fading car dome light

[lazynok]

that last one looks way too confusing. damn i need money so i can buy parts and start trying myself.. soon... soon...

 

[Phasor]

that last one looks way too confusing

It's not, really! It's just that the symbol for a mosfet looks a bit daunting, if you've never used one... :lol:

 

[lazynok]

heh.. i don't even know what a mosfet is

 

[kinjalgp]

MOSFET is one the member of electro-tech-online :lol:

Well, Jokes apart. Read this tutorial:
https://www.americanmicrosemi.com/tutorials/mosfets.htm

 

[lazynok]

i looked on my friends panasonic cd/mp3 stereo in his car and it sed MOSFET on the top of it.. weird... never seen that word in my life.,.. then twice in one weekend

 

[Phasor]

kinjal - that tutorial looks very technical, for someone who has never even heard the word "mosfet".

If I may explain, very simply:

A mosfet allows current between the 'drain' and 'source' terminals. The amount of current allowed to flow depends on the voltage between the 'source' and 'gate' terminals.

For an "N-channel" mosfet (more common), an increasing gate voltage increases drain current. For a "P-channel" mosfet (less common), a decreasing gate voltage increases drain current.

 

[kinjalgp]

Ok before reading the MOSFET tutorial, read this JFET tutorial to get the basic idea of these voltage controlled unipolar devices.

https://www.americanmicrosemi.com/tutorials/junctionfet.htm

 

[lazynok]

after learning about mosfets... it seams that that when the switch isn't triggered... it will stay on? hmmm...

 

[Phasor]

after learning about mosfets... it seams that that when the switch isn't triggered... it will stay on? hmmm...

Explanation:
When the switch is closed, 12V is directly applied to the gate, the light turns full on, and the capacitor charges via R3. When the switch opens, the charge on the capacitor keeps the mosfet turned on, and it slowly discharges via R3 and R1. As it discharges, the gate voltage drops slowly, so the mosfet turns off slowly, and the light fades out.

 

[Roff]

after learning about mosfets... it seams that that when the switch isn't triggered... it will stay on? hmmm...

Explanation:
When the switch is closed, 12V is directly applied to the gate, the light turns full on, and the capacitor charges via R3. When the switch opens, the charge on the capacitor keeps the mosfet turned on, and it slowly discharges via R3 and R1. As it discharges, the gate voltage drops slowly, so the mosfet turns off slowly, and the light fades out.

Right on, Phasor. I might add that the value of R3 controls the turn off delay. The value shown allows the lamp to start turning off pretty much instantaneously. If you make R3=0, there is about a 5 second delay before it starts to turn off.
Ron

 

[lazynok]

after seeing what everyone is saying about the mosfet diagram. i think i shall try that one. question.

what is the R3 1meg? i'm guessing its a resistor. on the packaging of the resistor it would say 1meg? never seen one of those...

and what are the D and G for?

 

[kinjalgp]

1meg = 1 MEGA OHM = 1 x 10^6 Ohms

D = Drain Pin of MOSFET (analogous to collector of transistor)
G = Gate Pin of MOSFET (analogous to base of transistor)

 

[Roff]

1meg = 1 MEGA OHM = 1 x 10E6 Ohms

D = Drain Pin of MOSFET (analogous to collector of transistor)
G = Gate Pin of MOSFET (analogous to base of transistor)

Actually, 1*10e6 ohms is 10 Megohms. 1 Megohm is 1e6 ohms. The "e" means "times 10 to the power of". For example, 2e3 is 2000, and 3.6e-2 is 0.036.

Ron

 

[kinjalgp]

ooops that was a typo. Now its ok.

 

[Relic1]

ummm, just one problem guys.... the interior lamps in a car are actually ground active.
ie the door pin/switch is grounded when the door is open and the other side of the bulb i connected to battery. Close the door and it opens the circuit.
the circuits that have been posted up are for a positive switched switch. It's possible to do this to a car but it means reconstructing the door and the paneling of the door jam. Or converting to a positive grounding system.... either way not an easy project.

using the bulb as the resistor in a very simple RC circuit you can use a 1 farad cap across the line to dim the lights very nicely, but discharging that much power when you open the door will cause the switch to die fairly quickly.
I know there is a way to do it with a transistor, but my transistor manipulating abilities have gone seriously south since I’ve been working with software for the last 8 years.

 

[lazynok]

go hatch!