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Basic Electronics » Why do we need a resistor going from the contrast pin to the ground?

May 31, 2009
by luisgarciaalanis
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I don't know electronics, that is why I got the kit :)

I was watching the MIT electronics course in youtube and they say that ground should be Zero volts, from what I understood. is this why we put the resistor? to eleminate the voltage going to the ground from the LCD contrast pin?

June 14, 2009
by BobaMosfet
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No, actually the resistor is their to apply BIAS to the circuit. Contrast is controlled by a transistor, and the resistor affects the bias of the gate.

In order to see this work best, look in your parts that you got with the kit-- did you get a little blue trim pot? (trimming potentiometer - aka variable resistor) with the #3323 on it? If so, you can use this to adjust your contrast with a small screwdriver.

Here is how it is connected with a single voltage source:

The datasheet for the display says that the LCD has 3 connections that have to be connected through the trim pots 3 leads:

Vdd - +5 Voltage (pin 2 on the LCD) Vss - GND (pin 1 on the LCD) Vo - Contrast Signal (pin 3 on the LCD).

Remove the 1K Ohm resistor that you are currently using on Pin 3. Leave everything else the same.

Put your trim pot on the right side of your board, so that none of the bread board leads connect it to the MCU or anything else you might have there. I put mine down just below the MCU, but above the Voltage Regulator (Row 26, 25, and 27) with the trim-pot adjustment facing the outside of the board. In other words, the pins in the trim pot are in these holes on the breadboard: h25, i26 and h27.

You'll need 3 short wires. For Vdd (+5), Vss (GND), and Vo (Contrast). Since my LCD pin 3 wire connected to a27 on my board, I put a blue wire that connects d27 to f26 - that essentially connects Pin 3 to the middle pin on the trim pot.

Next, since the entire circuit is running off of +5 and GND out of the voltage regulator (LCD and all), you don't need to mess with pins 1 and 2 on the LCD - leave them be as they are. Instead, connect the +5 rail on the right side to j25, and GND rail on the right side to j27.

Check your connections.

If you've done everything right, go ahead and connect your battery. NOTE: You might not see anything on the display-- that is probably because your trim pot needs adjusted. Take a small screwdriver (philips) and turn the trim-pot GENTLY to the right (clockwise). It might take awhile, but eventually the text starts to show up.

I found on mine that the contrast adjustment was on the high-end of the trim-pot, but it works GREAT.

Hope that helps!

June 15, 2009
by BobaMosfet
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I want to amend my above statement slightly.

It is not a transistor controlling contrast on the LCD package. After looking at it and the proper data sheet for it, I find that it is actually 5 in-series resistors that control the level of the LCDs contrast. The pull-down resistor you add between Pin 3 and GND determines where between each of the 5 resistors the voltage level is set to control the contrast. Which makes it a pull-down resistor.

Everything else above is correct, as regards using the trim-pot to adjust the contrast- it works GREAT!

July 30, 2009
by luisgarciaalanis
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Thanks for the responce :)

July 30, 2009
by rajabalu21
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LCD has a viewing and bias angle. Please see this link for details.

We need the resistor to adjust the bias angle. Please see this link for details. Actually it is a variable resistor. NerdKit includes a variable resistor for this purpose. But from experience they have arrived at the 1K resistor for ease of assembly.

July 31, 2009
by luisgarciaalanis
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but my question was more inregards on why is a resistor grounded, whats the effect of the resistor on that pin

August 01, 2009
by wayward
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+5V   R1    v     R2
     10k         10k

This is a voltage divider with two equal resistors. Input voltage is distributed equally between the two resistors and we have a 2.5V voltage drop on the first, 2.5V drop on the second. If one resistor was higher, then the voltage drop on its terminals would be proportionately higher.

Now imagine that we take R2 out of the circuit:

+5V   R1    v
>----VVV----o <-- this is the LCD contrast pin!

What remains is our LCD contrast pin. :) Attach a resistor to it, ground the other side of the resistor, and you will form a voltage divider. LCD can read the voltage on the terminal, which is under our control (by altering the attached resistance), and adjust the bias angle accordingly.

Hope that helps!


August 05, 2009
by BobaMosfet
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The Bourns Trimming Potentiometer (3323) included with the NerdKit is (if it has a 104 on the side) a 1 Ohm to 100 KOhm variable resistor (a voltage divider). Mine was actually up to 105 KOhm. If you turn it all the way clockwise, you're at 100K. All the way CounterClockWise and you're at 1 Ohm.

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