I designed the circuit to be built on a double-sided 4x6 cm protoboard using mostly 0805 SMD parts. The schematics presented are based on this build, but it can be adapted easily for through-hole components (tip: to minimize space, solder through-hole resistors vertically).

The following parts are required:

- Resistors: 330 Ω x 1; 470 Ω x 1; 680 Ω x 1; 1 kΩ x 1; 10 kΩ x 3; 470 kΩ x 3;

- One 10 µF ceramic capacitor

- One logic-level N-channel MOSFET (e.g. RFP30N06LE or AO3400)

- One red and one yellow LED (or other colours if you like).

- Two-wire screw terminal connectors x 3 (they are not absolutely necessary, but they make it easier to connect and disconnect periphery during testing)

- A loud piezo buzzer that's good for 3.3 V

- An ATtiny85 microcontroller (PDIP version)

- An 8-pin PDIP socket for the microcontroller

- An ESP-01 module (it can be substituted by another ESP8266-based module, but there will be a lot of changes in the layout in that case)

- A 3.3 V DC-DC boost converter capable of delivering 200 mA (500 mA burst) currents at 2.2 V input. (I recommend http://www.canton-electronics.com/power-converter... because of its ultra-low quiescent current)

- One 3 pin female header

- Two 4-pin female headers or one 2x4 header

- 22 AWG solid wires for the water sensor

- 22 AWG stranded wire (or another type of thin exposed wire to create traces)

I recommend the resistor values listed above, but you could substitute most of them for similar values. Depending on the type of the LEDs you want to use, you may need to adjust the current-limiting resistor values to get the desired brightness. The MOSFET can be either through-hole or SMT (SOT23). Only the orientation of the 330 Ohm resistor is affected by the type of the MOSFET. A PTC fuse (e.g. rated for 1 A) is recommended if you are planning to use this circuit with NiMH batteries. However, it is not needed with alkaline batteries. Tip: the parts required for this alarm can be purchased cheaply from ebay or aliexpress.

In addition you will need a breadboard, several through-hole 10k resistors, multiple male-male and female-male jumper ("dupont") wires and a USB-UART adapter in order to program the ESP-01 module.

The water sensor can be made in a variety of ways, but the simplest one is two 22 AWG wires with exposed ends (1 cm long) spaced approximately 1 mm apart. The goal is to have less than 5 MΩ resistance between the sensor contacts when water is present.

The circuit is designed for maximum battery economy. It draws only 40-60 µA in the monitoring regime ( with the power LED removed on the ESP-01 module). Once the alarm is triggered, the circuit will draw 300-500 mA (at 2.4 V input) for a second or less, and after that the current will drop below 180 mA. Once the ESP module is done sending notifications, the current consumption will drop to below 70 mA until the buzzer switches off. Then the alarm will disarm itself, and the current consumption will be under 30 µA. Thus a set of AA batteries will be able to power the circuit for many months (likely over a year). If you use a different boost converter, say with a quiescent current of 500 µA, the batteries will need to be changed much more often.

Assembly tips:

Use a permanent marker to label all traces and component on the protoboard for easier soldering. I recommend proceeding in the following order:

- top side SMT LEDs and insulated wire bridges

- top side MOSFET (note: if you have a SOT-23 MOSFET, place it diagonally as in the photo. If you are using a through-hole MOSFET, place it horizontally with the gate pin in position I3.)

- top side through hole parts (note: the buzzer isn't soldered and doesn't even have to be mounted to the PCB)

- reverse side SMT parts and traces (e.g. individual strands from AWG22 wire)