Low Voltage Circuit

Automatic Changeover Switch

Many electrical and electronic appliances require either DC or AC electricity to operate. Typically, AC power is sourced from the mains, while DC power is obtained from batteries. However, situations may arise when there is a shortage of either AC power (due to a blackout) or DC power (due to battery depletion). To address this challenge, various solutions are available. In emergencies, generators or inverters can be used to generate AC power when the mains supply is unavailable. For DC power, options include using batteries or an AC to DC power supply.

This article explores the principles, design, and operation of an automatic changeover circuit. This circuit is designed to power a DC load, such as a series of LEDs, either from a battery or an AC-DC power supply.


  • Automatic Changeover Switch Circuit Principle:
  • Automatic Changeover Switch Circuit Diagram:
    • Automatic Changeover Switch Circuit Design:
    • Automatic Changeover Circuit Operation:
    • Applications of Automatic Changeover Switch:
    • Limitations of this Circuit:

Automatic Changeover Switch Circuit Principle:

This circuit utilizes the bistable mode of operation of the 555 Timer. In this mode, the Timer’s output is in either a high or low state, determined by the conditions of the trigger and reset pins. The Timer’s output is linked to a transistor functioning as a switch, enabling control over whether the Timer’s output is active or inactive. Two LEDs connected in series serve as the load. When the transistor is switched off, the LEDs are powered by the AC-DC power source, but when the transistor is switched on, the LEDs are powered by the battery.

Automatic Changeover Switch Circuit Diagram:

Automatic Changeover Switch

Automatic Changeover Switch Circuit Design:

1. Design of AC – DC Power Supply:

The initial stage involves selecting a voltage regulator. To accommodate the two series-connected LEDs and a Schottky diode, we opt for an LM7809 voltage regulator with a 9V output. To meet the regulator’s input voltage requirement of at least 12V, we decide on an input voltage of approximately 20V.

The subsequent step is the selection of the transformer. Since the primary voltage is 230V and the desired secondary voltage is roughly 20V, we can utilize a standard 230V/20V transformer.

The third step is to choose the diodes for the bridge rectifier. Given that the peak voltage across the transformer secondary is around 28V, the total Peak Inverse Voltage (PIV) of the bridge should be approximately 112V. Therefore, we need diodes with a PIV rating exceeding 112V. We’ve settled on 1N4007 diodes, which have a PIV of around 1000V.

The final step involves selecting a filter capacitor. With a peak voltage of 26V and a minimum regulator input voltage of 12V, the allowable ripple for a capacitor is roughly 14V. Using the formula C = I (t/V), where I represents the sum of the voltage regulator’s quiescent current and the required load current, we calculate a value of around 17uF. For this application, we’ll use a 20uF electrolytic capacitor.

2. Design of Bistable Multivibrator Circuit using 555 Timer:

The 555 Timer, operating in bistable multivibrator mode, produces either a high or low logic signal. Grounding the trigger pin results in a high logic signal at the output, while grounding the reset pin results in a low logic signal at the output. In this circuit, the output of the 555 Timer is linked to the base of the BC547 transistor.

Automatic Changeover Circuit Operation:

Once switch S1 is positioned, the circuit initiates its operation. In the case of switch S1 being set to position 1, the 555 Timer’s reset pin, which corresponds to the SR Flip-Flop’s reset pin internally, is connected to ground. Consequently, the 555 Timer’s output registers a low logic signal. With the base-emitter connection of Q1 being reverse-biased, it is cut off. The load LEDs are then directly connected to the voltage regulator’s output via the Schottky diode. This is when the AC to DC power supply circuit comes into action. The transformer reduces the AC power, and the bridge rectifier transforms it into an unregulated and fluctuating DC voltage. The filter capacitor eliminates AC ripples from the fluctuating DC voltage. Finally, the voltage regulator converts this unregulated DC voltage into a stable, regulated DC voltage.

On the other hand, when switch S1 is set to position 2, the 555 Timer’s trigger pin is grounded. Consequently, the 555 Timer’s output goes high in logic. This forward-biases the base-emitter junction of Q1, driving the transistor into saturation and turning it on. Two important points should be noted here: firstly, the Schottky diode ceases conduction as there is no potential difference at the junction since the voltage across the diode’s cathode and anode is zero. Secondly, the LEDs are now powered by the battery voltage and are biased through the resistor and transistor.

Applications of Automatic Changeover Switch:

  1. This circuit can be used as a home lighting system with few modifications.
  2. It can be used to drive other DC loads like a DC motor of any electronic appliance or other toy applications.

Limitations of this Circuit:

  • This is a theoretical circuit and may require few changes when implemented on PCB.

Related Articles

Leave a Reply

Your email address will not be published.

Check Also
Back to top button