How do you separate the LCD power transistor, to run on vehicle backlighting?

[nogard]

I'm going to be wiring in my MPGuino the afternoon. I've read some of the concerns with overheating. I never thought that would be an issue. But when you scale it down it makes sense.

I'm going to be installing this on a 1988 camry with 4 cylinder (3S-FE). I put an aftermarket illuminated vacuum gauge in. To get the back lighting to work on the with the factory controls I had to connect the positive and negative of the gauge reversed, I guess there was no diode or this would not have worked. Since the vehicle circuit uses constant positive and the rheostat is controlling current through the negative. This works great.

Now how do I integrate the LCD to the vehicle back lighting circuit. I would have to use this same reverse wiring to make the dimmer part work.

I just found out I can't post images, because I have a low count of posts. I will try to explain what I want to do in text and maybe later post a picture.

1. Is the power transistor on the upper right of the circuit with the 220 ohm resistor?

2. Do I need to disconnect all three wires from it? Or just the orange one feeding down to the LCD?

3. What are the pins on the LCD for the back light? (The one that the orange power transistor went to? and the ground next to it?)

4. If there is a separate back light ground then I could run that through the rheostat?

5. I also read about needing a 270 ohm resistor on the power off the vehicle backlighting (illumination) circuit?

Thanks.

[nogard]

Just wanted to see if posting a reply counts toward my 5 required to post a picture.

Yes it did, I wonder if this edit does?

[nogard]

It seems I finally got picture posting to work

My questions about relate to wires #1, 2, 3.

6. Is #2 the "back light" power to the LCD?

7. Would I need to disconnect wire #1 also to keep from heating/using the power transistor?

8. Is wire #3 the ground for the back light of the LCD?

Thanks again easier this time with pictures.

[dcb]

I "think" all they did was break the orange lead at 2, then connect the lcd end to the dashboard circuit via a resistor (i.e. 270 ohms).

You still need to identify the right dashboard lead however.

[nogard]

Which are the two power leads for the backlight, what pins? What are the lead(s) for the display power. I ask because I might have to flip the polarity around if it is possible to get the back light to work with my car.

I also don't like the fact that the display stays on, even when the backlight goes out. I wan to be able to shut the display off and still keep power to the unit for memory.

[dcb]

I like the lcd display on, shows me my previous current trip. doesn't take much power for just the numbers. but if you want to get fancy with coding you can tweak it however you want, maybe drive the lcd off an output pin and put the cpu into sleep mode. You might want to look at switching regulators and whatnot too.

[bobski]

The LCD backlight is a simple LED/resistor series circuit designed to run at 5V... It should be separate from the rest of the display electronics. #2 in your pic is the positive lead, #3 is negative. Since you want to use your dash circuit for dimming, you can disconnect the backlight from the Arduino all together and eliminate the 2N3906 transistor and 220 ohm resistor.
Pick up a 150 or 180 ohm 1/2 watt resistor and a 1N4001 rectifier diode (in case you get the wires backwards) from radioshack or where ever and hook them up in series with your backlight. Line end of the diode to your backlight positive, resistor on the negative (resistor orientation doesn't matter). You should now be able to hook it up to the lighting circuit just like your vacuum gauge... The remaining diode lead to lighting circuit positive, the remaining resistor lead to the dimmer/negative.