Disclaimer: I am not engineer. Use at your own risk.
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 it reaches the input voltage, the cap acts as an open circuit, and stops "pressing".
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 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
C = 5 mfd
R1 = 10000 ohms
R2 = 27000 ohms
R3 = 2200 ohms
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