A4: High(er) voltage and transistors

Geon Soo Park

Schematic

For Schematic I only drew 12 LEDs instead of 30 LEDs; to make the schematic simple

For the photoresistor, I used 10k resistor. The maximum current allowed to go into the ground pin is 200mA and I chose 5V for the power. R = V/I = 5v/0.02A = 250a. We need a resistor bigger than 250a. Furthermore, we need to use a resistor divider to allow the high impedence Anlog input to measure the voltage. We need to use the formula: Vout=Vin*(R2/(R1+R2)) to find the value for Vout. Vin is the 5V, R2 is 10k ohm, and R1 is the photoresistor (10k ohm).

For the LEDs, I used 30 LEDs. Since 300 LEDs equal to 18 watts, 30 LEDs euqal to 1.8 watts. I used 12V for the power.
Therefore, by using the formula P = IV, we can find the current flow through the transistor. 1.8 = I(12), so I = 0.15A. Since the transistor can provide max of 0.6A of current, the circuit is safe.

For the resistor for base (connected to pin 9), I chose 10k ohms, because even without considering the voltage drop from the LED strip; R = P (12v)/I(o.o2A) which is 600 ohms. To secure my Arduino I chose 10k ohms for the resistor. I also calculated the current flow for each LED strip.

Circuit

The photoresistor is connected to the 5V and connected to the ground pin through a resistor. It is also connected to pin A0 which functions as a trigger the input for the photoresistor.

The positive side of power supply (12v) is connected to the positive side of LED strip and the negative side of the power supply (12v) is connected to the ground. The base of transistor is connected to pin 9 through a resistor, collector part of transistor is connected to the negative side of LED strip, and the emitter part of transistor is connected to the ground.

Firmware

Two integels are initiated one for LED strip (#9) and one for the photoresistor (A0)).
There is one output from the Arduino board pin modes (#9) and one input for the photoresistor (A0).
If the photoresistor is less or equal to 300, it turns the LEDs on.
If the photoresistor is bigger than 300, it turns the LEDs off.

Operation

When the photoresistor value is equal or below 300 (dark) then it turns the LED strip.
If the photoresistor value is over 300 (bright) then turns off the LED strip

Thank you and have a nice day!