Another problem with the lcd

I had my first heatermeter 4.0 with the raspberry pi working and in the case, and while trouble shooting the blower power out with a meter I goofed and let the smoke out.

Upon rebooting it the heatermeter board actually booted but was not driving the lcd and while I could see values for the probes via the web interface it was glichy and cutting on and off. I purchased a replacement atmega, lcd and 16mhz resonator and now the heater meter appears to be working correctly via the web interface but I still get nothing from the lcd.

I have tried adjusting the contrast pot but it does not appear to do anything. Can anyone point me in the right direction for checking that pot or what else I may have messed up?

This was meant to be a birthday present for my father, but now it looks like it will not be finished in time

Thanks for any help

Jason

The pot itself would be something that would be really difficult to blow out. Your most likely suspects would be Q2 (BC337, drives the LCD backlight), or IC3 (74HC595, sends the data).

-- Is the backlight lit at all? Does changing the LCD brightness in the web interface have any effect? If not-- replace Q2
-- Backlight is on. The contrast pot does nothing at all? You don't even get blocks when at the extreme end of the pot's setting? Sounds like a bad LCD.
-- Backlight on and you can get blocks when adjusting the pot. IC3 is probably dead.

1 - GND
2 - 5V
3 - Roughly a volt. Should change when you adjust the pot
4 - Digital signal*
5 - GND
6 - Digital signal. Usually GND but should blip once a second
7 - Floating
8 - Floating
9 - Floating
10 - Floating
11 - Digital signal*
12 - Digital signal*
13 - Digital signal*
14 - Digital signal*
15 - 5V
16 - Variable

*= Might read 0V might read 5V might read anywhere in between. It just stays at whatever the last data sent was

The web interface changes the backlight correctly. The contrast pot appears to have no effect. When measuring voltage between 1 and 3 I get 3.5 or so however the contrast pot does seem to effect the voltage there.

The lcd is brand new I just replaced it, looks like it could be a soldering mistake shorting between 2 and 3?
Edit: just checked resistance between 2 and 3 and on the lcd I had around 10k on the pcb however only 1k
Also I have 5 volts everywhere I should and and the digital signal on 6 is blipping once a second

Thanks for your help Bryan and thanks for the time and effort to document how to do all of this

The voltage between 1 and 3 should be close to a volt when the contrast pot is in the proper position. I can't tell you what the resistance between 2 and 3 is because you can't measure the resistance of a component while it is in-circuit (unless at least one of the sides is disconnected). You'll have to pull the potentiometer if you want to measure it. If you do, two of the legs should measure 10k ohms and the other two pairs of legs should add up to 10k ohms.

Good news is that if the digital signals as blipping every second, that the shift register sounds good. I can't imagine a potentiometer getting fried, those things are pretty bulletproof because they've basically got built in short circuit protection.

Thanks bryan

I went ahead and ordered another shift register, lcd and pot. Somethings got to be wrong and the shipping is going to be more than the parts if I guess wrong. It really sounds like the problem is the lcd, I'm just really confused as to how I could have messed up the new lcd... I never had 12 volts on it just 5 and even if you reverse the ribbon cable it shouldnt mess it up. Oh well maybe I will have more answers when I get some parts in

After you pull the LCD off, you may want to check the traces on the board to make sure you didn't short something to the 12V line. You can do a continuity test between the 12V supply and each of the pads for the LCD. I'd definitely do this prior to hooking up the new LCD. Maybe the board got toasted as well.

dave

D Peart said:

After you pull the LCD off, you may want to check the traces on the board to make sure you didn't short something to the 12V line. You can do a continuity test between the 12V supply and each of the pads for the LCD. I'd definitely do this prior to hooking up the new LCD. Maybe the board got toasted as well.

dave

Click to expand...

Excellent suggestion... this makes a lot of sense

Still no luck in making the display work. I have however figured out what I did to mess everything up. I found a short from 12 volt supply to the common ground. With that in mind and considering how cheap the parts and board are I have decided the easiest/best way to proceed at this point is to just reorder and start over.

I changed the contrast post and can now get solid black squares in the lcd, I see the digital signal ticking every second, I have tried a known good lcd and the new ATMEGA has power everywhere its supposed and appears to be doing what its supposed to. The raspberry pi can see the heatermeter and is getting good and accurate readings so I believe the processor is good. The only thing I can think of is that there is a problem on the board itself or a problem with one of the caps or resistors.... and considering how much of a pain it is to desolder some of this stuff its easier to start over. It doesn't look like I'll have time to get it done for dads birthday, but depending on how long oshpark takes it shouldn't be too late.

Thanks again for everyones help