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Basic Electronics » What is the purpose of 1000 ohm resistor for LCD's pin 3?

March 12, 2013
by oshjdf
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In the nerdkit guide, the reason given is to control constrast. But it doesn't give any detail explanation of how different value of resistor can control the LCD's contrast. What happens if I use 100 ohm/500 ohm/2000 ohm?

Another thing is why place it before connecting it into ground? Isn't resistor normally placed before a component?

Thanks

March 12, 2013
by Ralphxyz
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What happens if I use 100 ohm/500 ohm/2000 ohm?

Well you might release the hidden smoke.

Actually I use a 1k potentiometer on most all of my setups.

Try it, you probable will not do any harm, just don't leave at a low setting.

Ralph

March 12, 2013
by oshjdf
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Hi Ralph,

"just don't leave at a low setting" Could you give more specific answer?

March 12, 2013
by Ralphxyz
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No

You can use a 1k or even a 10k potentiometer.

do not leave it at say 100 ohm for a long time.

Just momentarily twist it down to see the effect on the LCD.

Ralph

March 12, 2013
by Noter
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I have one LCD display that works best with no resistor when VCC is 5.1v, the contrast line just goes directly to GND. If I do the same on my other LCD the display is just black boxes, too dark to read. So on that one I need the ~1k resistor to lighten it up enough to see the characters. If you continue to increase the resistance the characters will eventually become so light you can't see them any more and the display looks like it's off. You can't hurt the LCD with different resistance values on the contrast pin, even zero ohms (directly to GND). Experiment and have some fun and then pick one that gives results you like best, it will probably be around 1k if that is what is working now.

March 12, 2013
by Noter
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And the reason the contrast pin has to go to GND via some resistance is because that's the way the LCD is designed to work. There's no other option. When you design your circuits you decide where things go, when you use someone else's design they decided for you.

March 12, 2013
by oshjdf
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Ralph: So, that is it. "Do not use lower than 1000 ohm".

Noter: Hi Noter. If we are looking at the perspective of design, let us say I want to build a simple 50mA LED circuit. Using 9V battery, the circuit requires 130 ohm resistor to produce 50mA. So, the question is, can I place the resistor after the LED instead of the resistor first (like in LCD case)?

Actually I do not quite understand when is the case to place a resistor after a semiconductor.

March 13, 2013
by Ralphxyz
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can I place the resistor after the LED

One time I had a resistor placed before the LED I also had probe wire before the LED for testing voltage drop across the resistor.

Somehow I inadvertently touched the probe wire with voltage and immediately blew the LED.

Lesson learned always put the resistor to ground.

If I had the resistor to ground I would not have blown the LED.

So now I always place the resistor after the component.

Functionally it probable does not matter unless you need a voltage drop before the component.

Ralph

March 13, 2013
by Noter
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In your led example it's like Ralph says, it doesn't matter. However that's not the case with transistors. Here's some pretty good docs on transistor circuits -

Learning About Transistors

Basic Transistor Circuits

Transistor Circuits

Basic Amplifier Transistor Circuits

March 14, 2013
by mongo
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My LCD has a pot in place. One end to +5V and the other GND. The slider of the pot goes to pin 3 and I can control the contrast/brightness of the LCD.

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