Dimming Pumpkin

I got the schematic for this project online.

A transistor is used with Arduino to dim a strip of white SMD LEDs. The timing can be changed by programming the Arduino. A transistor can be used either as a switch or as an amplifier. In this application, its used to slowly amplify voltage from 0-12V and gradually back to 0V in a set amount of time. The transistor used here is a TIP120 from Fairchild, which is perfect for Arduino and this application because the Emitter-Base voltage is 5V. Basically this voltage acts like a tap and modulates the input voltage, in this case its 12V-0V.

Arduino can give PWM (Pulse width modulation) from 0-5V , which in the program in the Arduino IDE is 0-255. The rate of change isn't perfectly smooth, but it will do.

Just remember, the current adds up as you add more LED. A typical 5 meter strip can run up-to (60ma X 150) which is 3 Amperes! Luckily the TIP120 can deliver upto 5A but the chip runs quite hot(about 80 degrees). I was only using 1A, so there was no problem there. I used a 20AWG wire (basically jumper wires) and they are rated to take at least 5A ,so that's safe.

For a 12V source , I used my lead acid battery. Make sure the leads don't make contact in the breadboard. Otherwise there will be lots of sparks and melted plastic.

You can tinker with the timing setup here to your level of spookiness. But this worked for me!

const int transistorPin = 9; // connected to the base of the transistor
void setup() { pinMode(transistorPin, OUTPUT); }

void loop() {

for (int brightness = 0; brightness < 255; brightness++) { analogWrite(transistorPin, brightness);

delay(5); }

delay(5);

for (int brightness = 255; brightness >= 0; brightness--) { analogWrite(transistorPin, brightness); delay(20); }

delay(2000); }