Measuring DC Motor Load Resistance - Selecting the Right Switching Transistor
I am an electronics noob, although I have done several Arduino based circuits, the stuff I have done is all a bit "painting by numbers" and I feel the need to understand more about what I am doing. I have searched the internet for an answer but can't quite get all the info I need. Time to ask for help!
I am building a circuit (without a microprocessor) where a PIR sensor will start a small 3v DC motor (from the junkbox) when the sensor is activated (PIR output pin goes high). The most helpful page I stumbled upon so far (because I think I understand it) is here:
Under the heading: Choosing a Suitable NPN transistor, where it gives a procedure for selecting a transistor.
The first step is to ensure that the transistor's maximum collector current is greater than the load current, where load current is calculated by dividing the supply voltage by the load resistance.
The thing that is tripping me up is: how do I measure load resistance of the motor? My hunch is that I just measure the resistance across the positive and negative terminals of the motor whilst it is disconnected. Can it be that simple I ask myself.
I appreciate, I could just try any old NPN transistor and see if it works, but that will leave me no better off for the next time.
Thanks for any help you can give, be they answers or other places to look!