Detecting presence of mains voltage using Microcontroller, low power solution

Question

I'm currently designing a 'smart' light switch, controllable from Microcontroller and physical switch. I am looking for a low power solution to convert signal so that the Microcontroller can detect if there is mains voltage at the input of the lamp (or arbitrary device). The device will also be embedded in the walls so required small form factor, as well as low power loss / heat due to limited ventilation. Thank you in advance for the advice, I am quite new.

Goal: Detect (from microcontroller) if the output from the relay has AC mains / connected to live wire. Should detect regardless of the lamp being connected or not (no load/open circuit). This is to notify user if the device being powered is safe to handle / exchange or not. It doesn't need to know the exact voltage, just the presence. Delay of few 10s to 100s ms is acceptable.

Additional info: The Microcontroller may be powered from mains or a 24v dc supply line, currently deciding.

Size: aiming to fit 3 sets of relays and detection components (as well as a power supply for the Microcontroller and the Microcontroller) into 8x4 cm.

Power: aiming for the detection solution to take on order of 10s of mW.

Cost: aiming for the detection solution to take ~<$3

From what I've read there are a few approaches.

• Voltage Transformer into rectifier and capacitor (low pass filter)

Pros: Isolate high and low voltage

Cons: Voltage Transformer may be large and costly.

Unsure: Rectifier may dissipate a lot of power.

• Optocoupler

Pros: Isolate high and low voltage. Low space

Unsure: Optocoupler may require consume lots of power.

• Difference Op Amp

Pros: Low space

Cons: No isolation

Unsure: Op Amp power consumption

• Hall Sensor

Pros: Low space, low cost

Unsure: Hall sensor power consumption. Operation under no load (no current flow -> no magnetic field?)

Circuit

Answer 1

This circuit provides an isolated HV-AC sensor with high impedance.

The MCU generates short positive pulses of some µs width at OUT. This is the green track in the pictures. 100..200 ns after the rising edge of OUT the input IN is read.

During the negative half wave or without any input signal the MOSFET T1 is not conducting. The yellow track shows the voltage at the secondary side of the transformer. It initially follows the positive edge of OUT and is falling because the current in the winding rises. The MCU reads a "high" 100 ns after the rising edge.

At a large percentage of the positive half wave T1 is conducting and shorts the transformer winding. This reduces the inductance of the secondary winding and the voltage there does not follow the rising edge. Only 50 mV peak can be seen here, the MCU will read a "low"

To detect the presence of an AC mains voltage the MCU should take such measurements with a sampling rate of 100 Hz or more. The width of the short pulses is not important, one µs is good enough. If a "high" is read for a longer interval than the AC period duration, there is no AC input signal.

With an input signal the MCU will read a "low" for around 30% of the AC period duration.

R05 and D2 protect the MCU ports.

The transformer isolation capability must be rated for mains usage. There are very small components available.