Great Forum - Much Read - Thanks All. I have issue with External power to following set up: UNO R3 SIM900 Shield TFT Screen Touch Shield V2.0b It's a Phone and all works fine when USB connected to UNO.
Will not work with external power connected to 2.5mm Plug. I have tried just about everything I can think of: 1. 9V Battery 2. 9V from Mains Adaptor 3. 12 V from Mains - UNO gets warm 4. 5V regulated from all the above 5. 3.7V 2000Mah Battery Direct - Brand New fully charged - giving 4.56V checked
What happens is: 1. UNO fires up 2. SIM900 fires up 3. Screen fires up but 'dimmer' than USB powered 4. When SIM900 gets connection to phone network - screen reboots
It is like there is just not enough power AMPS getting through - hence Dim screen when any external power is connected. I have tried several UNOs and all same result.
Being a Phone I really need a Battery supply of some sort connected to either the 2.5mm conn or VIN/GND.
Any suggestions welcome. Thanks