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Author Topic: Driving bare LCD  (Read 1338 times)
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I hope this is the right place to post this. I have a trip computer in my car that I wish to use/repurpose, but I have fitted a different ECU, and changed the size of the injectors, etc. This means that the data it displays is either missing or just downright wrong. I have searched online for any information about driving similar bare LCDs, but have not found very much.
Using Microchip and Atmel app notes as well as this I have a pretty basic idea of how to physically drive the screen, but my problem is in trying to identify which pins are backplanes and which are segments and how many of each there are. The LCD has 26 pins, two appear not connected to anything on the original PCB and two are connected to each other. The LCD has 42 segments as far as I can tell, but that's about as much information as I've been able to glean from it. Have any of you had any experience trying to drive a similar LCD? Any information you can provide would be great.
Thanks.
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Use your arduino to generate two opposing polarity signals. Apply one to a pin and then the other to all other pins, one at a time. Segments will start to light up if one of them is a com.

Do this to all pins.
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something like:
Code:
void loop ()
{
  digitalWrite (pinA, LOW) ;
  digitalWrite (pinB, HIGH) ;
  delay (1) ;
  digitalWrite (pinB, LOW) ;
  digitalWrite (pinA, HIGH) ;
  delay (1) ;
}

And add 1k resistors on the pins so you can't accidentally burn them out.
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Dont worry about burning them out: lcds are tiny little capacitors.
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Dont worry about burning them out: lcds are tiny little capacitors.
Rightttt.....  smiley-roll

When a capacitor is empty, it acts like a short. (check with a multimeter if you don't believe this)

If something is a short, its resistance is 0.

If the resistance is 0, the current is given by Ohm's law:  I=V/R

Subbing in 5 for V and 0 for R you get: I = 5/0

I = infinity

How much current are the digital pins on the Arduino rated for?
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@ *dhenry* as usual you are testing our meager little minds: Quote...
Quote
When a capacitor is empty, it acts like a short. (check with a multimeter if you don't believe this)
We don't have the Obvious luxury of your Magnificent Test Laboratory, and clearly not Sample and Hold Ohmmeters"
Every time I measure one it just reads whatever capacity is marked on the case.
I know that new parts are not defective very often so why bother for 'shorts' and since a good capacitor (By your definition is 'shorted' can you please tell me how to make that obviously 'new' measurement? ... Please?

Bob
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Use a simple analog ohm meter and test a capacitor. The needle will flick towards the 0, then quickly increase.

Now I know it's not a short like you would imagine with a wire and a battery. The point is the initial charge current into a capacitor is high. Connecting a capacitor directly to a uC pin could easily allow a current to flow that is higher than the specs. Although it would be very brief, it would cause damage to the pin which over time would cause the pin to fail.

Same thing with connecting a mosfet directly to a uC pin. A series resistor never hurts.

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Dont worry about burning them out: lcds are tiny little capacitors.
Rightttt.....  smiley-roll

When a capacitor is empty, it acts like a short. (check with a multimeter if you don't believe this)

If something is a short, its resistance is 0.

If the resistance is 0, the current is given by Ohm's law:  I=V/R

Subbing in 5 for V and 0 for R you get: I = 5/0

I = infinity

How much current are the digital pins on the Arduino rated for?

An LCD display has tiny capacitance values and significant series resistance and inductance, you won't blow up a Arduino driving one!
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Most people design for 1000pf - 1500pf / cm2. My experience with segment lcds typically used in a multimeter is about 500pf / cm2.

At the switching frequencies for those lcds, nothing to write home about.
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Thanks guys. I'll give that a go.
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