The car radiator should contain a sufficient amount of water. If the driver fails to notice a significant reduction of the water in the radiator, the motor will overheat.

A device for monitoring the water level in the radiator (see diagram), has the advantage over similar devices that no electrolysis occurs, leading to the progressive destruction of the walls of the radiator. The use of silicon transistors makes the device a little sensitive to significant changes in temperature.

A device for monitoring the water level in the radiator

The basis of the device - the multivibrator from one stable state to the transistors T2 and T3. His load is a warning light L7. The transistor T4 contributes to a more precise operational status (opened / closed) of the transistor T2.

When the probe in the radiator submerged in water, to the base of transistor T1 receives the bias voltage and it was open. In this case, the base and the emitter of the transistor T2 have the same potential and this transistor will be closed. As a result, the multivibrator is not working, and the alarm lamp L1 is de-energized. Diode D1 protects the base of transistor T2 from overvoltage.

By lowering the water level in the radiator, the probe is in the air. As a result, the transistor T1 is closed, and T2 opens. Now multivibrator will operate with a frequency determined by the time constant of the chain R4 C1 (about 2 Hz). Signal lamp L1 will flash with the same frequency, attracting the attention of the driver.

Capacitor C1 should be a paper, as when the polarity of the charge on it is reversed. The probe is made from stainless steel and the tube for the dipstick from plastic with a high melting point. For these purposes it is possible to use nylon, PTFE or Dacron.

The device should only be used silicon transistors and diodes. For example, transistors T1, T2 can take type MP, T3 - CT and T4 - KT315 with any letter index. Diode type D or D with any letter index.


  • "Funk-Technik", 1970, No. 12
  • Publication: N. Bolshakov,

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