Simple Non Contact AC Mains Voltage Detector Circuit

Electricity powers our world but it can also be dangerous.

This article explains how to build a handy tool that can detect live wires without touching them.

This is a great safety tool for people who work around electricity but also for anyone who wants to be careful at home.

WARNING: Even with this tool always be careful around electricity.

If you are not sure what you are doing call a qualified electrician.

Circuit Working:

Parts List:

The antenna captures AC signals from the air.

AC voltage induces a small current in the antenna.

The base of the first transistor is connected to the antenna.

As AC voltage is applied to the antenna it generates a separate current in the base of transistor Q1.

Q1 boots this signal and its emitter voltage follows the input signal.

The emitter of Q1 is connected to the base of Q2.

This design enhances the signal even further.

Q2 acts as an emitter follower providing high input impedance.

The emitter of Q2 is connected to the base of Q3.

This arrangement continues the signal amplification.

Transistor Q3 as the final emitter follower provides a high impedance output.

The collector of Q3 is connected to an LED with a current limiting resistor.

When AC voltage is detected the LED lights up.

The LED serves as an indicator of the presence of an AC voltage which nearby a antenna.

Formulas:

The following simple formulas will help to design and understanding the non-contact AC mains voltage detector circuit that uses transistors:

Q1 (R1) Collector Resistor:

For a typical red LED the resistor R1 connected in series with the LED and collector of Q1 transistor is chosen to keep the current flowing through the LED within a safe range of 10 to 20 mA.

Use ohms law to determine R1:

R1 = V_cc​−V_LED​​ / I_LED​

where,

• The supplied voltage is Vcc 9V battery voltage.
• The forward voltage drop of an LED is represented by VLED which is usually around 1.8V for a red LED.
• The required current flowing through the LED is represented by ILED e.g. 10 mA = 0.01 A

You may better understand and improve your circuit design with the help of above formulas which are based on the particular needs of your application and the components you have in hand.

Based on component characteristics and real world testing adjustments are required.

How to Build:

Below are the steps to build a Simple Non Contact AC Mains Voltage Detector Circuit:

• Place the three BC547 transistors on the PCB.
• Connect the collectors of the all the transistors to the positive supply using resistors.
• Connect the other end of the resistors to the positive terminal of the 9V battery.
• Wire the emitter of the transistor Q1 to the negative of 9V battery.
• Connect the emitter of the transistor Q2 to the base of the transistor Q1.
• Connect the emitter of the transistor Q3 to the base of transistor of Q2
• Connect the collector of the transistor Q1 to cathode of the LED.
• Connect the anode the LED to the positive supply
• Connect the base of the transistor Q3 to the antenna wire or conductor.
• Connect the positive terminal of the 9V battery to the positive supply on the PCB
• Connect the negative terminal of the 9V battery to the GND supply on the PCB.

Testing:

• Power the circuit by connecting the battery.
• If there is an AC voltage nearby the antenna the LED should light up.

Important Notes:

• Make sure to check twice the transistor pinouts and connect them correctly.
• Choose resistor values based on the specific requirements of your transistors and LED.
• Experiment with the length and positioning of the antenna to increase sensitivity.
• This is a simple design and depending on specific component features one should adjust resistor values for best performance.
• Always refer to datasheets for your specific components for accurate information.
• Also one should be cautious when working with electricity and ensure proper safety measures are followed

Conclusion:

The circuit can detect the AC mains voltage without making direct contact by using an antenna to pick up the AC signal from the air.

Once the signal has passed through three transistor stages an LED will flash to show that there is AC voltage present.

To find AC power sources or live wires without coming into contact with the this type of non contact AC mains voltage detector circuit is often used.

References

AC Mains contactless detection with DIGITAL