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Old 04-08-2003, 08:56 PM   #26
jjunior887
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Damn Radio Shack, who closes a store at 8:30p anyway. I got there right at 8:30, I know because my clock said 8:31 when I got back in. Anyway the guy was out front and said "sorry we're closed what did you need?" I said "a couple of resistors" and he said"sorry man we're closed". What an ass, why did he care what I wanted if he was just gonna tell me they were closed again? Oh well I guess I'll have to try again tomorrow. I can't stand people sometimes. Sorry for the rant, I'll let you guys know how it goes tomorrow.-James
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Old 04-09-2003, 03:11 AM   #27
J-Spec Tuning
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Default Tried something out

I've been reading this thread and tried a little experiment. With a 13.6V power supply, I hook the power source to a potentiometer. I hooked the positive to one lead and then hooked a wire from the middle lead of the potentiometer. This wire is now my positive wire. My negative wire remains the same. Hooking this together -> accessories to pos and black to negative, I get power to the backlight. Turning the potentiometer increases or decreases the resistance depending on which way I turn. I experimented with three different potentiometers and found the 10 turn 0-50k Ohm one worked really well. I was able to adjust the turns until the light was basically spot on.

So you don't need a 50K ohm potentiometer (pot) but that was the only ones I could find. The cost of the pot is really dependant on the sensitivity of the pot. The 10 turn pot ($10) one was twice as expensive as the 1 turn pot. The 1 turn pot was very difficult to adjust to get the light spot on.

I know this may sound confusing but think of the pot as nothing more than an adjustable resistor. Put it in series between the positive lead power source and the accessories wire leading to the gauge. The negative lead of the power source is connected in the same fashion to the black wires leading to the gauge.
Cheers,
Thaison
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Old 04-09-2003, 08:09 AM   #28
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How many watt is the "pot" you have & can you provide a link...
Here is something I found at a place I used to get catalogs from...

http://www.partsexpress.com/pe/showd...D=117570&DID=7
http://www.partsexpress.com/
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Old 04-09-2003, 08:25 AM   #29
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I'm not promising anything, but I am working with a local circuit supplier on a design to isolate the dimming voltage coming from the factory dimmer used on the interior lights and convert it to be used with the Omori gauges. This circuit will convert the voltage difference between the variable light ground and 12 volts into a voltage referenced to ground. The circuit would be designed to power 4 Omori gauges (because that's how many I have).

I don't have an expectant date or if this is even going to happen. We are both doing this on our own time as a project and I told him that if we could package it in a nice, not too expensive package, he could probably sell a lot of them.

How many of you would be interested in something that you could just connect a constant Batt+ and ground, plus the variable ground for illumination and it would output a variable voltage output referenced to ground (with a trim so you could adjust it if your gauges are a little brighter than your dash lights)? We might also include some filters and connections so the power for the four gauges would go through this module as well and be sure it is filtered so no noise can affect the measurement.
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Old 04-09-2003, 10:07 AM   #30
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Quote:
Originally posted by 2000vfr800

How many of you would be interested in something that you could just connect a constant Batt+ and ground, plus the variable ground for illumination and it would output a variable voltage output referenced to ground (with a trim so you could adjust it if your gauges are a little brighter than your dash lights)?
I would be interested, for one!
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Old 04-09-2003, 12:38 PM   #31
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Very interesting stuff indeed! Here's just a quick question to make sure of something, though, and you can call me a total n00b since I know nothing about this stuff...

But have you guys been able to successfully dim the gauges, but STILL have them perform as normal? The reason I ask is because I recall somebody else doing something very similar to this, but even though they seemed to ONLY be messing with the lightbulb current, the gauges started acting really goofy, sluggish, and inaccurate. I could be completely making this up, but wanted to throw this out there in case somebody got really far along in this project and forgot to check the acuracy of the gauges after all this hard work was completed.

I'm sure they would be just fine, as the mech. Omori boost gauge I have can dim, I don't see why they would've totally changed the setup for the electronic gauges.
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Old 04-09-2003, 12:47 PM   #32
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The problem is that the dimming circuit adjusts the - side (can't really call it ground since it is being adjusted) up to the + side to dim the lights. So, you can do too things.

1. Connect the dimming wire to the + of the gauge illumination. This will allow your gauge to dim with the factory switch, but it will be opposite from your stock gauges. So, when you brighten your dash, your gauges will dim. When you dim the dash, the gauges will brighten.

2. Connect the dimming wire to the ground wire of the gauges. You would have to remove the ground from gauges to the car because the ground wires are connected together inside the car and if you didn't, you would short out the dimming dircuit. If you did this, your gauges would act funny since you wouldn't be getting the required voltage all the time and it would vary. This could really affect the accuracy of the gauges.
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Old 04-09-2003, 01:15 PM   #33
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I was toying with the idea of some voltage regulators LIKE THIS...
I would have to sit down & do a little caculating (math )...
For me to spend that kind of time on something I don't need isn't going to happen.
The fixxed option isnt that bad & if you picked up one of these ---> HERE it would work fine instead of the resistors/diodes...
unless there is something funny about these all you have to do is hook up the incoming positive to the center terminal & the out going to the light to either one of what's left...
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Old 04-09-2003, 01:26 PM   #34
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Except I want my gauges to dim with my dash lights and not with an additional external pot.

That is why I am trying to work on this circuit with the guy I work with.
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Old 04-09-2003, 02:13 PM   #35
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i've thought about this a bit, and i'm pretty sure i can come up with a simple circuit that does what we need it to do. i just have to think about it for a while--i'm a diy at heart.

the issue is that we need to sample the voltage differential from main b+ (the battery) to the "top" of the factory dimmer circuit, which varies from 0vdc to 11vdc. if we assume b+ is about 13vdc, that gives us a range of differential from 13v to 2v.

that is the voltage across the factory lighting circuit. it is NOT ground referenced--it is b+ referenced. that's the kicker.

we need to sample this voltage and create a GROUND referenced output.

right now i'm thinking a couple of op-amps could do the job, since they have differential, or "floating" inputs. we don't need voltage gain from them, just isolation.

i might have something workable by the weekend. my birthday's in two days, so then again, i might not!

ken
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Old 04-09-2003, 02:21 PM   #36
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Cool...maybe you'll get it done before me (see six posts up). You are doing exactly what we are... Some op-amps to invert the signal and reference it to ground with some power transistors for power... Maybe a filter or two as well...
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Old 04-09-2003, 04:40 PM   #37
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well since neither of my local radio shacks had the resistors or the diodes i was looking for I still have nothing. I may end up waiting to see what you come up with ride5000, maybe you can give us an idea of how to do it once you figure it out. -James
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Old 04-09-2003, 05:14 PM   #38
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Quote:
Originally posted by GLwagon
How many watt is the "pot" you have & can you provide a link...
Here is something I found at a place I used to get catalogs from...

http://www.partsexpress.com/pe/showd...D=117570&DID=7
http://www.partsexpress.com/
The wattage rating was very low on mine - only 2 watts @ 70 celcius. I walked away for dinner, forgot to turn it off and came back two hours later. No issues whatsoever. Now for people powering three meters, two watts may not be enough. The link that you provided is rated at more than enough watts to handle three gauges. Just place one in series with the three gauges and tap the middle lead for the positive terminal for all three gauges.
Thaison
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Old 04-10-2003, 11:02 PM   #39
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I went ahead and ordered the L-pad Glwagon suggested as well as the 4ohm resistor, they didn't have the diodes from what I could tell so I'm going to have to scratch that idea for the moment. I still wouldn't mind using the circuit you guys are talking about making though, it would be nice to dim the gauges with the dimmer switch. I'll keep you guys updated.-James
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Old 04-16-2003, 10:42 AM   #40
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I didn't know about this thread here... Someone asked a similar question on the Legacy Central board, so I threw together a circuit that dims my aftermarket boost gauge with my dash lights:

http://www.graphics.cornell.edu/~v/dimmer/
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Old 04-16-2003, 04:30 PM   #41
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Quote:
Originally posted by vrg3
I didn't know about this thread here... Someone asked a similar question on the Legacy Central board, so I threw together a circuit that dims my aftermarket boost gauge with my dash lights:

http://www.graphics.cornell.edu/~v/dimmer/
excellent job, vrg3. it looks exactly like what i had been effing around with in my head.

ken
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Old 04-16-2003, 04:42 PM   #42
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OK, I understood just enough of that to be able to still stutter out a "huh?" at the end. I understand enough to probably make one on the second try. How about someone put it in a nice little box with plugs for me.
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Old 04-16-2003, 04:58 PM   #43
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Thanks, Ken... Yeah, it's basically an op amp set up as a differential amplifier with unit gain, except the feedback is taken from the output of the transistor, which is set up as an emitter follower. The transistor's there because the op amp can't source enough current to drive light bulbs.

jehcpa - Does your car have the same kind of connector as mine for the ashtray light (or any other dimming light)? I'm referring to the one that mates with the 2-pin blue connector in the picture on the aforementioned web page. If it does, I could build you one; as a precedent, I sent one to Igyee4 on Legacy Central for $10. A box is kind of overkill; as you can see in the pictures, it's a really small circuit. I just dunked it in epoxy to insulate it.

But if you know how to solder, I'd just say to go over to Radio Shack, buy one 741, one TIP31, a 5-pack of 10k resistors (for a total of about $5), and put it together, using the close-up pictures as a guide. It's pretty easy. The trick is that I used pin 8 (which isn't connected to anything internally) of the 741 as a tie point for the emitter of the transistor. That way the transistor can just be soldered straight to pins 6, 7, and 8 of the IC. Then, the resistors and wires can be soldered straight to the IC pins, too.
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Old 04-16-2003, 05:05 PM   #44
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Yes, the WRX dimmer controls through what would commonly be thought of as the "ground".

Yes, I can solder, know what the components are and can read the schematic and recreate it. I just won't know the why. I probably don't need to know but then again I "need" to know what and why it works, just a flaw of mine I guess. No biggie, the semester of electronics in high school a decade or so ago didn't get me this far is all.
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Old 04-16-2003, 05:18 PM   #45
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Oh, the "why?" I can explain that... This is going to be a really high-level explanation, but I don't think any of it is wrong [i]per se[/]i:

Whenever you're dealing with an op amp with negative feedback, basically the amplifier does everything in its power to make both inputs equal.

When you have two equal resistors in series, the voltage in the middle of them will be halfway between the voltages at the ends; this is a type of voltage divider.

So, the noninverting ("+") input to the op amp gets half the IllumPositive voltage (with respect to ground). The inverting ("-") input gets the voltage halfway between the output voltage and the IllumNegative voltage.

IllumPositive/2 = (IllumNegative+Output)/2
IllumPositive = IllumNegative+Output
Output = IllumNegative-IllumPositive

The transistor is set up as an emitter follower (positive voltage at collector, input at base, output at emitter), which is basically a current amplifier. As long as some kind of load is connected between the emitter and ground, the voltage at the emitter is constantly 0.6 volts less than the voltage at the base, but at much lower impedance (higher current capacity). The 0.6 volts is made up by the op amp.

...was that any help at all?
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Old 04-16-2003, 07:10 PM   #46
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Definitely enough for me to almost understand - as long as noone ever asked me to explain it myself. Thanx.

Now for the crucial question - what specs on this thing need to change if I am running 4 gauge lights off of this gizmo?

And to make sure I understand -

Illum + in the schematic is the wire that has 12V+ when my lights are on.

Illum - in the schematic is the wire that varies based on the dimmer from 0V (brightest) to 12V (dash lights off).

I ground the gizmo.

And the output goes to my light + on the gauge.

The problem is Omori shares the ground between the gauge and the lights and if you connect the Illum (-) to it then you are dimming the gauge power when you dim your lights! (or worse)

Thanks for the great info on this.
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Old 04-16-2003, 07:24 PM   #47
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Oh and how much would you charge someone getting ready to graduate from Columbia (post-grad)

Without a plug on it though since I don't know the plug design in the WRX gauge pod yet, I could put one on later for that.
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Old 04-17-2003, 12:01 PM   #48
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If your bulbs aren't too ridiculously bright, this circuit should work okay... The TIP31 is technically rated up to 3 amps, which is around 35 watts in your car. The only modification you'd need would be a heat sink on the transistor.

You could equivalently use two or four of these circuits to put less load on the individual transistors. They're cheap and small.

If you do put a heat sink on it, be careful. The heat sink is attached to the collector, which is connected to the Illum+ line. Use electrically nonconductive thermal compound so that the heat sink doesn't short out to ground accidentally.

I can put the circuit together for you if you want; PM me.
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Old 04-23-2003, 12:02 PM   #49
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Hey All,
Just thought I'd jump in with a question of my own since I'm getting a couple of omori's. You say that the red wire is the gauge V+, and the white is the gauge Illumination.. So theoretically, inverting the ground side of the dash lights is the final goal correct? Meaning if the dash uses a floating ground from 12 when off to 0 at full bright (guestimations at best) you want to flip that so the gauge will have 0 when off and 12 when full bright? Gotta pull out a few books, but I'll get back to you on it, sounds like its not that bad. Sucks that you can't get neg voltage in a car.. would make life so much easier.. stupid DC..
Anyways, what was the deal with the Gauges acting funny, was that when you hooked up the floating ground of the dash to the actual ground of the gauges?

-- Steve
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Old 04-24-2003, 02:18 PM   #50
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Steve, that's pretty much what the circuit we're talking about does.

daverup just PM'd me about some confusion regarding the pictures of the circuit on the web page. I've added more information now so it should make more sense. Basically, I left out the information that the circuit pictured took advantage of the unused pin 8 on the LM741 op amp chip.
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