None of us likes bugs at home. To kill these flying insects, they should first be attracted and then electrocuted. An electric fly swatter is one such device with a high voltage electrocuting circuit and an insect attracting UV lamp of 365 nm wavelength. This ultraviolet fluorescent lamp is mounted in the middle of the cabinet and a pair of carefully spaced, electrically insulated, charged wire grids surround the light.
When an insect comes close enough to the mesh pair, an electrical arc is formed, the dielectric breaks down and current flows through the insect’s body. Electrocuting the insect doesn’t require it to touch both the wires as an arc forms in the air gap over 1800 V.
Electric fly swatter circuit
In the schematic, the grid is charged to 1800V by the voltage multiplier built around diodes D1 through D6 and capacitors C1 through C6. When a bug passes through the grid, the high grid voltage causes a discharge though the bug, the grid voltage reduces and the stored voltage kills the bug instantly.
To ignite the UV lamp, a voltage tapping is provided from the volt-age multiplier circuit. At switch-on, around 700V ionises the gas in the tubelight and capacitor C7 discharges through the tube, lowering the resistance enough to sustain continual AC current flow. C7 then continues to act as the ballast.
Construction & testing
Assemble the circuit on a general purpose PCB and enclose in a small rectangular cabinet. Fix the UV lamp and the assembled PCB inside the cabinet. Mount the high voltage grid outside the cabinet on the front panel. Now your electric bug zapper is ready to use.
As the circuit works on a high voltage, don’t touch the grid with bare hands to avoid electric shock, which can be dangerous!
The article was first published in May 2010 and has recently been updated.