The 555 timer is one of the most widely known and simple integrated circuits (IC) on the market. It can be used in many ways using different modes (monostable, astable, and bistable) with each mode used to produce a specific effect. For this project, we're going to focus on using the 555 timer in astable mode for pulse-width modulation to produce a fading LED effect. The 555 timer's astable mode outputs an oscillating pulse signal that switches between on and off states at a frequency and wavelength dependent on the circuit's resistors and capacitors. The Highs and Lows of the pulse signal will create the fade in and fade out effects of the LED.
Step 1: Components
Here's what we need to get started:
-20k ohm resistor
-2K ohm resistor (Depending on your LED and how bright you want it, you might need a lower value.)
-NPN transistor (I used a 2N2222)
-jumper wires and breadboard
Step 2: 555 Timer
The 555 timer has 8 pins, each having a specific function. The pins are oriented according to the indicator dot on the chip. In the image, the dot is located on the bottom left corner of the chip, representing the location of Pin 1. The rest of the pins follow in order from left to right on the bottom, and right to left on the top with Pin 8 being directly above Pin 1.
Step 3: Wiring
Follow the circuit diagram making sure that the 555 timer is positioned correctly and all polarized components are oriented in the right position. Pin 1 is connected to ground(-). Pin 2 is bridged with Pin 6 and connected to the + end of the 220 µF capacitor. Pin 3 is connected to the 20K Ω resistor and linked between the NPN transistor's Base and the capacitor. Pin 4 is bridged with Pin 8 and connected to positive(+). The NPN transistor's Collector is connected to positive(+), while the Emitter is connected to the 2K Ω resistor. The LED is connected to the other end of the 2K Ω resistor and to ground(-). Pins 5 and 7 are not used. Check your circuit for any errors and connect the 9V battery. The LED should begin to fade in and fade out slowly.
Step 4: Changing the Circuit
The rate of the LED fading in and out can easily be modified by playing around with different resistor or capacitor values. You can even add a linear potentiometer or photo resistor in place of the 20K Ω resistor and see how it affects the LED. Removing the transistor will produce a blinking LED affect instead of the fading affect. You can also connect the circuit to an oscilloscope and visually see the pulse signal being produced by the 555 timer. Tinker around and have fun with your new pulsing LED!