PAGE 091 OF 150 · DISPLAYS & INDICATORS
Power Indicator LED
A small LED confirms a circuit has power.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Simplest indicator
A power indicator is an LED and resistor connected across the supply, showing that power is present. Place it after the switch if you want it to show whether the device is on.
Choose low current
Modern LEDs are bright at 2 to 5 mA. On 5 V with a red LED at 2 mA, R = 3 ÷ 0.002 = 1.5 kilohms. This saves battery life and avoids dazzling users.
Cautions
Use the right resistor for the supply voltage. Never connect an indicator directly to the mains.
REMEMBER
A seven-segment digit is seven small LEDs arranged to draw numbers.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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