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LED Fader – Easy



Building the circuit: A pack with all the parts is available here

To build this circuit you will need:

Ω      One 9V battery

Ω      One 9V battery clip

Ω      One tactile switch

Ω      One 4.7KΩ resistor

Ω      One 150Ω resistor

Ω      One 50KΩ potentiometer

Ω      One 470μF capacitor

Ω      One NPN transistor (BC548)

Ω      One LED (any colour)

Ω      One breadboard

Ω      Some hook-up wire


The circuit:

LED Fader circuit diagram

Above you can see the circuit diagram for the circuit. You can see the 4.7KΩ resistor after the switch which the capacitor is charged through, and the variable resistor (for which we are using a potentiometer with only two pins connected) around the capacitor for it to discharge through. The 150Ω resistor is placed before the LED as a current limiting resistor after the transistor.


Start by placing some wires in the breadboard as follows. With two wires one space apart, bridging the central gap in the breadboard. A wire from positive voltage should then be placed into the space one to the right of the rightmost of these. A wire to ground should be connected to the left most of them. A wire to positive voltage should then be placed onto the bottom half of the breadboard somewhere to the right of the others.


LED Fader Breadboard photo


Then place the tactile switch bridging the middle gap on the breadboard from the red wire to the 4.7KΩ resistor which should be connected to the rightmost green wire. Add in the 150Ω resistor from in-between the two green wires to a space to the left of them and an LED from that space to ground (making sure the long leg joins the resistor. Finally add in a transistor with its middle pin on the same track as the right-most green wire.


LED Fader Circuit Breadboard image


Add in the capacitor with one leg going to each of the green wires, make sure the white stripe is facing left!


LED Fader Circuit Breadboard image


Finally add in the potentiometer with the middle pin and one of the outer pins connected to the green wires, it doesn’t matter which way!


LED Fader Circuit Breadboard image


Plug in your battery and press the switch, the LED should fade in slowly then once you release it, it should fade out, the fade out time can be adjusted by turning the potentiometer!




LED Fader Circuit Breadboard image

Overview/How it works:

To understand this circuit you first must have a good basic understanding of what a capacitor is and how it works, a capacitor is like a little battery that charges when a voltage is applied and releases that voltage when the charge is taken away! Capacitors charge quickly at first then get slower as they get fuller, this means that the voltage on the positive pin starts off at 0V (when the capacitor is charging), and as it takes in less and less voltage to charge the capacitor the voltage becomes greater and greater, this makes them great for timing circuits because the time taken to do this can be controlled very easily with a resistor and different value capacitors.

 When the switch is first pressed the capacitor starts to charge through R1 (4.7KΩ) and so as it charges the voltage on it’s positive pin is slowly raised, thus raising the voltage on the base of the transistor and allowing current to pass through increasingly to the LED through R2 (which is a current limiting resistor). This makes the LED fade in from nothing! Once the capacitor is fully charged and the switch still held the LED will simply stay on. When the switch is released the voltage on the positive pin of the capacitor slowly goes down again as it discharges through R3 (which is a variable resistor), we can vary this resistor to change the time it takes to fade out each time. The higher the resistance, the slower the capacitor discharges!  The slowly lowering voltage is also applied to the base of the transistor, which lets less and less current through to the LED and so fades the LED out.

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