Magnetic Sensor with LM324

[kmdineshece]

I need to measure RPM of motor. I have magnetic sensor, (magnet with coil) which is interface with LM324, the output of lm324 given to ATMega88PA, Input Capture Pin, so i need a exact pulse signal, i have designed some circuit, but this circuit does not produce constant pulses, if any expert in this area, help me!...

Thanks in advance!...

 

[barry]

Not quite sure what you're trying to do there, but it looks like there's a lot wrong.

1) You've got a diode in series with sensor output which will allow only negative signals through, but your opamp is referenced to ground. I think you want to turn that diode around and maybe put a resistor to ground on the opamp input.
2) Your opamp is open loop; what you want is a comparator with some hysteresis.
3) You're using a resistor and two diodes to (presumably) set the negative input to 1.2 V. Why not use a resistor divider?
4) What's up with the resistor and zener on the output? You've got a 5volt supply on your opamp, and a 5.1 volt zener. Do you expect your opamp output to exceed 5v?(Hint: it won't)

 

[FvM]

Although the diode is forward biased by LM324 input current (pnp transistors), it's no good circuit. Simply remove the diode and adjust the threshold voltage set by D1 and D2 if necessary.

 

[chuckey]

You have to be careful of the bias current flowing from the IC to earth via the transducer. It will be too low to burn out the transducer, but it might (will) affect the magnetic circuit. You could try to reverse the connections to the transducer to see if you get the same waveform out of the IC. This is based on magnetic heads used in tape recorders, they are ALWAYS AC coupled with a high quality low leakage capacitor.
Frank

 

[kmdineshece]

I have modified the circuit , but still, i didn't get exact output, pls share any reference, example links

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could you explain about hysteresis ? (you mentioned)
how it affects the signal?, is it due to open-loop?

 

[chuckey]

Hysteresis is like back lash in a mechanical system. If your amplifier gives a 1 when the input is above 100mV, it will stay at a 1 until the signal drops below, say 80 mV. The output will then remain at a 0 until the signal rises to above 100 mV. This way if the signal has 5mV of noise on it, there will be no multiple bursts of noise at the output.
Frank

 

[FvM]

I have modified the circuit , but still, i didn't get exact output.

Unfortunately worse than before. Still no correct input bias.

It would be interesting to see the ouput level of the magnetic.

 

[kmdineshece]

how to minimize those noise?

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then, how could be correct?

 

[barry]

You need to use a comparator circuit. As was previously noted. Google it. That resistor in series with the diode does nothing.