I have a project that uses both 16x2 character LCD (part LCD-00791) and one of the large 20mm 4 digit 7-segment LED displays (product COM-11644). Both are wired to an Arduino Mega 2650. Three POTs, one toggle and 12 LEDs are involved as well. The primary problem I am having is that it appears the Mega will not effectively power both the 7-Segment LED display AND the 16x2 character display. With 7-seg LED on, the output on the character LCD is all garbled, if it shows at all. However when I disconnect the 7-segment, the character LCD works just fine. I’ve made sure that pin-outs are correct, but both are sharing the Mega’s common 5V and Ground circuits.
I’m considering using a small NPN (S9013) transistor to boost the output to the LCD, but I don’t think will address the main problem. I need both to function simultaneously, so I’m think I need to power the 7-segment display externally to the Arduino. It is pretty bright. Data sheets for both would indicate they well within the 40mA range for the digital I/Os, but I think I am, or close to, exceeding the power amperage for 5V circuit. Please help!
The current consumption of both devices should be easy enough for the voltage regulator on the Mega to supply it. Provided the Vin and Vcc of the devices isn’t powered by an output pin directly. Make sure they are supplied by the 5 volt supply of the board itself. Can you measure the voltage on the 5 volt pin when each of the devices are connected alone and together? Put a multimeter in series with one of the devices to make sure there is no over current consumption dragging the voltage of the other down.
The datasheet of the LCD display suggests to attach a 10k-20k potentiometer between Vss and Vdd with the wiper to V0 to adjust the contrast. Did you do that? Partially displaying characters does look quite similar to not enough display contrast voltage.
You can use a transistor to shut them down at will. But I wouldn’t worry about that until you got it working properly.
I have a similar problem also with a mega when I am powering from the USB. When I power from an external source (with plenty of current capacity) , then all is well have you tried that?