# Thread: Help with Constant to Momentary circuit (for power switch) please.

1. ## Help with Constant to Momentary circuit (for power switch) please.

I'm trying to build a circuit to turn on my laptop when the car is on. I got the diagram from here.

This is how I built it using components from Radioshack.

The problem is it doesn't work consistently at all. Like it works for a couple times then stops working (no pulse sent to laptop). Did I do anything wrong?

(The red wire from the wire to the power switch is connected to 30, btw).

Thanks.

2. The problem could be the duration of the pulse.
Try using this formular and subtitude component if needed.
Time (seconds) = Resistance (ohm) X Capacitance (farad)
I am not 100% sure if that formular work for your case, but you can try.

3. The the12volt.com site says that circuit will give about 1/2 second of continuity between 30 and 87. Is that correct according to your formula?

So if I want to get 1 full second, I need a bigger capacitor, right? How about 2200 &#181;F?

4. perhaps reducing the resistance would also increase time and allow you to keep the circuit as small as possible.

5. Actually, taking a second look, the resistor in the diagram is in parallel with the capacitor so it wouldn't matter at all, would it?

The job of the 10K Ohm resistor is just to bleed off the charge in the capacitor when power is removed so that next time, when power is applied, there will be current through the capacitor again (until it's charged up).

Am I correct?

6. Yeah, i think you are right. I'm a bit confused though. Due to the presence of the resistor, couldn't current always flow through this thing? I'm no electronics expert as you can see.

7. The resistor is so big that there's no current through it. (Otherwise, the power switch would always be shorted despite the capacitor there).

8. *delete*

9. Originally Posted by b8bboi
Actually, taking a second look, the resistor in the diagram is in parallel with the capacitor so it wouldn't matter at all, would it?

The job of the 10K Ohm resistor is just to bleed off the charge in the capacitor when power is removed so that next time, when power is applied, there will be current through the capacitor again (until it's charged up).

Am I correct?
Actually not. The resister is there to delay the discharge of the capacitor. The higher the resister, the slower it will discharge, in essence, keeping the relay energized. The other problem with increasing the value of the capactiro or resister, to increase the relay stays energized, is that it will take longer for the capacitor to charge. This may not be an issue in your setup, like if an extra second before the computer starts doesn't make a difference, then that would be the way to do it. It would probablly be best to increase both slightly.

Michael

10. and buy a soldering iron

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