## My laptop power on circuit

Disclaimer: I am not engineer. Use at your own risk.

Got the car mount for the docking station just about done. So I need to mod the power button to press when the key is in the ignition. Here's the circuit I'm using.

The 4n25 is an optoisolator. Basically an led and a phototransistor in one package. They are cheap. Should be able to get one for under 50 cents. There are 6 pins. We shall ignore pin 3 and 6.

Here's how it works. The cap charges through R1. While it's charging, current flows through the led inside the 4n25 which let's current flow on the transistor side. Effectively pressing the switch. Once the cap charges fully, it acts as an open circuit so current no longer flows, "unpressing" the button.

R2 is a high ohm resistor. >20,000Kohm. This lets the cap discharge after power is removed. Otherwise, it would be awhile before it could "press" again.

Basically, by varying R1 and C, you can decide how long it will "press" for.
The formula is:
R(ohms) * C(farads) = seconds till ~63% charged (This is the RC constant, or so I'm told )

The one thing to be careful of is that the 4n25 can't take much current without burning out. Around 5ma. (You can go higher for short periods.)

Which leads me to R3. R3 is optional. As I kept playing with different values for R1 and C, I once let too much current flow through the isolator and broke one. So R3 is going to limit current to about 5.4ma to protect the 4n25.

You don't need it if your values for R1 and C don't let too much current flow.

From my pile of parts, I used the following.
4n25
EDIT
The following is WRONG, WRONG, WRONG
C != 3300 mfd cap. (If you use a polarized cap, make sure it's in the right way!)
R1 != 510 ohms

R2 = 27000 ohms
R3 = 2200 ohms

I used an led as the test load circuit. The power button needs much less power. So when hooked up, it would press too long and keep rebooting.

I ended up using
C = 5
R1=10000
and it worked fine. BUT, when in the car, it doesn't not. The car is actually around 13.5 volts so it's too quick. I'm going to rebuild today with R1 as a pot so I can tune it.