Lots of these on the aftermarket which dim out the light after a set period. But any which fade it *on* as well - like modern BMWs and others? Or even a circuit of one? Lots of circuits for dimming out ones found on Google, but not any for both on and off.
WTF for?
Just wire a capacitor in line with the +12V. That way you'll get both fade in and out. You can set the time it takes using a variable resistor. I think I'd be wanting at least a few thousand uF for the capacitance and a 10k Ohm variable resistor.
Hardly rocket science.
Except of course that it wouldn't work.
Quite.
Let's see if I remember my electronics, it's a long time ago, but...
You'd want the capacitor to be wired across the bulb, e.g. from the +12V to ground. That way the cap would initially act as a short circuit, leaving 0V across the bulb, and as it charged the voltage across it would increase until it was at +12 when fully charged. The bulb would stay off at first and light as it charged up.
The delay then becomes the crucial bit. this would depend on the size of the capacitor and on the resistance in the circuit - since you need the bulb to be on full when the cap is fully charged I'd have the resistor in series with the cap between the supply lines, and the pair of them across the bulb.
You'd definitely have to play with the values to get a sensible delay. And what it would do when the soft-off circuit kicked in I have no idea...
Words fail me.
But you don't know about that either.
But the resistor would be in series with the bulb & stop it coming on at full. Or if the delay was more tham microseconds at all.
WTF not?
That would be the 'since you need the bulb to be on full when the cap is fully charged I'd have the resistor in series with the cap between the supply lines, and the pair of them across the bulb.' bit then...
I'd do ASCII art but it *always* goes wrong.
Basically imagine C+R in series, then take the pair and put them in parallel with the bulb.
The time constant of a CR circuit is t = C x R.
Lets set the fade in period at 2 seconds and the bulb at 18 watts. That makes the capacitor needed about 0.1 *Farad* - to bring the capacitor to approx 60% of the supply voltage. And that just covers - badly - the fade in and out. If you want the capacitor to be large enough to keep the bulb on for, say, 20 secs you're into a whole new ball game. The lamp would also never reach its full output.
Of course obviously you can use your CR timing circuit to drive a high impedance device - transistor etc - to switch the load to avoid using massive values but the snag is a CR circuit isn't linear in its operation or between charge and discharge.
In which case there'll always be 12V acroos the bulb, wether the capacitors charged or not. You're going to need an active device.
To look cool when you go in and out of tunnels.
It is if you don't know the first thin about electronics.
It's a switched circuit - so you start with the cap discharged and 0 volts anywhere. Then when you switch it on the bulb and discharged cap have about the same resisitance - i.e. 0 but as the bulb comes on it heats up and the resistance goes up rapidly. At that point the cap is the easiest route so the majority of the current goes thattaway, leaving less for the bulb, so it starts up slowly. As the cap charges, the path through the bulb gets more equal and when the cap is fully charged, there is 12V across the cap *and* 12V across the bulb.
I hate dynamic analogue non-linear systems, though. Much easier to use an active device, as you say.
For the OP - you could try this one:
Only if you limit the current. A resistor in series with the cap limits the current through the cap. It would have to be inseries with the bulb as well to limit the current through the bulb.
So basically you have 12V across the bulb and a variable voltage on the capacitor? To vary the voltage on the bulb you need the resistor in series with the bulb and the cap in parallel with the bulb but then you won't get full voltage on the bulb.
I'd stick to posting stuff that you actually have the slightest bit of knowledge about Conor. Not of course that it's stopped you before.
John
I'll stick with what I know from my BTEC Electronics Engineering. Your expert opinion is based on what knowledge?
I assumed the OP wanted one that did both.
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