How to wire up LCD LED Interface Pins

Discussion in 'General Electronics Chat' started by Hurdy, Mar 18, 2014.

1. Hurdy Thread Starter Senior Member

Feb 27, 2006
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Hi,

Firstly, thank you for taking the time out to read this post.

I recently purchased a MIDAS MC21605E6W-SPTLYI 2x16 I2C LCD. The datasheet doesn't really provide any information regarding the connection of the LED+ & LED- pins to Vdd (3v) and GND.

Should I connect the LED+ pin to Vdd (3V) via a resistor? If so, what value of resistor should I use?

Again, thank you very much for your time.

Rob

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2. Brownout Well-Known Member

Jan 10, 2012
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I can't tell either.

3. spinnaker AAC Fanatic!

Oct 29, 2009
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That is just the backlight. Add an appropriate current limiting resistor and connect to your 3V. Datasheet should give forward voltage of the LED so you can calculate the resistor.

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4. spinnaker AAC Fanatic!

Oct 29, 2009
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Well looks like it does not have max current for forward voltage of the LED. You could take a guess based of the color.

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5. Hurdy Thread Starter Senior Member

Feb 27, 2006
137
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Hi both,

I really appreciate your prompt responses.

The datasheet states the following:

LED Forward Voltage: 3.8V - 4.2V
LED Forward Current: 15mA

So assuming I use ohms law I get the following:

R = 4 / 0.015 = 266 ohms.

My next question is, if the Vdd for the LCD controller is 3v, is it common for the LED backlight to require a 4v supply? Currently everything in my circuit design uses 3v. Does this mean that I'm going to need a 4v supply just for the backlight LED?

6. Brownout Well-Known Member

Jan 10, 2012
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It might. 3v isn't enough forward voltage for some types of LED's. You datasheet also shows two LED's in series. Are they 4V wired together, or 4V each? Can't seem to tell. I would connect a 15mA current and measure the voltage, and measure the resistance with a DMM.

7. spinnaker AAC Fanatic!

Oct 29, 2009
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Sorry I must have missed that in there. Your forward voltage is going to be based off color. Are you certain that is the only range? If that is it then you won't be able to light the backlight. Blue does have a pretty high forward voltage. What color is the backlight? If it is yellow, green or red, I would be surprised if it was really that high.

8. Hurdy Thread Starter Senior Member

Feb 27, 2006
137
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Thanks Brownout. I think this will be the best option. But of a bummer that I will need to supply another Voltage rail to my circuit. Such is life though.

9. spinnaker AAC Fanatic!

Oct 29, 2009
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If the display is 3V then your backlight might light with 3V. I would give it a try. Use 20ma as your current just to play to safe.

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Feb 27, 2006
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12. spinnaker AAC Fanatic!

Oct 29, 2009
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Yep on closer inspection of the datasheet, looks to me like they are in series. Bummer.

Strange it says LED 1 with 2 dies.

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13. Hurdy Thread Starter Senior Member

Feb 27, 2006
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yea a real bummer.

Oh well, I will need to create one power rail for 3V for my chips and another 4v rail just for the backlight LED.

Some testing is required first though.

Really appreciate your time and help with this.

Rob

14. Brownout Well-Known Member

Jan 10, 2012
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If you just supply 5V for the LED, you can just connect a 3V regulator to the 5V supply for the chip. Or else just run the display on straight 5V. But before you do anything, I would supply 15ma to the LED and verify the voltage.

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15. Hurdy Thread Starter Senior Member

Feb 27, 2006
137
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Just had a response from MIDAS regarding the backlight LEDs.
The LEDs typically need 4.1V at 30ma

I was advised to use a 5V supply with 30R resistor.

So how do you calculate the resistor required?

Ohms law tells me the following:

R = V / I = 5V / 0.030A = 166.66R

166.66R certainly isn't 30R.

Here's a copy of the datasheet that was emailed to me from MIDAS support (download PDF)

Many thanks,

Rob

/************* EDIT *****************/
I found an LED calculator here when Googling.
It recommends 33R, which matches MIDAS' response.

I guess I didn't consider supply voltage as well as the diode forward voltage.

Last edited: Mar 23, 2014
16. Brownout Well-Known Member

Jan 10, 2012
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(5 - 4.1)/.03 = 30 ohm.

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