I wanna run a dc motor 2 speeds and off , hi low off, manually switching between two speeds with spdt switch. How do use the Arduino to do this?
Have Arduino control a simple relay.
https://www.amazon.com/Tolako-Arduino-Indicator-Channel-Official/dp/B00VRUAHLE/ref=sr_1_18?crid=2U9IOZ325LO7I&keywords=relay+board&qid=1567177919&s=gateway&sprefix=relay%2Caps%2C149&sr=8-18
(forum might some extra characters on the beginning and end of the link)
Think
I would use PWM to achieve the two speeds . Drive a transistor from an analog output to switch the motor.
Hi,
Welcome to the forum.
Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html .
A spdt switch only has two positions, how do you aim to get two directions and OFF.
The spdt switch will have to have a centre OFF position.
Can you post a link to specs/data of the motor please?
Can you tell us your electronics, programming, arduino, hardware experience?
Thanks... Tom... 
A single pole, double throw, center off switch and a resistor will easily accomplish that. Why post on an Arduino forum????
Paul
Please don't wander around the forum with this particular topic or your stay here may be short lived.
Bob.
Thanks