Q1 in that circuit is redundant, because of the value you have chosen for the base resistor of Q2. You could just as well drive Q2 from the Arduino pin through a 1k base resistor. Although I would reduce the base resistor to about 470 ohms, to make sure that Q2 turns on fully.
dc42:
Q1 in that circuit is redundant, because of the value you have chosen for the base resistor of Q2. You could just as well drive Q2 from the Arduino pin through a 1k base resistor. Although I would reduce the base resistor to about 470 ohms, to make sure that Q2 turns on fully.
TBH that circuit comes from an experiment I made (too) many years ago... I was using the parallel port of 286 and writing to it with qbasic 50-or-so lines "program" 
I'll try your suggestion.
Thanks!
Ok, so could I do this circuit with a TIP120 Darlington?
I'd imagine the circuit would be Pin 6 to Base, Fan Ground to Collector, Emitter to Ground? Would I need a pull-down resistor to Ground on the Base?
dc42:
Q1 in that circuit is redundant, because of the value you have chosen for the base resistor of Q2. You could just as well drive Q2 from the Arduino pin through a 1k base resistor. Although I would reduce the base resistor to about 470 ohms, to make sure that Q2 turns on fully.
I tried you suggestion and it worked (I had no doubt 
)
It seemed to make no difference if I used a 1k, 470 or even 220 ohm resistor between pin 6 and bd135 base.
Thank you
Edit: for the record, with a 100uF 25V connected between the fan wires I could silence it at low speeds. The relationship between pwm raw value and fan speed changed - now I can go as low as 16, provided the fan is already running, and the start pwm value is now about 50.
tuxduino:
Edit: for the record, with a 100uF 25V connected between the fan wires I could silence it at low speeds. The relationship between pwm raw value and fan speed changed - now I can go as low as 16, provided the fan is already running, and the start pwm value is now about 50.
Be careful, with that capacitor in place you may easily exceed the current rating of the BD135 at the start of each PWM pulse, especially if you use a low-value base resistor. Also, the capacitor will make the fan run faster at low PWM values - which is why you can now go down to 16.
To silence the fan, it would be better to use a PWM frequency above the audible range, say 25KHz. If the fan doesn't work well at that PWM frequency, then an inductor and capacitor can be used to smooth the voltage. Or (more simply), buy a PC fan with a separate PWM input.
Thanks for the info!
Or (more simply), buy a PC fan with a separate PWM input.
The "buy" option is good when you don't want to waste time in details and get a project done. But I'm just experimenting with some electronic "waste" I have accumulated over the years from my dad's shop... 
sorry for me to interrupt. but for me since the motor would be always on then why not use a relay?
connect the red wire to the comment of a relay and the black to ground. on the one of the N.O. just put a 12VDC or the V max of the motor while the other use a well any kind of volatage converter you know to power that motor. just put a transistor and a diode to activate the relay. this will make it run at different speed.
As you can see I'm no expert when it comes to electronics, let alone motor control, but thinking about the pros and cons of a relay vs. pwm, some words come to mind: cost, noise, reliability (no mechanical failure for the transistor option).
I'm just thinking out loud... I'm curious too about the answer from the gurus 
my 2 1 cent.
technically i would say that yeah i would always choose the pwm way. but if your the type that is result oriented then the method i suggest will get you far enough.
I set a PWM circuit up using a FET and any time I try to slow the fans down (4 120mm PC fans, 350mA) at all they start making an unhappy noise. Any idea's? The fans are definitely a brushed motor.
Thanks