# How to Add Delay to an LED

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A simple way to add turn-on and turn-off delay to an LED circuit is to use an RC circuit.

A simple way to add turn-on and turn-off delay to an LED circuit is to connect a resistor and capacitor in series to form an RC circuit. It takes time for a capacitor to charge/discharge through a resistor. The time is determined by T = R x C, where R is the resistance in ohms, C is the capacitance in Farads and T is time in seconds. The value of RC is actually the time it takes for the capacitor to be charged to 63 percent of the supply voltage.

## Step 1

Calculate the resistance and capacitor values based upon the desired delay time. For example, assume the supply voltage is 3 volts, the LED forward voltage is 2 volts and the current through the LED will be 10 milliamps. Then the LED forward voltage will be close to 63 percent of the supply voltage. If we use a 300 ohm resistor and 10,000 microfarad capacitor, there will be a delay of approximately 3 seconds (RC = 300 ohms x .01 Farad) after the power is applied before the voltage at the junction of the resistor and capacitor is equal to the LED forward voltage.

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## Step 2

Connect one lead of the 300 ohm resistor to the power strip at the top of the breadboard.

## Step 3

Connect the other lead of the 300 ohm resistor to the positive lead of the capacitor.

## Step 4

Connect the negative lead of the capacitor to the ground strip at the bottom of the breadboard.

## Step 5

Connect the positive lead of the LED to the point where the capacitor and resistor are connected.

## Step 6

Connect the negative lead of the LED to the 100 ohm resistor.

## Step 7

Connect the other lead of the 100 ohm resistor to the ground strip.

## Step 8

Connect the negative (black) lead of the battery holder to the ground strip.

## Step 9

Connect the positive (red) lead of the battery holder to the supply strip. There should be a delay of approximately 3 seconds between the time you connect the red lead and the time the LED turns on.

## Step 10

Remove the red lead of the battery holder from the supply strip. There should be a shorter delay of approximately 1 second between the time you remove the lead and the time that the LED turns off.

#### Things You'll Need

• Yellow LED

• 2 resistors

• Electrolytic capacitor

• 2 fresh C or D batteries and corresponding battery holder
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