Manual voltage stabilizers are still popular because they are easy to make, cheap, and reliable because they don't have any relays. They can also handle a wider range of mains AC voltages than automatic voltage stabilizers. Most of the time, these are used in homes and businesses to power things like lights, TVs, and refrigerators. They are also used in places where the mains AC voltage goes from very low (during peak hours) to very high (during non-peak hours). Some of the manual stabilizers on the market have a high-voltage auto-cut-off feature that turns off the load when the output voltage of the manual stabilizer goes over a certain high voltage limit that has been set. The output voltage may go up if the AC mains voltage goes up or if the rotary switch on the manual stabilizer is not used correctly.

One of the biggest problems with using a manual stabilizer in places where the voltage changes a lot is that you have to keep an eye on its output voltage, which is shown on a voltmeter, and keep changing it with its rotary switch. Or, the output voltage could reach the limit set by the auto-cut-off feature, which would turn off the load without the user knowing. To turn on the load again, the rotary switch on the stabilizer has to be used to change the voltage. This is a very annoying and inconvenient way to work for the user. This under-/over-voltage audio alarm circuit, which is an add-on circuit for the existing manual stabilizers, solves the above problem. When the stabilizer’s output voltage drops below a preset low-level voltage or rises above a preset high-level voltage, it makes different beep sounds for "high" and "low" voltage levels. For "high" voltage level, it makes short-duration beeps with short gaps between each beep. For "low" voltage level, it makes slightly longer beeps with longer gaps between each beep. With the help of the rotary switch and these two different types of beep sounds, it is easy to read just the stabilizer's AC voltage output. There's no need to check the voltmeter reading often. It is best to set the high-level voltage 10V to 20V lower than the required high-voltage limit for auto-cutoff operation. In the same way, low-level AC voltage can be set 20V to 30V above the minimum operating voltage for a certain load. The output terminals of the manual stabilizer are hooked up to the primary winding terminals of step-down transformer X1. So, the 9V DC across capacitor C1 will change depending on the voltage at the output terminals of the manual stabilizer, which is used in this circuit to sense whether the voltage is high or low.

Together, the zener diode ZD1 and the preset VR1 are used to sense and adjust the high-voltage level that makes the beep sound. In the same way, transistor T2 is used with zener ZD2 and preset VR2 to detect and adjust the low voltage level for the beep signal. When the DC voltage across capacitor C1 goes above the preset high-level voltage or below the preset low-level voltage, the collector of transistor T2 goes high because transistor T2 is not conducting. But if the DC voltage measured across C1 is between the high-level and low-level voltages that have already been set, transistor T2 conducts and its collector voltage is pulled to the ground level. The astable multivibrator circuit, which is made up of transistors T3 and T4, uses these changes in the collector voltage of transistor T2 to start or stop oscillations. Through resistor R8, the base of transistor T4 is linked to the collector of transistor T5, which drives the buzzer. So, the buzzer goes off when the collector voltage of transistor T4 goes up. Set VR3 is used to change how loud the buzzer is. When everything is normal, the DC voltage measured across capacitor C1 is within the window voltage zone. Because diode D2 and transistor T2 are conducting, the base of transistor T3 is pulled low. Because of this, capacitor C2 is drained. Because transistor T3 is in cut-off state, the astable multivibrator stops oscillating, and transistor T4 starts conducting. The buzzer doesn't make a beep sound because transistor T4 is on and transistor T5 is off. When the DC voltage across capacitor C1 goes above or below the window voltage level, transistor T2 turns off. Its collector voltage goes up, and so does the voltage across diode D2. So, there is no way for capacitor C2 to be drained through diode D2. The astable multivibrator starts to beep, and the time between beeps is controlled by the DC supply voltage, which is low when low-level voltage sampling is taking place and high when high-level voltage sampling is taking place. For an astable multivibrator to work, it takes less time to charge capacitors C2 and C3 when the DC voltage is high and a little more time when the DC voltage is low. So, when the voltage is low, the buzzer goes off several times in a row, while it only goes off once when the voltage is high.