Troubleshooting

[Tony]

Not as yet. If there are enough requests I may have to draw one.....

[sonicwarhawk]

Tony, got a new dht11 and the thermo is running great. very happy.

[ig65]

Tony, I got the kit put together and everything worked (all blue LEDs lit and tubes displayed temp in F and humidity) except I couldn't change modes using the IR LED. I unplugged it, checked all the connections, re-powered it and everything worked except no mode change and now it displayed in K. The next time I plugged it in only the blue LEDs lit up steadily. Now when I plug it in it does nothing (nothing lights up). I'm hoping it just changed to 'off' mode, and I am wondering where I should look to troubleshoot the mode change problem.

[Tony]

Starting with the easy things, did you make the solder bridge on pin 10 of the pic (last post in the assembly thread), it's easy to miss. Check the IR LED is lighting (there should be a faint red glow) and that the sensor is pointing slightly to the left of up (inclined towards the LED)

[ig65]

Ah, I had missed the solder bridge. Unfortunately that didn't fix the issue though. It looks like my IR LED is not working. What specifics should I look for in a replacement?

[Tony]

The LED used is an epitex l850-36UP but almost any medium power IR LED could be used in its place. They're usually very reliable though so make sure it's in the right way and there's nothing else that may be causing it to not work. What's the voltage across R2?

[ig65]

Across the pins of the IR LED I get 4.4, but across R2 I get 0.0.

[Tony]

Looks like it's either open circuit or in the wrong way - try turning it round just in case. If it still reads the same can you find a replacement locally or would you like me to send you one?. Any IR 10-60 degree LED will work though you may need to inrease the value of R2.

[ig65]

I turned it around, and now am reading 2.8 across R2, and 1.15 across the IR LED. Still won't display anything.

[Tony]

Is the IR LED glowing now?, if so could you post or email me a photo of the PCB solder side. The problem is almost certainly around IC3 or the IR LED.

If the solder bridge is missing noise from the inverter is picked up by the floating pin on IC1 and eventually tricks it into thinking it's a valid signal, it'll cycle until it switches off and the noise stops.