# 12V variable-speed fan as a function of temperature using 555 timer

I would like to turn the car fan into a variable-speed fan using a the analog temperature sensor of the car, and a 555 timer.

While temperature decreases, resistance (of the sensor) increases. Can I use this information in a ratio R1/R2 (below) to change the pulse width of the 555 timer, and by that changing the speed of the fan? Can this be done?

• R1: the resistance of the sensor
• R2: determined resistance
• – Jack Creasey Oct 22 '17 at 16:06
• This is unlikely to work as you propose. When you slow the fan the air will pass more slowly over the heater coil and will come out hotter. When you speed it up the temperature will be cooler but there will be more of it. You need to control the temperature of the heating coil instead and this probably means motorising the valve. – Transistor Oct 22 '17 at 17:11
• What heater coil do you mean ? The fan is mounted on the car, it's main objective is to cool the radiator of the engine, news cars are mounted with this system, old cars are not, it's just an idea or an application I'd love to do, i hope that we are on the same page(idea) – sufyan Oct 23 '17 at 9:01
• You should have said which fan you are talking about. @Transistor thought you were talking about the passenger compartment heating fan rather than the cooling fan. – HandyHowie Nov 14 '17 at 8:23
• Handy's interpretation of my comment is correct. – Transistor Nov 15 '17 at 21:40

Assuming you're using the standard 555 Astable PWM circuit that makes use of two diodes to permit duty cycles lower than 50%, you can figure out what resistance you need knowing that the duty is $D=\frac{R1}{R1+R2}$.