My issue here is that my Arduino Uno shuts down right after plugging it in. I have DHT11, an i2c LCD display, and a 2 channel relay plugged into it. Connected to the channel relay is a heat pad. I have already read that the possible solution for this is to add an external power supply. How do I add an external power supply and specifically, what battery voltage should I use?
I've been using the 5V pin and connected it to a breadboard, then all of the components are connected to it.
Can you please post a copy of your circuit, a picture of a hand drawn circuit in jpg, png?
Hand drawn and photographed is perfectly acceptable.
Please include ALL hardware, power supplies, component names and pin labels.
Do you have a DMM? (Digital MultiMeter)
What are the specs/data for the heat pad?
Where is the power for the heat pad coming from?
Your relay is at this moment powered from the Arduino. You will need a separate 5V supply and remove JD-VCC. See below image (taken from Relay module effecting Arduino).
Can you please post a copy of your circuit, a picture of a hand drawn circuit in jpg, png? Hand drawn and photographed is perfectly acceptable. Please include ALL hardware, power supplies, component names and pin labels.
It will be so much easier to understand, especially if you LABEL pins and components.
If you leave the mains to the heater disconnected, does the fault occur?
Do the schematic from what I can see you have shorted a lot of things out. Here is some simple guidellines: Gil's Crispy Critter Rules, they apply to processor hardware:
Rule #1. A Power Supply the Arduino is NOT!
Rule #2. Never Connect Anything Inductive (motor, speaker) to an Arduino!
Rule #3 Don't connecting or disconnecting wires with power on.
Rule #4 Do not apply power to any pin unless you know what you are doing.
LaryD's Corollary's
Coro #1 when first starting out, add a 220R resistor in series with both Input and Output pins.
Coro #2 buy a DMM (Digital Multi-meter) to measure voltages, currents and resistance. Violating these rules tends to make crispy critters out of Arduinos.
Hint: It is best to keep the wires under 25cm/10" for good performance.
The heater can work as long as it's the only one connected to the Arduino. But, when the DHT11 and the LED display was connected along it, it becomes faulty.
It's about mA, not mAh. You can measure the coil resistance; divide Vcc (5V in case of the Uno) by the coil resistance and you will have the peak current. If you cater for that it will be sufficient.
If you power from e.g. USB, mAh does not come into play. If you use a battery, mAh comes into play. You will have to think "I want this to work for "X hours"; multiply the required current calculated above by the hours and you will have the required mAh rating of the battery.