Nokia 5110 causing ADC reading to increase.

Hi

I have a bit of an enigma with a Nokia 5110 LCD screen. The basics of it are this. When the back light of the screen is active ADC readings jump by 3 or 4. Very very basically I can demonstrate the problem by simply wiring up the Arduino (It's a Nano) to a 12V source (using VIN of course). Then use a 22K and 10K resistor as a voltage divider between the power supply with the node of those two resistors connected to A0. This should give me a reading of about 768 ish depending on how close the supply is to 12V and the accuracy of the resistors. This is indeed the case, however, if for example I get a reading of 770 with the LCD back light turned off, this reading will jump to 774 when I turn the back light on. Weird. The display is wired to the Arduino with all pins but ground connected by 1K resistors to limit the current (no I do not wire VCC to VIN, VCC is wired to +5V). The back light is wired to a data pin so I can switch it on and off. If I power the LCD display back light from an external source, it doesn't happen, the reading stays steady. So my guess is that using the Arduino power to light the display is causing a slight voltage drop on the +5V rail of the Arduino, making the AVCC drop slightly. If this is the case then the 5V regulator on the Arduino must not be particularly good.

Does this mean that my "box" will have to use its own dedicated 7805 regulator to AVCC to prevent this drop happening?

Cheers, Da

As far as I know , the only proper way to wire the backlight to a data pin is to use the data pin to turn on an NPN transistor (or N-channel Mosfet) that has the collector of the transistor connected to Vcc (+5Vdc) and the emitter connected to the backlight pin (lcd pin-15 of a parallel lcd). Alternately, the backlight can be connected to +5V with an 82 ohm or 91 ohm resistor. Connecting the backlight directly to +5V is a matter of personal preference (I've never heard of any being damaged that way , not that it means they haven't) but if you did it would draw about 14 -15mA. I realize that is within the current spec for a data pin but I believe that practice is frowned upon. If you connect the backlight to +5V using one of the above methods INSTEAD of connecting to a data pin you might not have that problem because the backlight will not adding to the load of the Arduino on board regulator.

raschemmel:
As far as I know , the only proper way to wire the backlight to a data pin is to use the data pin to turn on
an NPN transistor (or N-channel Mosfet) that has the collector of the transistor connected to Vcc (+5Vdc)
and the emitter connected to the backlight pin (lcd pin-15 of a parallel lcd). Alternately, the backlight can
be connected to +5V with an 82 ohm or 91 ohm resistor. Connecting the backlight directly to +5V is a matter
of personal preference (I’ve never heard of any being damaged that way , not that it means they haven’t)
but if you did it would draw about 14 -15mA. I realize that is within the current spec for a data pin but I
believe that practice is frowned upon.

Well, it took 2 minutes to test using a transistor instead of directly to a data pin on a breadboard (although I put a 1K resistor to the base and a 220R resistor from the emitter as well just to be safe). And damn, the ADC error has totally gone away. Thanks, I have no idea why that worked but it did, and that’s what matters. Nice one. I am somewhat confused however. Isn’t the load of an NPN supposed to be on the collector not the emitter? Why does that work?

Look at the symbol for an NPN transistor and compare it to the symbol of a diode (or LED) . Note that the Anode (or collector) is always positively biased with respect to the cathode (or emitter). Consequently the current flows from the collector through the junction to the emitter. The configuration I recommended is called SERIES PASS in the jargon and it means the load is connected to the emitter and the input for the transistor is the collector.

You can replace the 220R resistor you have for the emitter with 100 ohm without worrying about the current.