But, everyone is saying I don't really need it. I'm trying to do everything I can to protect my GPIO pins which is why I was sticking the zener diode in there as one last line of defense. Also the same with the base resistor for a transistor, I'm trying to study and understand the "how" of that sizing.Īs for my input circuit, maybe I should take a step back and explain what and why of what I'm looking for. I am probably not looking in the right place in the data sheet but I would like to understand the "how". R2 goes into the base of T1 which will turn on D2 led from the GPIO 5v line.īased off feedback above here is my comments and questions.ġ) The datasheet for D2 says it has an internal resistor for 5v, does that mean I don't need a current limiting resistor in that loop?Ģ) I already have the D2 LED's, and they are 5v (13mA), but if I was doing something else doesn't the 5v header have room for more total current than the 3.3v header?ģ) It was suggested I remove D1, is there any reason it would hurt to leave it in there?Ĥ) How is the current limiting resistors R2 and R3 supposed to be calculated?Īs what was said by gordon77 that loop is 24v and not 3.3v, but what I'm really trying to understand is where the 10mA came from. OC1 will then allow current to go from the GPIO 3.3v to R2 and R1, R1 obviously going into the GPIO. That will trigger the optocoupler relay (OC1). That is the external circuit shown to the left of the red dashed line. I will have a hall effect switch that will be triggered on a 24vdc loop. At any rate, hopefully this helps explain what I am wanting to do better. ![]() I used ProfiCAD to draw the circuit which was way better than. Ok, hopefully I have made this a little bit easier this time.
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