Water level alarm with three transistors.

This is a water level alarm with three transistors is simple and small due to the few components it has. Another advantage of this alarm circuit is its very small demand for current, since it can work with a 9V battery for a long time.

The alarm signal will serve as a warning to deactivate the water source.

Alarm circuit operation

The alarm emits a sound through a loudspeaker when the water level reaches the maximum level. Transistor Q1 acts as a switch. When this transistor is saturated, it allows the passage of current to the relaxation oscillator composed of an UJT transistor (unijunction transistor) Q2, a capacitor C1 and a resistor R1 and R3.

The sound of the alarm signal has a frequency that depends on the values ​​of the capacitor C1 and the resistance R2. If you want to change the frequency of the alarm signal, you can change the values ​​of R1 and C1 for others of different values.

Pulses produced by the relaxation oscillator activate and deactivate transistor Q3. Transistor Q3 passes from its cut-off region to its saturation region continuously, applying the signal to the speaker.

Bipolar transistors Q1 and Q3 suggested in the component list work correctly, but can be replaced by virtually any other NPN transistor with similar characteristics. The UJT transistor (Q2), can also be replaced by another of similar characteristics.

The normally open “test” momentary contact switch (NO) is used to test the correct operation of the circuit.

Water level alarm component list

• 1 NPN BC107 bipolar transistor or similar (Q1).
• 1 NPN BFY51 bipolar transistor or similar (Q3).
• 1 2N2646 UJT (unijunction transistor) or similar (Q2).
• 1 100K resistor (R1).
• 1 6.8K resistor (R2).
• 1 120 ohm resistor (R3).
• 1 0.01uF (microfarads) capacitor (C1).
• 1 normally open (NO) test switch (test).
• 1 16 ohms or more miniature loudspeaker.
• 2 metallic conductors of electricity, which are used as probes to sens the water level.
• 1 9-volt battery or a power supply of the same voltage.