EmonGLCD - red LED not working

I have just finished building an EmonGLCD, which appears to work OK following testing with the tester sketch and solar PV sketch (although I have not set it up to work with the base station and EmonTX yet). The only problem is that the red LED does not light during the 3s period of the tester sketch when it is supposed to (both green LEDs light OK).

As far as I can tell, all passive components are in the right place and all appear to be working OK. However, the voltage across the green LED terminals is 2.7V and across the red LED terminals is 3.24V. I can't see that they should be different as the only passives between the ATMEGA and LED are 100R resistors. Would anyone know what voltage is supposed to be supplied?

Thanks, Martin

Robert Wall's picture

Re: EmonGLCD - red LED not working

LEDs are current-driven devices - hence the 100 R resistor. If you download and look at the circuit diagram, you'll see that the Arduino is run off 3.3 V so 3.24 V across the LED indicates to me that it is drawing no current. Is the orientation correct? Check the circuit diagram and the assembly instructions.

If in doubt as to whether it should be on or off, you could always write a "one-liner" sketch (actually about half-a-dozen lines) to drive only the LED output to test it - or temporarily modify the sketch to write a permanent "HIGH" to the LED.

DrGannet's picture

Re: EmonGLCD - red LED not working

Thanks for the reply. Placing the contacts of a new LED in parallel with the contacts of the bi-colour LEDs (which are soldered into the board) lights the new LED. Therefore, it looks like I fried the red side when soldering them in (but surprised both went). I'll replace them at some point once I get the rest of the system configured, and the next Farnell order goes in.

Robert Wall's picture

Re: EmonGLCD - red LED not working

That sounds unlikely, but possible. Was your soldering iron not hot enough? I know it sounds silly, but there's less chance of damaging a component when you make the joint with the iron at the correct temperature and quickly, than if the heat is applied at a lower temperature for a long time to get the metal up to the melting point of the solder.

DrGannet's picture

Re: EmonGLCD - red LED not working

Thanks, that's an interesting observation. I was probably soldering with the iron about 50°C above the solder melting point. However, when soldering the LEDs, both partially slipped out of the holes to sit on long legs on the board, so I re-soldered them. Melting the solder on all three legs at once probably resulted in the LEDs becoming a bit warm...

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