I have a system composed of a 230VAC zone isolated from a human contact zone with a 7.5mm 2k5 space.

On the human side there is a dry contact switch:

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I am searching for a less than 1€ solution to test if the current flows or not.


I found the solution by using a pulse transformer schematic below:

enter image description here

The circuit is powered with a 3.3V PWM.

  • L1-L2: Is a transformer 1-1 (2K5V isolated)
  • U2: Is a zener 2.2V (200uA leakage)
  • Vs is the signal measured by an ADC.


  • R1 is present: Vs=840mV
  • R1 is absent: Vs=140mV (comes from the zener leakage)

This circuit is based on the impedance transfer to primary properties of a transformer. As a reminder: https://en.wikipedia.org/wiki/Transformer

Basically the load seen as the primary is Z(load)=Z(sec)/m². Having a 1-1 transformer imply m=1. So the load of the secondary is transfered directly to the primary.

The equivalent circuit is below:


simulate this circuit – Schematic created using CircuitLab

In my case i want to detect if my switch is closed or open, therefore if i have a load to the secondary or not.

If ZL is,

  • Present: The inductance is short circuited and the the voltage of the resistor in series is the 3V3 pwm.
  • Absent: The circuit is an R-L being a low-pass filter, the voltage of the resistor is the DC voltage of the pwm, 1.65V.

To reduce the size of the transformer I choosed a 1MHz PWM.

To have a difference signal that an ADC can measure, I added a small measure circuit with a zener of 2.2V. When the load is present the zener let the signal pass (with a -0.7V offset) and when absent the signal is blocked.

The capacitor is charged when the load is on and discharged in the resistance when the load is off.

The series resistance of my zener is 30ohm and it leaks 200uA so it is trivial to modelize the equation of the voltage of the capacitor and derive it to find the values to have the best delta between Load on and Load off.


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