No one is safe from pipe leaks. But if the incident is discovered in time, the negative consequences can be minimized. The author of Instructables under the nickname TheRadMan came up with a way to convert a very cheap household siren into a flood alarm. They can be made several and placed on the floor next to sinks, radiators.
The master claims that such sirens over the hill are sold in stores under the names Dollar Tree, Dollarama, etc. In our Fix Prices and other stores, they also periodically appear. You probably paid attention to them: a small box with electronics and a reed switch inside, and a magnet is included in the kit. You put three LR44 batteries in the battery compartment of the box, and now, if you move the magnet away from the reed switch (for example, when opening the door, window), a piercing signal sounds.
But this device reacts to an open circuit, and also with a fairly low resistance, and therefore, as a flood signaling device, it can be used only after refinement. That's what TheRadMan did. Let's see how?
As you can see, the master cut the conductor connecting the YL3710 chip with the minus of the power source, and placed the BS170 field effect transistor instead of this conductor. Now you can drop the reed switch or simply move the magnet away from it - until the transistor opens, the microcircuit's power circuit is open and the signal does not sound.
In order for the transistor to open during leakage, the master assembled a divider of two resistors: 10 MΩ and 1 MΩ. In parallel with the first of these resistors, a flooding sensor consisting of two electrodes is connected. When water enters the sensor, its resistance decreases significantly, the voltage at the gate of the transistor increases relative to the minus, and the transistor opens.
A little lower on the diagram separately shows an additional unit with a sensor consisting of two 10-kilo-ohm resistors, the conclusions of which are not shortened. A 10-megaohm resistor is not needed in this case, and when there is no leakage, there is no additional load in the form of a divider, which makes the power supply longer.
The master removes the batteries from the siren, opens the case and bites the conductor connecting the board to the negative terminal of the battery compartment:
He bites the reed switch - it will come in handy in some other alarm:
Connects the transistor and resistors according to the scheme, displays the electrodes. Everything that can touch the board is insulated with a heat shrink tube:
Although ... Now everything is for sure:
These recesses had to be done in the housing in order to bring out the electrodes:
TheRadMan closes the case, returns the batteries to the compartment in the correct polarity, and makes sure that there is no sound:
Tightens the screw, closes the compartment lid and makes sure that there is no sound. Another siren on a chip in another building and with a switch is being finalized according to the second version of the circuit:
Trying to use a sensor of a different design, consisting of a counterpart for a comb and two pieces of thin wire:
After tightening the shrink, the sensor looks like this:
The master checks the signaling device with an improvised miniature cuvette with water:
Having made several signaling devices with built-in and remote sensors, places them like this:
So, for comparison, ready-made, rather expensive signaling devices with 2xAA compartments and indicators for discharging the power source look like:
In homemade signaling devices, everything is fine, but there are no such indicators. Therefore, the batteries must be periodically checked with a voltmeter, briefly connecting a small load (something consuming about 30 mA, which corresponds to a working siren). The voltage under load should not sag much.