# Easiest method to convert analog ac voltage or frequency to digital 0-5v dc?

#### Circuitz

Dec 7, 2013
24
Hello there-

I have a permanent magnet inductive sensor and tone ring setup for measuring rotational speed that outputs AC sine wave voltage in the range of 0.65V to 8.5V and frequency from about 37Hz to 1.5KHz.

I am wanting to convert this output to be microcomputer controlled so I need 0-5v DC or at least a safe frequency input. Could I just use an opticoupler for the frequency directly or do I need to scale the voltage to 0-5 volt dc with a frequency to voltage chip? Or rectifier and voltage divider from 8.5V ac to 5 volt dc?

Thanks!

#### AnalogKid

Jun 10, 2015
2,860
For a constant 5 V rectangular wave at whatever the input frequency is, you need some kind of comparator circuit. This could be a simple as one transistor, or as complex (actually, still pretty simple) as a comparator IC circuit.

Is the voltage range RMS, peak, peak-to-peak?
Does there have to be galvanic or ground isolation between the sensor and the 5 V output?
Where are you located?

ak

#### BobK

Jan 5, 2010
7,682
Do you need to measure both the frequency and the amplitude, or just the frequency?

Bob

#### Circuitz

Dec 7, 2013
24
I am in Ohio. I do not know if I have a square wave or sinusoidal wave as I do not have a scope. I am reading this with a digital multimeter. I would think it would gradually build like a sinusoidal wave as it is an induced current from a magnetic field. I do not know why it would ever go negative though. I would like this to be passively monitored, yes, if that is what you are referring to about ground or galvanic isolation (opto coupler?) I could measure either frequency or amplitude, I do not need to monitor both. Thanks in advance for your help!

#### BobK

Jan 5, 2010
7,682
it is an induced current from a magnetic field. I do not know why it would ever go negative though.
An increasing field produces a voltage in one direction, a decreasing field produces a voltage in the opposite direction. So, unless your field is always increasing, or always decreasing, it will produce both positive and negatve voltage.

Bob

#### Circuitz

Dec 7, 2013
24
Thanks for the explanation BobK- that makes sense! Looking at voltage I am not interested in the negative amplitude as it should be just a mirror of the positive amplitude so could a rectifier/comparator circuit clip that?

#### BobK

Jan 5, 2010
7,682
Depends on the voltage, a rectifier would need a signifcant fraction of a Volt before it would conduct.

Bob

#### dave9

Mar 5, 2017
1,188
Considering that this circuit seems to be for a track race car, I would advise you to buy an off the shelf product that does whatever you're trying to do.

I am doubtful your insurance covers homebrew mods to the braking system and one mistake in testing - even if the circuit works as intended, could mean thousands in losses if not life and limb.

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