I have (I guess) I simple problem about electricity. I want to power a laser pointer through Arduino. This laser pointer works at 3V and 50mW. I am going to explain what I have done to solve the problem (but obviously is not working). Sorry if my english is not enough correct.
I put and OUTPUT pin on HIGH and the voltage is 4.9 V which is 5V. Then I thought that putting a resistance in front of the laser pointer make decrease the voltage until 3V. I counted the resistance in that way:
Voltage of decrease = Voltage of Arduino - Voltage of laser -> V = 5 -3 -> V= 2V
W = V * I -> 0,05 = 2* I -> I = 0,025 A
R = V/I -> R = 2/0,025 > R = 80 Ohms
So I put a 80 resistor in front, but it seems that the resisitor is not decreasing the voltage, and the laser pointer doesn't work.
You are trying to construct what is called a "voltage regulator" by using a resistor, which is not a good way to do it. You will see a lot of variation depending on current. Also your calculations are not quite correct. You are computing 2*I = 0.05 but the "2V" represents the voltage dropped across the resistor, not by the laser. Your calculation should be I=P/V --> I=0.05mW / 3V=16.7mA. Then, R=V/I = 2/0.0167=120 ohms.
But you should not put so much faith in the 50mW number. It is probably an upper limit and not a guarantee.
It would be better to build a voltage regulator out of a device like an LM1117. Then you will know you have 3V no matter what.
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If you mean the laser pointer is 50mW output then it will take far more current than the Arduino pin can supply. Laser diodes are typically specified by their light output, not the electrical input power. Efficiencies are low.
What is needed to drive a laser diode is an accurate current source (its very easy to burn out a laser diode by over current).
As others have pointed out this is totally the wrong way of doing things. Are you sure you have not damaged your laser pointer, you will be very lucky if you have not.
For driving lasers look at this:- http://www.repairfaq.org/sam/laserdps.htm
Thank you for al your replies. I've never thought that it could be that difficult. The laser pointer is still working. I took a look in that website and as I am not very good in electronics, I changed the way I will power the laser.
I will keep on powering trough 2 AA batteries but I will turn on and off trough a relay controlled by arduino. In that way I make sure that the voltage and everything remains the same.
