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I would like to create a voltage tester like the one described in this video, using 555 ic, and then connect it to Raspberry Pi, in order to log the frequency a 220V waterpump is switched on.

My questions are:

  • How should I connect it to the Raspberry Pi? Should I connect it to a PWM pin?
  • Do I need some extra components to protect the device or get more accurate readings?
Paradox
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Marinos An
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2 Answers2

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Question

Let me first summarize the OP's question.

He has a 220VAC water pump. He knows that a 555 IC can tell AC current on by blinking a LED. His question is how to let Rpi talk to 555 to find the pump is on or off.

Research

I am guessing how 555 detects AC current.

As soon as the AC current passes a wire, electromagnetic field is created. The electromagnet field induces a small 50Hz voltage/signal on the ugly looking DIY copper wire antenna which is connected to 555's trigger input.

When the AC signal is high enough, 555's monostable timer starts and stops in less than 1/50 second. The AC signal cycles every 1/50 second, so is the timer, which now outputs a 50Hz square wave signal, when water pump is switched on.

Answer

Now let me suggest a quick and dirty answer.

Let Rpi repeats reading 555 output, say 10 times, in 1/50 second, and taking the average. If water pump/current is on, the average should be somewhat high, else very low.

References

OP's Original Video Reference on 555 Timer Based AC Current Detector https://www.youtube.com/watch?v=vp-IbQC6KK0

NE555 Timer Datasheet - Texas Instruments http://www.ti.com/lit/ds/symlink/ne555.pdf

555 Timer Tutorial - Electronics Tutorials https://www.electronics-tutorials.ws/waveforms/555_timer.html

555 Timer Circuits - All About Circuits https://www.allaboutcircuits.com/textbook/experiments/chpt-8/555-ic/

ACS712 AC/DC 5A Current Sensing Module Summary https://penzu.com/p/e0b38806

Schematic

enter image description here

NE 555 timer in monostable mode

tlfong01
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Use either a microcontroller with built-in ADC (ATMega/Arduino/etc) or dedicated I2C/SPI ADC chip, then design input protection and voltage divider circuitry suitable for the input voltages required to convert them within your ADC's range (0-3.3V or 0-5V are common). A voltage divider works for DC circuits, probably want a clamping diode to protect your circuit if the voltage goes over the maximum value, and an inline resistor to limit current into the ADC input. For AC, you'll need more sophisticated circuitry to convert the AC to DC for measurement (see true RMS multimeters).

stephen4312
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