Can realize 6. 7 v-12 v dc speed a bit circuit basis can make very simple
As for 24 V dc governor can realize everyone please hand begins to practice it
The components and general price:

The price difference but should not all are a lot

1 the number four op-amp lm324 * 1 0. 8 yuan
2 rotating potentiometer * 4 variable resistor b10k best buy to take a switch function that can realize the function of closed fan without the switch 1 yuan a belt to switch the more expensive 3 yuan
3 led two take over by five
4 resistance
(1) 470 1/2 w 4
(2) the 12 k 1/4 w 4
(3) 6.8 k 1/4 w 1
(4) 470 1/4 w five
All is 0. 1 yuan
5 level 2 tube
4148 a 0. 1 yuan
Lh-zd 6. 2 v 1 0. 1 yuan
6 capacitance
(1) 25 v 330 uf 4 0. 2 yuan
(2) 25 v 1 0 100 uf. 1 yuan
7 grade 3 tube
Tip42c 4 1 yuan one
8 level 3 tube heat sink 4 0. 2 yuan a
9 3 needle fan socket four
10 4 needle 12 v input jack one
11 bread plate or experiment 8 yuan a board

A circuit diagram. 4:

Offered words of complex can see the single road:

324 is the number four op-amp 1-3 5-7 8-10 12 to 14. 4 legs and 4 legs for 11 12 v power input for 11 solution is negative level first single road and then put the other three road well is simple picture of the first of several more led to don't mind first part well test can speed and then add led is comparatively easy

Finished product figure: