Lcd screen problem????


 
J

John Bostwick

TVWBB Wizard
Well, I guess you can never make enough Heatermeters and not have a problem that you seem to not be able to figure out. I recently made two new v4 boards one for me(my 2nd-version4) and one for someone else. The one I made for someone else worked fine and had no problems at all. The one I had build for me, has a problem and I Cant seem to figure out.

After I finished soldering up the board and installed a new Atmega, I plugged the power in and I noticed the MCP1700-33 getting hot to the point of smoking. So, I unplugged it and checked all solder joints and could not find a thing wrong with any of them. tried again and it again got hot. since I was not going to run it without the RPI, I took out the mcp1700-33 and tried power again. I knew I would not get anything on the lcd and everything looked ok so I unplugged it again and hooked the RPI to it and turn it on.

I did not get anything on the lcd but I was able to get the heatermeter working and on my network and it functions without problems all the probe connections work and thermistor works fine. when I tried to adjust the backlight it also works fine be it either on the board itself or on the config page but nothing is displayed on the lcd. I thought maybe it was a bad Atmega and I changed to a working chip that works on my older version 4 board and same problem with the lcd. I rechecked and soldered the board again and I tried what I believe is another mcp1700-33 that I had available and it did not smoke or get hot and the board works except for the lcd(backlight is working fine).

I thinking maybe I have a bad shift resister at this point im not sure what else it could be.

Any suggestions?
 
hmm, odd indeed. The voltage goes from 12 to 5 through that dc-dc regulator board, then the 5v goes to (either) the rPi which feeds 3.3 back to the HM or to the 3.3 regulator in question (that was getting hot). Real world experimentation has shown that it's ok to have both the 3.3v regulator and the rPi connected at the same time... Your first symptom was the 3.3v regulator getting hot, that would make me think "SHORT" or perhaps a backward diode or capacitor, solder bridge etc. It seems you've been down that road, verified component orientation etc, at this point your (second) 3.3v regulator is not getting hot, so perhaps you cleared up a solder bridge during your troubleshooting? If I were you I would disconnect the rPi then power the HM up and measure the 3.3v, if it is good I would resolder your shift register one last time, check connector and/or ribbon cable for the display, and if she still dont work I would swap the shift register. If you have a spare display I might try swapping displays before going down the road of desoldering the shift register IC, which could be a PITA... Good luck getting your problem solved....
 
I'm not sure what else it could be either. The LCD isn't complicated compared to the v3 boards, there's just a straight set of traces to the shift register and then straight traces to the LCD. They're also connected to 2 of the 3 LEDs so check there for shorts as well. If you've got a solder bridge on something on the shift register it could overheat the 3v3 regulator and damage the shift IC. I don't think you can damage the LCD though, because the inputs are high impedance so they're somewhat tolerant of bad connections. If all the 5V stuff is working, the Pi and the backlight, the parts that are in play to make the LCD work are the shift register, the LCD itself, the contrast pot, and the LEDs with their resistors.
 
Thanks Ralph and Bryan, I'll take a look at the LEDS. well in fact, on hindsight, I had one of the LEDS in backwards But even after removing that one completely it did not help. The other two LEDs will light up though, so I sure there was no solder bridge them. I have to wait to get some de-solder braid, the local Radio shack was out.

Thanks
 
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