Simple Non Contact AC Mains Voltage Detector Circuit

Electricity powers our world, but it can also be dangerous.

This post shows you 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 Operation:

Parts List:

The below mentioned are the circuit operation process:

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 induces a varying current in the base of Q1.

Q1 amplifies this signal, and its emitter voltage follows the input signal.

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

This configuration amplifies the signal 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.

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 in the vicinity of the antenna.

Formulas:

The following fundamental formulas will help with the design and comprehension of the non-contact AC mains voltage detector circuit that uses transistors:

Q1 (R1) Collector Resistor:

In order to limit the current flowing through the LED to a safe value typically between 10 and 20 mA for a normal red LED the resistor R1 connected in series with the LED and collector of Q1 was selected.

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 intended current flowing through the LED is represented by ILED e.g. 10 mA = 0.01 A

You may better comprehend and improve your circuit design with the aid of these formula and recommendations, which are based on the particular needs of your application and the components you have on hand.

Based on component characteristics and real world testing, adjustments can be required.

Construction Details:

• Place the three BC547 transistors on the PCB.
• Connect the collector of the first two transistors to the positive supply using resistors.
• Connect the other end of the resistors to the positive terminal of the 9V battery.
• Connect the emitter of the first transistor Q1 to the base of the second transistor Q2.
• Connect the emitter of the second transistor Q2 to the base of the third transistor Q3.
• Connect the emitter of the third transistor Q3 to the negative terminal of the 9V battery.
• Connect the collector of the third transistor Q3 to one leg of the LED.
• Connect the other leg of the LED to the negative supply through a resistor.
• Connect the base of the first transistor Q1 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 negative supply on the PCB.

Testing:

• Power the circuit by connecting the battery.
• If there is an AC voltage in the vicinity of the antenna, the LED should light up.

Important Notes:

• Make sure to double check 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 optimize sensitivity.
• This is a basic guide, and depending on specific component characteristics, you might need to adjust resistor values for optimal performance.
• Always refer to datasheets for your specific components for accurate information.
• Additionally, be cautious when working with electricity, and ensure proper safety measures are followed.

Conclusion:

The circuit detects AC mains voltage without any direct contact by using an antenna to capture the AC signal from the air.

The signal is then amplified through three transistor stages, and the presence of AC voltage is indicated by the illumination of an LED.

This type of non contact AC mains voltage detector circuit is often used to detect the presence of live wires or the proximity of AC power sources without physical contact.

References

AC Mains contactless detection with DIGITAL