Power Supply: 12V DC to 5V DC
The main purpose of this power supply is:
- to provide connections for 12V devices;
- to convert the 12V DC (from the 230V-AC-to-12V-DC power adapter) to the 5V DC required by the LCD display and the ultrasonic sensor.
Any 12V to 5V step down voltage converter capable of providing a few hundreds of mA will be OK:
| Device | Voltage | Current |
|---|---|---|
| Ultrasonic sensor | 5V | 15mA |
| LCD display | 5V | 100mA (backlight ON) |
Another reason this power supply is required is to avoid excessive current (and heat) in the 5V linear regulator (NCP1117) on the Arduino board. At 12V @VIN, if the sensors/actuators were powered by the Arduino board, the 5V linear regulator would dissipate more than 1W of heat [(12V - 5V) * .115A = 0.805W + Atmega328 + LED + RFID]. The NCP1117 in DPAK package has a thermal resistance (junction to ambient) of 67°C/W resulting in a junction temperature of 92°C at 25°C.
Another advantage of this arrangement, the microcontroller and the 5V peripherals have separated power lines.
(next - Arduino UNO)