off the battery (non-switched) to get the circuit to work correctly.<p>Ron

[jonjonr6]

Thanks.

[bridgen]

Ron's suggestion will certainly work.
However, it's possible to dispense with the inverter. <p>Wire the 555 as a monostable. Connect pin 2 to the +supply via 100k, and connect 100uF between pin 2 and 0V (chassis). When power is applied pin 2 will be held low (and therefore act as a trigger) for a short time. The C charges up to +12 to 14V and the chip will not be tiggered again unitl power is disconnected then reconnected.

[jonjonr6]

Well, obviously there are several different ways to get the same results. Here's the circuit diagram I found and was planning to use (on the left). However, he has different values for caps and resistors, and he has a diode and another resistor in his design. Are they needed?<p>http://ourworld.compuserve.com/homepage ... 55mono.gif

[bridgen]

Hi Jon,
the page which that link takes me to has several circuits and it's not obvious which one you mean.

[jonjonr6]

Ooops, sorry, It was supposed to go straight to the right one. Its the one titled:
555 timer Mono stable (one shot) circuit<p>There are two diagrams. The one on the right is designed for the long input. However, I'm thinking that in my situation, the 2K resister connected to the + supply and the trigger in front of the cap is not needed. I believe it is there to bleed off the caps charge. In my situation, the ignition will supply the + bleed off on the trigger.
Sound about right?

[jonjonr6]

ok, I built it without the extra 2k resistor and diode. I used a 556 timer, and ran the output to the trigger of the second half of the 556. It seems to work just fine. However, I've got to get the cap/resistor combination on the first hreshold/disharge so that it does about one hour. I have a 4700uF cap and a 470k resistor at this point. I got only 10 minutes out of 4700uF and 100k. I'd be happy with 30 minutes if that's all I get.

[russlk]

A CD4013 dual D flip flop connected as divide by 4 will give 4 times the timing delay. Connect NOT-Q output to D input.

[bridgen]

Ok Jon,
Yes you are correct, you don't need the 2k or the diode if you are triggering automatically at switch-on as I suggested above. <p>The problem with the large values of C, which are necessary for long pulses, is leakage current. Unless you can find a large enough one with very low leakage the best route to take is that suggested by Russ.

[jonjonr6]

Thanks.<p>Actually, I have never worked with or looked at flip-flops. Honestly, I "learned" (term used very loosely here) 555/556 timers just for this one project. So far, I $30 into th 556 design, and almost have it all soldered down. I'm not sure what leakage current is. I'm guessing that it uses current from the power source slowly even when it's fully charged. As long as it's pretty low, like .05A, I'm fine, the car battery can handle it.
Thanks