7″ TFT mon

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I am using this display with my Pis. Thanks to a tutorial they put some time ago, I have been able to control the brightness on the screen. Recently, I was testing some nodes on the Pi using a continuity tester and while there is no apparent damage to the Pi or any change in its behavior, suddenly the brightness control for the Waveshare 7in is not working.

I tried another one of the displays I have around and brigthtness control does not respond on it either. I tried both displays with a different Pi with no results. Then I tried doing these using a different GPIO pin and have still come up with nothing.

Because of this, I am unable to diagnose the source of the problem.

Is it possible the the displays are now somehow damaged?

  • What does “testing some nodes” mean? What is the “continuity tester”? – Milliways 19 hours ago
  • Ah, let me see. (1) Pi 1, Mon 1 used to be OK, now (2) P1, M1 not OK (3) P2, M2 not OK either. So my quick and dirty conclusion is that Pi is broken. But then even (4) (a) P1 + M1, (b) P1 + (c) M2, P2 + (d) M1, P2 + M2 all no luck. What would you conclude now? One possibility is that P1 has become a serial mon killer, any mon connected will be killed. – tlfong01 19 hours ago   
  • And are you doing PWM brightness control? If yes, then I will see if I can give your a postmortem. 🙂 waveshare.com/wiki/File:PWM_control_backlight_manual.pdf – tlfong01 19 hours ago   
  • And are you using the default GPIO pin #18 as the PWM pin? And has it ever worked once or twice? And now any other pins not working? Are Rpi and mon are still working otherwise? If yes, perhaps only the Rp PWM output and Mon PWM input part is fried. Or you just made the wrong connection and everything is alive and well. There is still some hope! 🙂 – tlfong01 19 hours ago   
  • did you use the same power supply for all tests? … did you use the same hdmi cable for all tests? – jsotola 19 hours ago
  • 1
    It is easy to check if GPIO 18 is alive. Use any GPIO command/program to set it high or low. If you don’t have a multi-meter, use a LED and 1k series resistor to see it can turn on or off. When doing PWM duty cycle check, you can see the LED goes from dark to dim, to bright etc. You should do this dry run before connecting to the mon. – tlfong01 19 hours ago   
  • So I would suggest to put the mon’s aside, and just test the GPIO pin #18 for PWM output. You can use terminal commands as suggested by WaveShare. You can also use a python program to do PWM. My answer to the following question has a fully debugged python program to out put PWM, also using GPIO #18, to control servos. You can of course use the same program not to control servo, but the back lit LED, or brightness of HDMI mon: raspberrypi.stackexchange.com/questions/99315/… – tlfong01 19 hours ago   
  • Hi @user942937, There are three versions of WaveShare 7″ HDMI Monitor. You might like to let us know which version you are using, and perhaps also show us a close up photo of the Rpi GPIO pin #18 connecting to the mon’s back lit PWM input. – tlfong01 18 hours ago

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