Mosquito Killer Bat Circuit – Complete Circuit Explanation and Working

Mosquitoes have always been a nuisance, not only because of the irritation they cause but also because they spread dangerous diseases such as malaria, dengue, and chikungunya. In recent years, mosquito killer bats (also called mosquito swatters or mosquito rackets) have become very popular as a handy household device.

Unlike chemical repellents or coils, mosquito bats kill mosquitoes instantly through high-voltage electric discharge, making them both effective and environmentally friendly. The schematic shown above explains the Mosquito Killer Bat Circuit, which is the heart of this portable device.

Overview of the Mosquito Bat Circuit

The working principle of a mosquito killer bat is quite simple:

• A small 3.7V rechargeable battery is used as the power source.
• The low DC voltage is boosted to several hundred volts using a step-up transformer and a voltage multiplier circuit.
• This high voltage is applied across the mesh or grid at the face of the bat.
• When a mosquito touches the electrified mesh, it gets zapped by the high voltage and dies instantly.

The circuit makes clever use of a BD139 transistor, a step-up transformer (TR1), and a voltage multiplier stage to achieve this conversion.

Circuit Diagram Explanation

Looking at the schematic in the image, here’s how the circuit works step by step:

1. Power Supply – Battery and Charging

• The bat typically runs from a 3.7V lithium rechargeable battery.
• The charging section is not shown here fully, but generally, a small DC charging module is connected.
• The +4V supply enters the circuit to power the oscillator stage.

2. Oscillator Stage (BD139 Transistor + Feedback Coils)

• The BD139 NPN transistor (Q1) is the main driver of the circuit.
• Along with the feedback windings (L2, L3) of the transformer, it forms a self-oscillating circuit.
• When powered, the transistor switches rapidly ON and OFF, creating alternating current in the primary windings.

Working of Feedback:

• L3 (13 turns) provides base drive through R3 (2.2kΩ).
• L2 (35 turns) provides necessary oscillation feedback to sustain oscillations.
• R4 (22kΩ) biases the transistor base to start oscillation.

3. Step-Up Transformer (TR1)

• The transformer is the most important part.
• L1 (1250 turns) is the high-voltage secondary winding.
• The primary has multiple coils (L2 = 35T, L3 = 13T) for oscillation and energy transfer.
• This transformer boosts the 4V DC into pulses of several hundred volts.

4. Voltage Multiplier Circuit

• The high-voltage AC from TR1 is further rectified and multiplied using a diode-capacitor network (voltage multiplier).
• This increases the voltage even further, up to around 1000V DC output.

5. Output Mesh (Bat Net)

• The output is fed across a metallic mesh frame (in the bat).
• This mesh is split into two separate layers.
• When a mosquito flies between them, it bridges the gap, causing a discharge arc and killing the insect.

Component Breakdown

• Battery: Rechargeable 3.7V – 4.2V Lithium Battery.
• Q1 (BD139): NPN transistor working as a high-frequency oscillator driver.
• R3 (2.2kΩ): Limits base current, ensures stable oscillation.
• R4 (22kΩ): Provides transistor bias.
• LED2 (3mm red LED): Indicates charging or ON condition.
• Transformer (TR1): Custom-wound ferrite core transformer.
• Voltage Multiplier: A Network of fast recovery diodes and capacitors to step up voltage.
• Mesh Grid: A Metal frame where high voltage appears to kill mosquitoes.

How the Mosquito Killer Bat Works

1. The 3.7V battery supplies power to the BD139 transistor circuitry.
2. The transistor, resistors R3 and R4, and transformer coils L2–L3 form an oscillator.
3. The transformer boosts this low voltage into AC pulses of several hundred volts.
4. These pulses are further increased using the voltage multiplier.
5. The mesh now has a 500V to 1000V DC potential difference across it.
6. When a mosquito makes contact between the two mesh layers, the circuit discharges energy rapidly, killing it instantly.

Applications

1. Mosquito Control – Main use in households and outdoors.
2. Portable Device – Works with a rechargeable battery, can be taken anywhere.
3. Eco-Friendly Solution – No chemicals or fumes like mosquito coils or sprays.
4. General Bug Control – Effective even against small flies, gnats, or other small insects.

Advantages of the Mosquito Bat Circuit

• Highly Effective – Kills mosquitoes instantly.
• Portable – Lightweight and rechargeable.
• Eco-Friendly – No harmful smoke or sprays.
• Low Power Consumption – Works on a small lithium battery.
• Durable Design – A Simple circuit with fewer components ensures longevity.

Limitations

• Manual Operation – You need to swing or place the bat where mosquitoes fly.
• Recharge Time – Battery needs charging periodically.
• Safety Risk – High voltage across the mesh can give mild shocks if touched accidentally.
• Limited Coverage – Works only in the area you can physically reach.

Safety Precautions

⚠️ Although the current is very low and generally not life-threatening, the high-voltage mesh can give a painful shock.

• Keep away from children.
• Do not touch the mesh while ON.
• Ensure the charging unit is properly insulated.
• Use good-quality diodes and capacitors in the multiplier stage to avoid breakdown.
• Do not use near flammable gases or liquids.

DIY Building Tips

• Winding the transformer correctly is crucial – insulation must be carefully handled.
• Always use enameled copper wire of suitable thickness.
• Ensure proper spacing and insulation between the transformer windings.
• Use fast recovery diodes (FR107, UF4007) for the multiplier circuit instead of regular 1N4007 for efficiency.
• Add a fuse or PTC for battery protection.
• Place components neatly on the PCB to reduce stray losses.

Conclusion

The Mosquito Killer Bat Circuit using BD139 transistor and step-up transformer is a simple yet highly efficient way to generate high voltage from a compact 3.7V battery. The generated high DC voltage across the mesh makes it possible to eliminate mosquitoes instantly, without chemicals or environmental pollution.

This circuit is a great example of how clever use of oscillators, transformers, and voltage multipliers can provide practical, eco-friendly solutions to everyday problems.

By understanding the core design and working principles, hobbyists can repair, modify, or even design their own mosquito elimination devices.