The LM3914N is a chip specially designed to drive a bargraph display. The basic IC operates as a linear voltmeter with a 1.2v range, using 10 leds as the scale. Our need is to get full power showing all 10 leds lit, with just the bottom led lit when the fan controller is turned right down to around 6v.
To get the maximum input scaled down to suit, we use a variable potential divider VR1 on the input from the fan controller.
To raise the zero point, we add a resistor R2 between ground and the "lo" pin (#4) of the IC. The chip contains a chain of 10 x 1k resistors marking off the lit steps between zero and full-scale; we add to the first resistor in the chain (between zero and the first led pin) so there is another potential divider with 9k above the "led1 on" point, 1k+R2 below it.
The only other components needed are the 1k2 resistor R1 that sets the led current to 10mA and a couple of capacitors to get rid of any noise on the power or signal lines.
Here's a stripboard layout and construction plan with the parts list.
| IC1 | LM3914 and 18-pin DIL socket |
| C1 | 2.2uF 16v tantalum bead capacitor. |
| C2 | 0.47uF 16v tantalum bead capacitor |
| R1 | 1k2 resistor, 0.25W |
| R2 | 6k8 resistor, 0.25W (see notes) |
| VR1 | 10k horiz preset eg Maplin UH03D |
| D1-10 | LEDs to suit. |