Evening gents, I am somewhat of a novice with electronics, but nonetheless am working on a temp. controlled computer fan but I've run into some trouble and need some help! My basic layout is a thermistor implemented as a voltage divider (with a resistor of course) which feeds into an amp (op-amp). Then that signal goes into a primitive ADC that I made out of 5 comparators for increments of 10 degrees celcius (matched different reference voltages with thermistor divider according to factory specs). The next step is where I'm having trouble. So now I basically have 5 bits (TTL at 5V), and I want to have different fan speeds for each bit on (I have tested this part of the circuit and everything is good). So when the first bit is a 1, I want the fan to start but be low speed, and when the second bit lights up I want the fan to increase in speed a little bit. This continues till I am at a full +15 volts across the fan for about 2W of power. Could someone suggest a way to implement this. I am thinking about using CMOS switches all leading into the fan lead. Or maybe just power mosfets or BJTs?