Solid state voltage indicator with high voltage cutout

Voltage fluctuations in the mains are very common phenomenon. These fluctuations can cause heavy damage to costly electronics and electrical gadgets operating for long period of time. For such purpose, an automatic stabilizer is used which is usually quite expensive.

A costly voltage stabilizer can be replaced with this low-cost “Solid-state Voltage Indicator with High Voltage Cutout” circuit. It can also be used to convert a manual stabilizer into an automatic one. It performs three functions in different conditions.

  1. When the voltage is normal, it keeps the load on and a green LED glows to indicate normal voltage conditions.
  2. When the voltage exceeds the safe limit, it stops the power going to the load and a red LED (LED1) glows to indicate unsafe supply conditions.
  3. When the voltage falls below a preset level, the green LED (LED2) starts flashing to indicate a low voltage.

Circuit Descriptions

X1 is a voltage step-down transformer that steps down 220V AC to 18V. This voltage is filtered and rectified by C1 and D1 respectively. As the output of the transformer is not well filtered and regulated, the output will be proportional to the input.

VR1 is adjusted to operate the relay at a particular voltage higher than the output voltage of the transformer at normal mains level. So, when the mains voltage increases, the output of the transformer also increases. Zener diode ZD2 starts conducting and provides bias to T1 which starts conducting. This in turn energizes the relay and light up the RED LED (LED1), to indicate a high voltage.. Diode D6 provides protection to the relay against induced voltages.

circuit diagram of solidstate voltage indicator with high voltage cutout

IC LM741 (IC1) forms the heart of the normal and low voltage indicating circuitry. The potential of the non–inverting input serves as the reference voltage across zener diode ZD4. Variable resistor VR3 set the DC gain of IC1. LED D5 is connected between the output (pin 6) and the positive supply. R5 limit the current going to diode LED5. Under normal conditions, the voltage as the inverting input will be higher than at the non-inverting input. So the output will be low, ad LED5 will remaining glowing. But, when the mains voltage drops, the voltage at the inverting input becomes lower than that at the non-inverting input. Thus the output goes high and the green LED (LED2) is extinguished.

But when the IC is in between the upper and the lower saturation limits, it will act as a high amplifier and produce a square wave output at pin 6. The square wave will make the green LED (LED2) blink in order to indicate a low voltage. The frequency is set by C2 and R6. The flashing rate can be adjusted by using VR3.


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 2.2 KΩ

R2, R5 = 1 KΩ

R3 = 75 Ω/5W

R4 = 1.2 KΩ

R6 = 470 KΩ

VR1, VR2 = 10 KΩ


C1 = 220 µF/25V

C2 = 2.2 µF

C3 = 0.01 µF


IC1 = 741

D1 = 1N4002

D2 = 1N4001

ZD1 = 10V/250mA

ZD2 = 5V/250mA

T1 = SL100B


LED2 = Green


X1 = 230V AC primary 18V, 350mA sec. transformer

RL1 = 12V 300Ω relay

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