HELP - Frequency to Voltage Convertor using a Retro-Reflective Sensor to Measure RPM

Thread Starter

JustinG_KNF

Joined Jan 6, 2020
16
So I went ahead isolated the sensor + pull up resistor as per post#5 and here are the results. Im now thinking my voltage divider is a little too low. It cuts the reference voltage down to 0.5V. Im thinking I should use two of my 47K resistors to set this reference voltage to approximately 7.5V.

1578430100484.png
 

Dodgydave

Joined Jun 22, 2012
8,809
So I went ahead isolated the sensor + pull up resistor as per post#5 and here are the results. Im now thinking my voltage divider is a little too low. It cuts the reference voltage down to 0.5V. Im thinking I should use two of my 47K resistors to set this reference voltage to approximately 7.5V.

View attachment 196377
Just put your sensor output directly into pin 6 of the chip and the 8K2 resistor ,without the 470pF capacitor, and see what output voltage you're getting
 

Alec_t

Joined Sep 17, 2013
10,902
...... but without the 470p cap the risk is that pin 6 is held low longer than the time constant RtCt, in which case the linearity of the F-to-V conversion may be compromised.
 

Thread Starter

JustinG_KNF

Joined Jan 6, 2020
16
So I rehooked everything up and used two 10k resistors I found to set the reference Voltage to 12V. I powered the chip on and got ~1.9V constant no matter my frequency signal. This was tested both with and without the 470pF capacitor and I got the same results.
1578494079994.png
 

eetech00

Joined Jun 8, 2013
1,835
Hello,

Try the values shown in the circuit below.
The response will be slow but will be a compromise between output ripple and response time
Cap CL affects the amount of ripple in the output.
R2 should drop 2V. Equation for R3: (VS-2V)/200uA=110k
I've shown 12K resistors to drop the voltage down from the sensor output, but you can use 10k if you wish.
The pot calibrates the output.

eT

1578506371462.png
 

Thread Starter

JustinG_KNF

Joined Jan 6, 2020
16
Hello,

Try the values shown in the circuit below.
[…]
Sorry for the delay -
Thanks eT! I bought some more stable components, sized per your diagram and was able to get it working!
I am getting an output ripple that ranges about 0.2V which is a little too much so I will be playing around with the CL capacitor sizing and also RL sizing as I don't need the full range of 200Hz so I am going to adjust this down to 150Hz @ 10V by increasing RL.

Shout out to everyone else, as this whole Forum has been very helpful!

Here is my current circuit (attached a 50K resistor in series with RL and does not have the 5k POT as I can calibrate this in Labview)
1579785619349.png
1579785578787.png
 

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eetech00

Joined Jun 8, 2013
1,835
Sorry for the delay -
Thanks eT! I bought some more stable components, sized per your diagram and was able to get it working!
I am getting an output ripple that ranges about 0.2V which is a little too much so I will be playing around with the CL capacitor sizing and also RL sizing as I don't need the full range of 200Hz so I am going to adjust this down to 150Hz @ 10V by increasing RL.

Shout out to everyone else, as this whole Forum has been very helpful!

Here is my current circuit (attached a 50K resistor in series with RL and does not have the 5k POT as I can calibrate this in Labview)
View attachment 197482
View attachment 197481
Nice! Glad you have it working.

eT
 
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