Finding correct voltage needed for something

mr cheese · May 13, 2018

I have an iPhone 5S and accidentally permanently damaged the backlight of the LCD.

But, if you shine a light on the inside of the phone, it will light up the LCD perfectly and make for a very cool device.

So, until my replacement comes later tonight, I want to find a way to wire it up to my Arduino Uno so I can separate and light up just the LED's from the backlight.

I don't know how many watts it pulls or how much resistance I need (I'm not to hardware-savvy yet, I'm getting there), so I want to know how I might test it with a digital multimeter to get the necessary info, and wire it up.

The Arduino Uno has several 5V and a 3.3V terminals and I have a 10K potentiometer (which would be great to include so I can control the brightness).

Thank y'all for your help! If I didn't give enough info please tell me!

mariushm · May 13, 2018

The leds are current driven not voltage driven.

You need a minimum voltage to get the leds to light up and then you need to limit the current so that the leds don't get burn out.

Blue or white leds typically have around 3v ... 3.2v forward voltage, so if you have 3 leds in series the minimum voltage would have to be 3 x 3.2v = 9.6v to light up the leds.

To limit the current use the basic formula Ohm's law : V = I x R .. voltage = current times resistance

So for example if you want one white led with a forward voltage of 3.2v to light up and be limited at 20mA (0.02A) and you're powering the circuit with 5v, then :

5v - 3.2v (forward voltage of led) = 0.02A (current) x Resistance

so you need a resistor equal to Resistance = (5-3.2)/0.02 = 90 ohm ... you can round this up to 100 ohm ... so:

5v + ----------[ resistor 100 ohm ]------------- [ + LED 3.2v - ] -------------- GND -

with 100 ohm , current I = (5-3.2)/100 = 0.018 A or 18mA

The power dissipated in resistor is P = I²xR = 0.018 x 0.018 x 100 = 0.0324w , so even a 0.125w rated resistor would be suitable (big enough to not overheat from the 0.0324w that are dissipated in resistor as heat).

the iphone uses a led driver, a circuit which boosts the voltage to whatever higher voltage is required while controlling the current. If you plan to just use some led in the back of the display then use what I said above.