Jan 082024

Voltage Drop Alarm

This voltage drop alarm is very useful for power supply monitoring or testing. Sometimes it is necessary to monitor the voltage and take action when the voltage drops below a set or acceptable level.

If all we have is a multimeter, we can supplement it with a circuit that monitors the voltage and triggers an audible alarm when the voltage drops or the power source fails.

The circuit will keep the buzzer active even after the voltage has returned to normal. The buzzer will remain active until the reset button is pressed.

Voltage Drop Alarm Circuit Operation

The circuit uses the LM393 I.C. (2 comparators on a single integrated circuit) (U1) with a programmable voltage reference of 2.5 V on its non-inverting input. A three-terminal shunt regulator provides the voltage reference (VR). See the figure.

Voltage Drop Alarm Circuit

The comparator compares the voltage reference (Vref) with the output of the voltage source by dividing the voltage with potentiometer R1 and resistor R5. Potentiometer R1 is used to adjust the voltage under test.

When the test voltage drops below the reference voltage, the comparator output goes from low to high, triggering the thyristor. The thyristor then turns on. This activates the buzzer, which begins to sound. The buzzer remains active (sounding) until switch SW1 is momentarily pressed.

LM393 Very Low Voltage OFFSET Comparators

LM393 Very Low Voltage OFFSET Comparators

The resistor R6 in parallel with the buzzer ensures that the current through the SCR (X1) keeps the buzzer ON even when the comparator output goes low. The circuit can be powered from a 9V battery or an external power source.

With the test voltage active, use a digital voltmeter to measure the voltage across resistor R5 until it reaches the desired level above the reference voltage.

You can then connect the digital multimeter in parallel with the test voltage for a visual display. When the test voltage drops below the reference voltage, the alarm should sound.

TL431 - Programmable Precision Voltage Reference

TL431 – Programmable Precision Voltage Reference

Component List of the Voltage Drop Alarm Circuit

  • 1 LM393 integrated circuit. (2 comparators on one chip) (U1)
  • 1 TL431. Programmable precision reference (VR)
  • 1 Thyristor 2N5060 or similar (X1)
  • 1 Diode 1N5060 or the equivalent (X1)
  • 1 Diode 1N4148 or equivalent (D1)
  • 1 47K potentiometer (R1)
  • 1 2.2K resistor (R2)
  • 1 10K resistor (R3)
  • 1 6.8K resistor (R4)
  • 1 4.7K resistor (R5)
  • 1 1K resistor (R6)
  • 2 x 0.01 uF capacitors (C1, C2)
  • 1 capacitor of 0.1 uF (C3)
  • 1 buzzer
  • 1 normally open (NO) switch (SW1)

Based on the article by: John Lo Giudice

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