I have the following problem: for some *** reason, my LCD is flashing when my fingerprint scanner turns on. Here is a video of the behavior: Dropbox - File Deleted.
More details: I have a membrane keypad. When I start typing on it, the LCD turns on, and when I type the correct code, the fingerprint scanner turns on, and the LCD prompts the user to press a finger upon the scanner. However, then the LCD backlight starts flashing badly. Interestingly, if I just plug the wall adapter into the arduino and try the scenario, the LCD does not flash. However, if let's say 10-15 have passed since the adapter is plugged in, the LCD flickering becomes visible and irritating (again, when the fingerprint scanner is turned on). So, what could be the reason and how to fix it? Why if I just plug the adapter and execute the scenario, the LCD does not flash, and if I wait some time with the adapter plugged in, and then execute the scenario, the LCD flickers?
Just to add: My fingerprint scanner is GT511C1R (Fingerprint Scanner - TTL (GT-521F32) - SEN-14518 - SparkFun Electronics) and I am using the standard scheme for wiring it (560/1000 ohm for the RX). I am not using lcd.clear() command as I know it's heavy, and even if I don't write any text, the still LCD flickers.
We have no idea how many devices you have connected in total.
We don't know how you are powering it. When you get backlights - which are presumably simply connected to your 5 V and not controlled by anything - flashing, that suggests your power is failing. OTOH, if it is controlled by something, then that something may be causing the flashing. But you haven't explained any of that.
You really should not be powering anything requiring more than 100 mA - in total - from the Arduino regulator. But we don't yet know, do we? How much does the fingerprint scanner draw? You tell me, rather than me chasing the specifications.
I can agree, I feel as though you have not supplied sufficient details about just how you have this set up. The power may be failing (I might connect an oscilloscope or a multi-meter and measure your 5 volt rail to the LCD backlight). Im assuming you have circuitry to control the backlight, and that could then be a software problem, check it; if it isnt, connect a scope to that pin as well. Of course, this may seem too much, but without a diagram and the code, this is the only real help I can offer. I would add a diagram (maybe Fritzing? Welcome to Fritzing) and also post your code. Thanks.
I think the problem is with wiring, there might be some sort short in the wiring or some faulty connection in the soldering of the 16x2 display, when I had done my first project with 16x2 lcd I encountered same problem , the LCD would turn off maliciously , I solved it by getting soldered from a mobile repair shop to a module I got it soldered with high precision, on a proto pcb. Now it works fine.
I just bought a nokia 5110 display. when I connect is vcc pin to 5v and backlight pin to 3.3v and leave gnd pin unconnected I get flickering of the screen
lastly if these donot work try to give arduino a separate 9v supply from 2.1mm barrel jack. power the lcd from it but not the other circuitry. Give it a different power supply
The code? Here it is: FPS-LCD ยท GitHub It's a bit complicated, so I wanted to avoid sharing it, but if you think that would help..
I would try checking the 5 draw (which is obvious as the LCD is flashing, right?) tonight with the multimeter (but even if I see it, what's the conclusion?). I don't have oscilloscope.
AND: If I just try the fingerprint scanner on another Mega board, I get the same flickering a few moments after the board is powered: Dropbox - File Deleted . The code is the standard fingerid code (basically, what I have in the CheckFingerAndOpen method).
So, any ideas?
@vedantparanjape: It's the second time I am soldering this, while trying to find where the flicker comes from. I am using a new LCD, of course, because the surface of the old one is done. This does not mean that my second soldering is perfect, but damn I can't find the issue in so many connections.
by the way I am no expert, I too am a hobyist I am a grade 10th student and was introduced to arduino a year ago
I think u should tryto power the lcd with a separate regulated 5v supply from the mains and not from the board.
if this also does not work then try the same with a new LCD display
atlast if all these fail then please consult some professor of a nearby university teaching electronics these people will surely help you or a electronic repair technician
posetitelqt:
this is the scheme of the lcd (it's just my backlight A pin with the resistor is connected to a digital pin rather than to the 5v):
OK so a little more of the picture is revealed!!
Summary
Microcontroller ATmega1280
Operating Voltage 5V
Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V
Digital I/O Pins 54 (of which 15 provide PWM output)
Analog Input Pins 16
DC Current per I/O Pin 40 mA
DC Current for 3.3V Pin 50 mA
Flash Memory 128 KB of which 4 KB used by bootloader
SRAM 8 KB
EEPROM 4 KB
Clock Speed 16 MHz
I currently had the same problem with the flickering of my backlight on a 2004A: Thanks to your help, I got the necessary constant 5V voltage via an additional StepUp Converter under control!
Ah yes, the ever-present problem with the old Arduinos - the UNO, Leonardo, Nano, Pro Micro etc.
The obsolete tutorials on the Arduino site and others imply that the largely ornamental "barrel jack" and "Vin" connections to the on-board regulator imply that this is a usable source of 5 V power. This is absolutely not the case. It is essentially for demonstration use of the bare board back in the very beginning of the Arduino project when "9V" power packs were common and this was a practical way to power a lone Arduino board for initial demonstration purposes. And even then it was limited because an unloaded 9 V transformer-rectifier-capacitor supply would generally provide over 12 V which the regulator could barely handle.
Nowadays, 5 V regulated switchmode packs are arguably the most readily available in the form of "Phone chargers" and switchmode "buck" regulators are cheap on eBay so these can be fed into the USB connector or 5 V pin to provide adequate power for all applications. Unfortunately, many tutorials or "instructables" are outdated or misleading and have not been updated to reflect the contemporary situation.
The "Vin" or "RAW" terminal is essentially a legacy part, some "clones" such as the "RoboRed" and more sophisticated Arduinos incorporate an actually functional switchmode regulator but it should simply be ignored on the older designs.
I am saying that I digitally control it, but if directly short it to 5v the situation is the same. 
