Hi everyone (also welcome back to the people who previously know the last post ;))
I’ve been working on a infrared comms board for my arduino robots (making more than one to work together) Focusing on the infrared comms part, there are two parts, the infrared phototransistors and the emitters, eight of both on that one board. The infrared phototransistors work fine, however, I’m concerned about the infrared emitters and amount of current they’re blasting through. The product I’m using for the phototransistors and emitters are: Infrared Emitters and Detectors - SEN-00241 - SparkFun Electronics There is an attachment below for the schematics for the whole thing. Anyways, the schematics reads that for each emitter there is a resistor. Each resistor value is 180 ohms, the voltage passing through is about 5 volts, and current is about 360 milliamps. As you can see, at the bottom of all the emitters is a NPN transistor. The one i’m using is: https://www.adafruit.com/datasheets/PN2222A.pdf. I have just read articles on saturation voltage and how there needs to be the right voltage for the base to be saturated and fully transfer current from the collector to emitter, literally turning the gate on and off. The schematics shows that there is a resistor at the base of the transistor, and I presume current is also coming through the base up to the I/O pin on my arduino. Also looking at this graph, its says something else, but the resistance needs to be correct for the transistor to be properly saturated.
Using ohms law, calculating resistance is easy as doing R=V/I. Then it is R=5/0.36, which gives me a resistance value of 13 ohms. To me, this does not look right, at all. 13 ohms to bring down 360 volts seems, off. Now, obviously, this isn’t the way to used ohms law, because all this is is calculating the current resistance with the parameters given. My question is how to find the resistance for a current that you are aware of, and want to bring down that current to something that the arduino can handle, which is 40 ohms for the I/O pins and 200 ma for the ground pin.
Another issue is besides trying to saturate the base pin, why isn’t there a resistor on gnd? is it because the current coming through the base to emitter will decrease if there is a resistor on base? I’m still following diagrams like the one below in the attachments, and the schematics.
Anyways, the biggest issue is limitting current for the transistor to supplement the transistor, and not blow up my arduino by overloading the gnd and I/O pins. I tried testing this on a breadboard with an extra arduino on hand, have a dysfunctional GND and I/O pin on the arduino, and the transistor does get hot, a little, before I turned the whole system down.
Sorry for bothering everyone again. I tried googling, and I think I need more help because I don’t understand how the transistors work. Sorry if I’m being unintelligent and not using enough google.