# DIY 555 LED Strobes

Written by joseph west
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The 555 timer is an old, but reliable integrated circuit primarily used as a timer or oscillator. Combined with only a few passive components, the 555 generates a wide range of frequencies and timer periods. The 555 operates in one of two general modes: astable and monostable. In both cases, the logic level of the output pin changes according to an input stimulus. In monostable mode, a high-to-low transition on the input pin produces one pulse on the output pin, whereas in astable mode the input transition produces a continuous series of pulses on the output. Astable mode is the appropriate choice for strobing an LED.

Skill level:
Moderate

### Things you need

• Three resistors
• Capacitor
• 5V supply

## Instructions

1. 1

Determine the length of time you want the light emitting diode (LED) to be on and the length of time you want it to be off. Call these values T-on and T-off.

2. 2

Choose a value for the capacitor (called C) in the circuit. You do not need a precise value for the capacitor because you will tune the circuit using the resistors. For many circuits, C = 0.1 x10^-6 F (or 0.1 uF) is a good choice.

3. 3

Compute the value for the second resistor (called R2) using the following formula: R2 = T-off / (0.693 x C).

4. 4

Calculate the value for the first resistor (called R1) using the following formula: R1 = (T-on - (0.693 x C x R2)) / (0.693 x C).

5. 5

Connect R1 between the VCC and discharge pins, R2 between the discharge and threshold pins, and C between the trigger and ground pins.

6. 6

Attach the ground pin to the circuit ground (or common), the trigger pin to the threshold pin, the reset pin to "5V," and the VCC pin to "5V."

7. 7

Conjoin a resistor to the output pin, then connect the LED between ground and the unconnected lead of the resistor. Make sure that the LED's cathode is connected to ground -- the cathode is usually the shorter lead. The proper value for the resistor depends on the size of your LED. Most small LEDs do well with 15 ma, in which case a resistor of about 250 ohms will work.

8. 8

Power up the circuit. The output should start oscillating, causing the LED to blink.

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