Automatic Changeover Switch Circuit Using 555 Timer IC

All the electrical appliances we use require DC or AC power for their work. We get AC power from mains and DC power from batteries and DC power supply. But there also we face some problems due to power cuts or the limited life of batteries when needed. Now we have a solution for this, we use generators for AC and extra batteries for DC.

In this project, we are going to build an automatic changeover circuit. In this circuit, a DC load like a series of LED are operated by a battery or AC-DC power supply.

Principle Behind Automatic Changeover Switch Circuit

This circuit is based on the principle of a bistable mode of 555 Timer IC. The Timer output is either low or high depending on the reset and trigger pin. This output is connected with a BC547 NPN transistor which works as a automatic switch. This switch is on or off according to the output response. We use two LEDs as a load in this circuit. The principle of this project is straightforward. In case of the transistor is off, LEDs get power from the AC-DC power supply and when the transistor remains on the LEDs get power from the battery.

Circuit Diagram

Automatic Changeover Switch Circuit Diagram

Components Required

  • 555 Timer IC
  • 220-20V Step-down Transformer
  • Bridge Rectifier Module
  • LM7089 Voltage Regulator
  • 1N5819 Schottky Diode
  • BC547 NPN Transistor
  • 0.01uF Capacitor (x2)
  • 20uF/25V Capacitor
  • 10KΩ Resistor (x2)
  • 100Ω Resistor
  • Single Pole 2-Way Switch
  • 9V Battery
  • LED (x2)
  • Veroboard

Circuit Connection of Automatic Changeover Switch Circuit

The circuit is divided into two parts.

The first part is the AC-DC power supply. The circuit is a simple AC-DC supply circuit with a transformer and a bridge rectifier.

In this circuit, we need to drive two LEDs in the series show we use a Schottky diode and LM7809 voltage regulator IC. The input voltage is at least 12V required so we set a 20V input voltage for this.

We use a 230V to 20V step-down transformer—the diode the IN4007 which has a PIV of 1000V. Now time to choose filter capacitors. For a capacitor, the peak voltage of 26V and the minimum regulator input voltage is 12V. The ripple is about 14V. We calculate the capacitance by the formula, C = I(t/V). where I am the sum of the quiescent current of the voltage regulator and the required load current. After calculation, we get the value of about 17uF. So we take a 22uF/25V electrolyte capacitor.

The second part is 555 Timer as a bistable multivibrator. This system output is either a high or low signal. When the trigger pin is grounded, the output is a high logic signal and when the reset pin is grounded output is low logic signal. The output of 555 Timer is connected with BC547.

PCB Design

After designing the schematic diagram of the Automatic Changeover Switch circuit, the assembled components and wiring are too clumsy and looked very unprofessional. In fact, the wiring also has a chance of loose connection. To give it a clean and professional look and also to compress the size, I decided to build its PCB prototype using EasyEDA software as it is so simple to use. Now come to the main part, where we need to order our PCB prototype. I always prefer PCBWay for their quality assurance, fastest delivery and also for 24/7 customer support.

You can view the Gerber file here Gerber File Viewer.

PCB View

Automatic Changeover Switch 2D View
2D View
Automatic Changeover Switch 3D View
3D View

Why I Choose PCBWay as My First Preference?

I’ve done several runs with PCBWay and am happy with the results, it’s good quality. The etching, solder mask, and hole sizes are all done well and that is what matters to me. It takes a few hours for a design to get approved and then a couple of days to finish and ship. With more than a decade in the field of PCB prototypes and fabrication, PCBWay has proved its assurance from time to time. They always look at the customer’s needs from different regions in terms of quality, on-time delivery, cost-effectiveness and any other demanding requests.

PCBWay Gold Finish
PCBWay Thickness

These PCBs were manufactured by PCBWay and the finish quality really impressive, especially with the gold finish path. If you want to finish your product faster you could also ask PCBWay to make a panelized order where you receive multiple PCBs in a single panel. With this, you will also receive SMT Stencil from PCBWay, saving you time and effort. All the orders are high quality and could select a lot of settings such as thickness, flexible PCB, the colour of the solder masks, the number of layers, material, surface finish and more.

PCBWay different types of solder masks

How to Order PCB from PCBWay?

Working Principle of Automatic Changeover Switch Circuit

The working of this circuit is very easy. The circuit will work for any position of switch1. When switch 1 is at position 1, the reset pin of the 555 Timer is grounded. Actually, the reset pin is the reset pin of the SR flip flop. Hence the output of the timer is a low logic signal. Now the base-emitter junction of BC547 is reverse-biased so it is in a cut-off position. The load LEDs are directly connected to the Zener diode.

In this position, the role of the AC-DC power supply starts. The AC power is dropped by the step-down transformer and then converted to fluctuating DC by the bridge rectifier. The filter capacitor eliminates the ripples of the AC power. At last, the voltage regulator LM7809 convert this fluctuating DC into regulated form.

When switch 2 is at position 2, the trigger pin of the 555 Timer is grounded. So the output of the Timer is a high logic signal. The base-emitter junction is now in forward-based mode. So the transistor is in saturation mode and it is in the on position. In this condition, the Schottky diode doesn’t conduct voltage because the voltage difference between both ends of the diode is zero. So there is no voltage in the junction. In this situation, the LEDs are driven through the resistors and the system works with the voltage of the battery.

Applications of Automatic Changeover Switch Circuit

This circuit can be used in various conditions like home lighting, DC motors of any appliances, and any loads of house.

Limitations of Automatic Changeover Switch


This circuit is just a theoretical circuit. Before applying across greater loads it has to be modified. The voltage regulator is for 9V but for higher loads it has to be increased.

Leave a Comment