Note: this has been edited to replace the ESP8266 internal Pullup at the GPIOs with a dedicated pullup resistor that alleviates some wet-dry issues due to currents in the water and electrolysis with too weak a pullup resistor (such as the internal one). I use a 47K Ohm between the 3.3V pin and each GPIO pin that is used for leak detection. 

This design is for a simple low-cost leak detector that can be used to detect water leaks and send notifications when they are discovered. I designed this for use with ESPHome and Home Assistant.

This design accommodates M5x12 machine screws and M5 fastening nut. I recommend using stainless steel parts. The length of the screws offsets them a small distance from the bottom - you can use M5 washers if you want a greater offset. I placed these in my A/C and Water heater drain pans to detect when problems arise!

To wire these up to a microcontroller such as ESP8266 - Connect one end to GND and the other to the GPIO that you want to use to detect water. Add a 47K Ohm resistor between 3.3V and the same GPIO pin to act as a pullup. You can use multiple sensors by utilizing multiple GPIO ports. In my example I am using two of these.

In ESPHome the .yaml code looks like below for a dual sensor system. Easy to modify for less or more sensors.

 binary_sensor:

  - platform: gpio

    name: "Leak Detector Kitchen A/C Pan"

    pin:

      number: GPIO5       # pick your GPIO pin This is D1

      mode: INPUT   # note that the internal pullup resistor is too small so do not use INPUT_PULLUP

      inverted: True   # so "ON" means water detected

    device_class: moisture

    filters:

      - delayed_on: 100ms     # must be wet for 100 ms before ON

      - delayed_off: 5s       # must stay dry for 5s before OFF  

  - platform: gpio

    name: "Leak Detector Kitchen Water_Heater Pan"

    pin:

      number: GPIO4       # pick your GPIO pin This is D2

      mode: INPUT   # note that the internal pullup resistor is too small so do not use INPUT_PULLUP

      inverted: True   # so "ON" means water detected

    device_class: moisture

    filters:

      - delayed_on: 100ms     # must be wet for 100 ms before ON

      - delayed_off: 5s       # must stay dry for 5s before OFF