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magnetic pickup waveform in Blue Magnetick pickup waveform is in Blue magnetic pickup waveform in Blue

I want to read the speed signal from a magnetic pickup.

At low speed the signal voltage is in millivolts and it has a frequency which is equal to the number of teeth crossing the magnetic pickup.

This signal must be read by a frequency to voltage transducer, but this transducer need a minimun 5 volts AC input to read the frequency.

How can I build an amplifier circuit to amplify the millivolts to at least 5VAC and maintain the original frequency?

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  • \$\begingroup\$ What's the minimum and the maximum frequency? \$\endgroup\$
    – Rohat Kılıç
    Commented Sep 12, 2021 at 9:38
  • \$\begingroup\$ Minimum is 1 HZ and maximum will be less than 2000 HZ, Lets say that the maximum Pump shart RPM is 2500, that will be around 41 Rotations per second and there are 26 Teeths in the flywheel so the frequency will be 41x 26 around 1070 HZ \$\endgroup\$
    – mxj262
    Commented Sep 12, 2021 at 9:43
  • \$\begingroup\$ You have to link the datasheets of the equipment you have. Usually the ECU would need a square pulse, also the VFC converter. \$\endgroup\$
    – Marko Buršič
    Commented Sep 12, 2021 at 9:44
  • \$\begingroup\$ I am able to succesfully read the engine speed which is providing an output of 6VAC at 700 RPM using the Phoenix contact transducer, but the pump speed I am unable to read because it has the output from the magnetic pickup in millivolts because the number of teeths being less- transducer link \$\endgroup\$
    – mxj262
    Commented Sep 12, 2021 at 9:52
  • \$\begingroup\$ Show low speed output voltage waveforms ranging from an equivalent of 1 Hz to maybe 1000 Hz. This is where the biggest problems will be in signal distortion and differentiation artefacts. \$\endgroup\$
    – Andy aka
    Commented Sep 12, 2021 at 9:53

3 Answers 3

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The Phoenix Contact module is an industrial device, for 24VDC. So it accepts many industrial sensors, but not a magnetic pickup.

The easiest way is to replace the magnetic pickup with an inductive sensor with HTL output and 24VDC supply. There are many of them.

Otherwise there is LM2917 that does amplification of the pickup and frequency to voltage conversion all in one.

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  • \$\begingroup\$ I am also using the same Phoenix Contact module to read engine speed (118 teeths) and have been able to read it accurately - the engine is also using a magnetic pickup and gives out a voltage of 6 Volts AC when the engine is running at the speed of 700 RPM which is also the lowest speed,higher speed is not a problem because the voltage rises - I was also expecting the pump magnetic pickup to produce voltage more than 5Vac but it does not because the teeths are lesser on pump shaft flywheel. Phoenix contact module needs minimum 5 VAC to work \$\endgroup\$
    – mxj262
    Commented Sep 12, 2021 at 10:42
  • \$\begingroup\$ LM2916 also need a minimum Input voltage of 6 Volts so how does that solve the problem ? \$\endgroup\$
    – mxj262
    Commented Sep 12, 2021 at 11:02
  • \$\begingroup\$ @mxj262 It's very unlikely that a pickup outputs 6VAC. \$\endgroup\$
    – Marko Buršič
    Commented Sep 12, 2021 at 17:26
  • \$\begingroup\$ well it depends on the number of teeth and speed, the magneticpickup I am using on engine is seeing 118 Teeths cutting at 700 RPM and in that condition it is producing 6VAC it is can go even higher (15-17 V) link \$\endgroup\$
    – mxj262
    Commented Sep 13, 2021 at 5:11
  • \$\begingroup\$ Probably I can use a inductive proximity switch with high switching frequency . \$\endgroup\$
    – mxj262
    Commented Sep 13, 2021 at 5:13
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If, as you say in your comment

The output Voltage from magnetic pickup rises as more number of teeth cross it when speed increases this also increases the frequency -however the increase in the voltage may not be linear but yes it does rise with higher speed and frequency is equal to the number of teeth that cross so that rise as well

then use a high pass filter well below 1 Hz to filter out the DC, then use an integrator. This will lowpass filter the noise, and create a falling response with frequency, to give you an overall flat frequency response. Then use a comparator with hysteresis.

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Use a op-amp in the non-inverting configuration as shown in the picture. The gain will be $$A = 1 + \frac{R_2}{R_1}$$ so just choose the resistor values to meet the necessary output voltage that you need based on the input voltage.

For example, if you had a 100 mV signal, you would want a gain of 50 to get to 5V. This is pretty low speed, so most op amps will work. You could use a hobbyist board like the SparkFun OpAmp Breakout - LMV358.

Non-Inverting Op-amp configuration

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