Small circuit design help?

[snwtoy]

Hi folks

Can anyone help design a circuit to the following specs:

Output when 'off' = 12V
Output when 'on' = variable between 0-12V - controlled by some sort of variable switch which I will mount on/under my dash.

Input = 12V, ground, switching 12V/0V to control on/off state.

This is to act as a dimmer for my pillar guages which are far too bright at night, but just right during the day

I'm sure it's simple, but I don't know the names/values of the parts I'll require

Also, what'd be the tidiest way of packaging this together? I'm thinking something like a charge pill pottle and hot glue - nice and small and easy to hide behind the dash.

[pervert]

I'm pretty sure a switch-style variable resister will be what you are after...available anywhere like Dick Smith for a small sum, you could probably get a style that will even mount drilled into the dash.

[fangsport]

many cars come out with a dimmer for the dash backlighting, i believe this would work.

[snwtoy]

This is what I want to do - but where do I get the powered switch (use a small relay? would a transistor do the job?), and the variable resistor(do dicksmith have anything which will look ok mounted in the dash?)? Do I need to include any diodes there?

[Bazda]

You will prob need a relay switch.
easiest way is to go to jaycar they got heaps of stuff like that.
if u want to go hard out use a darlington (sensors light) and dims your gauge to how dark it is, which would be better cos soemtime su use ur lights when its only a little dark and u can even notice ur dash board lights

[Perky]

Snwtoy,
I hope you get a response from someone who knows what they're talking about. Unfortunately that's not me. I think that just adding resistance in series will work but your diagram is not quite correct. You need to connect either the wire going into the pot, or the wire coming out, to the wiper of the pot (the arrow) to wire it as a variable resistor.

However:
1)Don't do this unless you are sure that the wiring only powers the lights in the gauges -- otherwise the gauges will read funny.

2)Remember that the resistor will have to dissipate all the power that's not going to the gauge. I don't know how much that would be, but I'd guess that an ordinary potentiometer will probably not do. Maybe a more expensive wirewound pot would be enough -- I think Dick(head)Smith has ones rated at 5W. Otherwise you might be better off paralleling high power non-variable resistors.

I can't see why you'd need a relay unless you want it to be switched along with another circuit, such as the headlights. Darlingtons are not sensors but a type of transistor arrangement. You wouldn't want any diodes in this circuit.

A more elegant, and slightly more complicated circuit might use a voltage regulator chip.

[sergei]

Go for voltage regulator chip (~5A 12V with V-reference), If I have time I will post a diagramm

[sergei]

http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm#dimmer.gif

you can use the swith to bypass the cricuit so it will give 12v when off

or there is much simpler version of same stuff
http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm#dimmer2.gif

[sopachrga]

Perky, you are basically right, just buy a Pot from Dick Smith for a few dollars, and put it in-line with you light feed.

[Bazda]

nice 555 timers

[sergei]

P.S. Make sure all power transistors in use have good heatsink (at least 50mm x 50mm x 5mm aluminium plate)

[Alex B]

mm dumbass question buy why would you use a 555 timer not sayin it wrong but why?

[sergei]

pot for 5 bucks will burn out pretty quickly driving even one 2w light, you can have switchable input using a relay... and dimmer using some sort of semiconductor control, otherwise the cricuit will generate too much heat (and even smoke)

as for dimmer here is other way of doing it
http://www.cpemma.co.uk/reg.html

You can use anything similar to 317 (most 3 pin regualtors) but no less then 5A (wich is 317 not)
You don't need the capacitors as you are not driving any motor device...

[sergei]

it drives the lights similar way as ECU drives the injectors, DUTY CYCLE, 100% duty cycle gives full bright, 50% gives half (not quite), close to 0% its off, adjust the duty cycle and you can adjust the brightness, without dirving high currents thru pot, or having transistor in inefficient mode (half bright), this way transistor either on or off, while on it has very low resistance hence low voltage drop, less heat, and when its off obviously there is no current going trhu it at all.
This way the efficiency is somewhere close to 90% in any position, while if you have just a pot controlling transistor, if you have really dim, say 5%, the efficiency will be less then 5%, and so on if you increase the brightness eficiency will increse to an almost 90% at full bright...

[snwtoy]

Wow, thanks for all the help sergei - I think I'm more inclined to just go out and purchase a pot, rather than building one. How does a factory dimmer work without all the heatsinks you mention?

If I have a pre-assembled dimmer/pot - what should the circuit look like? Am I right in thinking i can use a transistor for the switch I have pointed to in purple?

This has turned out to be more complex than I orignally thought?

[Stealer Of Souls]

The factory resistors are usually reasonable big suckers.
The best method is the latest one Sergei put up.
You can even go one better by buying a variable regulator chip. I haven't looked at exactly how they work but I'm lead to believe all you need is one resistor then. The capacitors aren't that important in a car since it's fairly well regulated DC. But you would be wise to put them in to reduce ignition noise...

I think you'd better find out how much current the guages draw before you do anything.
If it's small, less than oneish amps(EL backlight, or LED lit), then you could just use a pot, if it's much larger then you're best off with a regulator circuit.

[sergei]

if u gonna use pot use small resistance, as big as possible, I've checked the wiring diagrams and in some cars they use transistor control, but other they use just a pot..
Use a wire wound pot (they can handle 3W)
http://www.dse.co.nz/cgi-bin/dse.storefront/42923bbd0eeb3fc8273fc0a87f990754/Product/View/R6909

the choice pf resistance depends on the amount of lights it is going to control, but you can go as low as 25-50Ohm... connect it by soldering middle pin to one of the side pins together and connect the two soldered pins to power source and other pin to lights (or oposite, does not matter), as for switchin on and off use a relay.. if you want I will post a diagramm...

[TrouserFxGt]

go and find a wrecked ford laser and get its instrument light pot and wire that in series with your gauge lights. easy.

[sergei]

I agree easiest one will be to go to "PickaPart" and get the real thing off a wreck...

[spencer]

You will prob need a relay switch.
easiest way is to go to jaycar they got heaps of stuff like that.
if u want to go hard out use a darlington (sensors light) and dims your gauge to how dark it is, which would be better cos soemtime su use ur lights when its only a little dark and u can even notice ur dash board lights

I love it, in a darlington pair The first transistor’s emitter feeds into the second transistor’s base to give big gains of weak signals not much use here. Maybe you meant a photodiode? anyway I agree with goin to a wreckers save alot of hassle