Level Shifting - Signal Scaling - Scale Analog Signal Voltage

Question

I have a variable voltage called Vreg (Voltage regulated) which is output of switching regulator (buck converter) and is less than 12v.

Also there is ANALOG Signal with voltage between 0 < V < Vreg < 12v. (Vreg is less than 12v). I need to scale my ANALOG signal from 0 < V < Vreg span to 0 < V < 5v span so that I can read it directly with Arduino. Also need it be economic with few parts or easily available parts like very common op-amps etc. Signal frequency is less than 50 KHz.

Further Clarification:

The problem is that I have a DC FAN with an integrated RPM sensor. It have three exposed pins.

• GND
• VCC
• RPM Signal (Tach OUT)

Module Contains a DC motor, and sensor output is analog. Internal schematic of module is like this:

image taken from here

I need to control the rpm of motor, but the module do not have such capability so i need to play with input voltage VCC to module. I've checked and it did work to control RPM. It can go down to near 0v, but the problem is with Tach pin's signal which have no sufficient voltage if module input voltage is near 0v. How i can overcome this?

I need an Intermediate circuit for this, and cannot use extra pin of UC like some answers below.

Thanks.

Answer 1

It sounds as though you want to know the ratio of your analog sensor's output in relation to your supply voltage.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Use a pair of 12:5 potential dividers to divide both the supply voltage and the analog sensor voltage by the same amount. Calculate the ratio in software.