New build, LCD does not light up

[Don_G]

Hi. We’re working on a the v2.1u (semi-assembled DIY kit). We finished the assembly without issue. However, when we switch the unit on, nothing displays on the LCD; no light at all. The transformer seems to be working correctly, outputting about 26V.

There is a little more information that I’d like to share. When we reached the testing section of the assembly instructions, we came across a possible issue. The voltage between VIN and GND measures around 5V with one meter and 10V with another; also the voltage between VCC and GND measures about 4V with one meter and around 7V with the other. You may ask how this is possible. It probably is simply a lack of understanding of how to read the analog meter – we’re new to this. 🙂

Since we weren’t sure which meter to believe we tried installing the ICs and LCD and plugging in the power. We purchased an Orbit 24V transformer, as suggested.

The person who soldered the components does that professionally. Several people have carefully studied the soldering on the back of the board and we’ve found no issues.

Do you have any suggestions of what to try next?

[Ray]

When using multimeter to measure VIN-GND or VCC-GND voltage, make sure you are measuring in DC, not AC. On a standard multimeter, DC voltage is marked as a V with a flat line above it; AC voltage is marked as a V with a curvy line above it. If VIN-GND is 5V, that’s correct. You said VCC-GND is 4V and 7V by your two meters, that seems to suggest a problem. VCC-GND should be 3.3V.

Going back to the instructions:
http://rayshobby.net/blog/?page_id=8074#part2
did you check the resistance between VIN-GND and VCC-GND? Are they within acceptable range?

Also, to isolate the problem, I suggest that you power the controller with a USB cable first, and check voltage, then move on to use sprinkler transformer. This can help diagnose whether the problem is in the 5V to 3.3V conversion and beyond, or it’s in 24VAC to 5VDC conversion.

[Don_G]

Hi, Ray. Thanks very much for your guidance. With your course in meter reading, we were able to determine that the voltage looks good. With the USB as a power source, VIN-GND is close to 5V and VCC-GND is close to 3.3V.

However, it appears that the resistance is way too low. For both VIN-GND and VCC-GND, the resistance is under 1K ohms. I’ve studied the back of the board closely with an eye loop and I can’t find a solder bridge. The only thing I can see is some filaments from solder wick (I believe). I’ve attached a couple of pictures – one each of the front and back of the board. Perhaps you can zoom in and see something I can’t.

Thanks again for your help! We’re building this as a family project so it’s a good learning experience.

[attachment=1:2yepb0oq]IMG_0708.jpg[/attachment:2yepb0oq]
[attachment=0:2yepb0oq]IMG_0717.jpg[/attachment:2yepb0oq]

[Ray]

The soldering quality seems pretty good, and I can’t see any obvious problem. One thing to double check if the four pins on the USB connector: those pins are very close to each other and are hence prone to solder bridges.

The resistances you reported are indeed quite low and indicate a problem. The VIN-GND resistance should be about 4.2 to 4.8 kOhm, and VCC-GND resistance should be even higher. So chances are that either there is a solder bridge somewhere, or a component has been damaged after connecting to the sprinkler transformer.

I forgot to ask: does the LCD light up when you power the controller via USB?

[Don_G]

Ray, thanks very much for your continued help. I have good news. I borrowed a more modern digital meter and the resistance measurements look good – VIN-GND is 8.7 kOhms and VCC-GND is 16.7 kOhms. Voltage is still correct. My old analog meter must have a problem with resistance.

I reinstalled the ICs and the LCD and powered it on with the USB. The LCD lit up and displayed appropriate information! I then tried it with the transformer and this time everything worked as expected. I can’t explain why it didn’t work previously since I didn’t make any changes to the board. But at this point I’ll move forward with additional testing.

I have one question for you. When powered with USB, the on/off switch has no impact; the unit is always on. Is this normal behavior?

Thanks again for your help.

[Ray]

Cool, glad to hear that it finally worked. Indeed it’s surprising that it suddenly just worked. Perhaps it was a wire connection problem?

The USB power thing is perfectly normal: the switch only controls 24V AC line, and does not control the USB power. So the unit will power on whenever you plug in USB cable.

[Author]

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