Arduino + Toyota bj40 1979


I have just finished a complete restauration of an old car.
Toyota bj40 from 1979

Now i want to add some automatition to the car.
First i want to change the heater blower.
The blower has only two speeds. Slow and fast.
Fast is 24vdc directly to the blower
Slow (1/2 fast speed) is a resistor 4 OHM 50 watt in serial with the blower engine.
I want to control the blower speed with an arduino.
What does i need betweeen arduino and blower engine?
Electronics is complete new for me!
Thans for the advice

Since the blower runs in only one direction a logic level MOSFET controlled with PWM would work. You will need to know or measure the stall current of the motor to be able to choose the right MOSFET.
A potentiometer or rotary encoder connected to the arduino could serve as the speed control.

Here is an example of the MOSFET motor driver.
low side driver.jpg

Does i place and ampmeter in serie with the blower motor, to measure the stall current? At high or slow speed?

Stall current is also called "locked rotor current". To measure with an ammeter you hold the motor so that it cannot turn and apply the full supply voltage. There is an easier way to estimate the stall current that will be close enough to use to choose a driver MOSFET. You measure the motor winding resistance with an ohmmeter. Then use Ohm's law to estimate the current.

Stall current = motor supply voltage / measured resistance.

When measuring the resistance rotate the motor slowly while watching the meter. Note the lowest reading.

Now you are thinking, "The fan will never be locked, why use the stall current to choose a driver?" The reason is that the motor draws stall current, briefly, every time that it starts.