# Tachometer

#### alanebro82

Joined Apr 4, 2007
3
I am using a LM2917 to sense input frequency, and give me a corresponding output voltage. Currently, it is working quite well and giving me an extremely linear output. Now I need to display this in a digital readout. I have an ICL7107 (3 + 1/2) chip. Basically, my problem is thus: how do I configure it such that the lowest necessary reading is 1000RPMs (this corresponds to a voltage of 4.14mV) and the highest necessary reading is 6000RPMs (this corresponds to a voltage of 6.29mV)?

So far, we were thinking of implementing it by having the readout be 10s of RPMs, such that a display of 0600 => 6000RPMs and a display of 1600 => 16000RPMs.

Any advice would be greatly appreciated.

#### alanebro82

Joined Apr 4, 2007
3
Here is a picture of my desired output based on the voltage I have from my frequency to voltage converter.

#### beenthere

Joined Apr 20, 2004
15,819
If you have a frequency input, why convert it?. Apply it to a counter chain for an accurate time period, and get RPM directly.

#### alanebro82

Joined Apr 4, 2007
3
Because it is a project to build a tachometer. We are simulating a rotating shaft by using a frequency to voltage conversion on a signal from the signal generator. In practice though, a sensing device would most likely not output a frequency, but a potential difference based on how fast the shaft is rotating. Therefore, we are trying to do it this way.

#### hgmjr

Joined Jan 28, 2005
9,027
alanebro82 said:
a sensing device would most likely not output a frequency
It is actually not that unusual for the speed output from a shaft to be in the form of frequency. One method of obtaining shaft speed is to attach a "shaft encoder". The output from a shaft encoder is a squarewave whose frequency is directly proportional to the rotating speed of the shaft to which it is connected.

There are even quadrature output shaft encoders that can be used to determine the direction of rotation when used with some fairly simple digital logic gates.

hgmjr

#### vane

Joined Feb 28, 2007
189
im loving the sound of this, from what i can understand why not just get a sticky label with a zero on and stick it at the end of the digital read out