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From: "Meindert Sprang"
Subject: Re: frequency to voltage converter
Date: Fri, 17 Jan 2003 09:08:14 +0100
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"anotherbrick" wrote in message
> i need a circuitry , which will build a speed signal from coming encoder
> for example i have a 2500 pulse encoder which is connected to a motor
> when the motor runs at 100 rpm - this makes 250000 pulses / second
> i need a IC which takes this pulses and builds a signal 0.5 V for example
> when the motor runs at 1000 rpm it will output 5 V.
> as you can see it must be very linear.
> does anybody have an idea ?
> is there such a IC on the market ?
> or any other methods ?
> (i dont want to use a tacho becouse it costs money )
Normally, you would use a mono-stable multivibrator for this. Triggered by
you pulse train, the mmv will output a pulse of fixed width with a
repetition rate equal to the pulse frequency of your encoder. Run the mmv
output through a low-pass filter and you have a DC voltage indicating the
speed. And it's linear.
But creating a reliable mmv with a very short pulse time, 1000 rpm equals
2,500,000 pulses, so the puls time must be shorter than 400ns, is very
A different approach was published many years ago in Elektor. They used a
single CMOS flip-flop, triggered by the signal to be measured. In series
with the VSS pin and power ground was a resistor and capacitor in parallel.
They claimed that the current draw of the FF was linear with the operating
frequency, thus creating a frequency dependent voltage over the resistor.
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