Feb 132020

This is a water level alarm with three transistors that is simple and small due to the few components it has. Another advantage of this alarm circuit is its very low power consumption, since it can operate for a long time on a 9 V battery. The alarm signal is used as a warning to shut off the water source.

Alarm circuit operation

The alarm emits a sound through a speaker when the water level reaches the maximum level. Transistor Q1 acts as a switch. When this transistor is saturated, it allows current to pass to the relaxation oscillator, which consists of an unijunction transistor Q2, capacitor C1, and resistors R1 and R3.

Water Level Alarm with Three Transistors

Water Level Alarm with Three Transistors

The sound of the alarm signal has a frequency that depends on the values of capacitor C1 and resistor R2. If you want to change the frequency of the alarm signal, you can change the values of R1 and C1 to other values.

Pulses from the relaxation oscillator turn transistor Q3 on and off. Transistor Q3 continuously transitions from its turn-off region to its saturation region, delivering the signal to the speaker.

The bipolar transistors Q1 and Q3 suggested in the parts 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 with similar characteristics.

The normally open (NO) test button is used to test the proper 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).
  • 1 100 K resistor (R1).
  • 1 6.8 K resistor (R2).
  • 1 12 Ohm resistor (R3).
  • 1 0.01 uF (microfarads) capacitor (C1).
  • 1 Normally Open (NO) Test Switch (test).
  • 1 16 Ohm or greater miniature loudspeaker.
  • 2 Metallic conductors to be used as water level probes.
  • 1 9-volt battery or equivalent power supply.

 Leave a Reply