Teardown | Electric Mosquito Racket

Welcome to the first product teardown by T Works. We decided that the popular electric mosquito racket would be a great candidate for our first teardown since it is a great example of how a simple working principle can be packaged into a product that is economical, effective, and fun to use. In this teardown, we will discuss the external design of the racket, the internal mechanisms and electrical components used, and the manufacturing techniques used to make the components.

The racket is a device used to kill mosquitos; the user waves the racket around to catch a mosquito between the metal grid, which shorts the circuit and electrocutes the insect. A Taiwanese inventor, Tsao-I Shih, is often credited with inventing the modern mosquito racket; he filed for a patent for an “electronic insect-killing swatter” in 1996. The device is extremely popular in tropical countries like India, where mosquitos are a nuisance and responsible for spreading lethal diseases like malaria and dengue. The racket offers an effective alternative to other mosquito repelling/killing solutions; it does not use harmful chemicals or require expensive refills, does not produce smoke or fumes, and ensures that the insect is dead.

There are many brands and variations of the racket available in the market. We purchased our racket for ₹400 and it comes with a 4V 600 mAh rechargeable lead acid battery and outputs a maximum of 2400 V at the mesh. The racket is made by Akari, a company based in Hong Kong. Most of the rackets available in the Indian market are made in China.

Product specifications:

  • Dimensions: 530 x 220 x 30 mm

  • Weight: 310 g

  • Price: ₹ 400

The device is packaged in a clear polypropylene bag with a sticker label which details the brand, device specifications, usage instructions, and warnings. The bag can be reused to store the device. This is a good example of cost-effective packaging that is useful throughout the life of the product.

The shape of the device is deliberately modelled after a tennis racket so that the user intuitively knows how to hold it and use it; the mosquito is the equivalent to a tennis ball that you have to hit. Designers employ the shapes of existing products already familiar to users when developing the form factor of a new device which requires user interaction. One such example is the pistol grip design, which is employed in other devices such as a power drill, IR guns, and glue guns.

There are two switches: one controls the main circuit and the flashlight, and the other button must be pushed down to activate the mesh circuit. The retractable plugs are a nice feature that can store the plugs flush to the handle when it’s not in being charged.

TEARDOWN

The device assembly can be divided into two subassemblies: the handle subassembly and the mesh subassembly. The handle enclosure sandwiches the mesh subassembly and is held together by self-tapping screws. After taking the enclosure top off, we can see the electrical circuitry. The PCB is carefully positioned to allow the slide switch and the push button switch to be mounted on the side of the handle without requiring an extra board and wires; this saves assembly time and cost.

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The handle enclosures are injection moulded ABS plastic parts. The design is optimised to allow the use of a single pull mould (i.e. without side actions) which simplifies the mould design and decreases mould cost. Ejector pin marks are visible on the inner surface. The thickness of the enclosure varies from 1.7 to 3.0 mm. A lip and groove feature is added to the edge for alignment between the enclosure shells. The standoffs are designed for self tapping screws. The special screws cut their own thread into the soft plastic during installation. Self tapping screws are cheaper than installing a metal screw insert into the plastic and using a normal screw but they are not suitable for multiple reassembly cycles.

The retractable plug uses a steel sheet that acts both a spring element and a sliding electrical contact. This is a clever and economical solution to provide a value added feature — a retractable plug. The plug is made by insert moulding two steel pins with ABS plastic. The plug pins have pointed ends that dig into the sheet to maintain a good electrical connection. The mechanism does not appear to be robust, however, and could be a critical point of failure if the contact between the pin and metal sheet is lost due to mishandling, resulting in the user not being able to recharge the battery. 

The sheet contacts are held in place by another small injection moulded part. This part is interesting as it showcases multiple injection moulding defects, such as flow lines, deformations, and sink marks. Since the defects do not affect the functionality of the part nor is the part visible to the customer, the defects were deemed acceptable.

The main printed circuit board (PCB) only has one layer. The board appears to be manually assembled, with through hole (TH) mounting type components placed on one side and then hand soldered on the other side, where the copper traces are visible. A separate board is used to mount the flashlight LED in the flashlight cup. Hand assembly and soldering takes longer than automated pick-and-place machines, is more expensive in high volumes, and is prone to errors; however, labour is relatively inexpensive in China, and the chosen components that require manual assembly may have been cheaper, which must have made manual assembly more economical. Note that no electronic components are used, which is something that is becoming increasingly rare in modern devices.

There are many electrical schematics for bug zappers available online; one example has been provided below. The circuit consists of three main parts. The power supply (red) converts AC input into DC to charge the battery. The battery feeds the oscillator circuit (green), which converts DC back to AC. A transformer steps up the voltage, which is further stepped up by a diode/capacitor ladder network (blue). The final voltage, which can range up to several hundred volts (2400 V for our racket), is stored inside an output capacitor (C7) across the output ends going to the metal mesh. The capacitor discharges when the circuit is shorted by a mosquito.

The mesh subassembly consists of 2 green plastic enclosure parts in the shape of the head of the racket, 3 metal wire meshes that serve as the terminals, and 2 yellow spacers that are sandwiched between the mesh to ensure that the mesh do not short.

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The enclosures and spacers are single pull injection moulded ABS plastic just like the handle enclosure. On close inspection, tool marks are visible on the inner and outer surface; this is an artefact of the machining done on the moulding tool. Generally, the mould surface is polished to ensure cosmetic surfaces appear smooth but, in order to avoid the added cost, the manufacturer decided to not polish it. Although these marks may not be noticed by the average consumer, it is good practice to ensure that the outer surfaces of the enclosure do not showcase visible defects. The tooling marks also suggest that the moulding tool was likely made of aluminium, which is faster to produce and less expensive than a tool steel mould. However, the aluminium mould will last fewer cycles.

The mesh used is called an expanded metal mesh, which is made using a special punch on sheet metal, as illustrated below. The mesh material is aluminium, which is a good electrical conductor.

Credit:  YouTube

Credit: YouTube

BOM ESTIMATE

The bill of materials (BOM) is a complete list of the raw materials and components required to make the product. The cost of the electrical components, metal parts, and plastic parts was estimated for mass production quantities. For the injection moulded parts, the mould tooling and material cost was estimated by a local Indian manufacturer as well as a Chinese manufacturer. The mould cost was divided by the mould life (generally 100k cycles) and added to the material cost to give the final cost. Assembly cost was not considered as this is dependent on the complexity of assembly, time taken per step, and region specific labour cost. The BOM cost is an important consideration for any product as it determines the final retail price. A general rule of thumb suggests that the BOM cost should be about 15% of the retail price

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Based on the cost of the plastic components from different suppliers, we get two significantly different estimates of the BOM of the mosquito racket. Using an Indian supplier, the BOM cost is estimated to be ₹307, which is 77% of the final retail price. However, if the plastic components are supplied by a Chinese manufacturer, the material cost of the large plastic components is significantly lower, resulting in a more reasonable BOM cost of ₹194, which is 48% of the final retail price. This is also a big reason why China is the current manufacturing hub of the world: due to the availability of low cost manpower as well as access to a variety of manufacturing technology at low costs. Competition among manufacturers may also force them to reduce their profit margins, which is beneficial for consumers.

SUMMARY

The device is a good example of an electrical product that packages its circuitry well into a useful form factor. From personal experience, I can vouch for the racket’s effectiveness at instantly electrocuting mosquitos that are caught in the mesh. The physical design appears structurally robust and will likely survive small drops and impacts.

The design also appears to be heavily optimised to reduce manufacturing costs for every component. This is expected, as the device has no IP to differentiate itself in a competitive market where price is a major purchase influencer. All the plastic enclosure parts are injection moulded, which is an economic solution for high volume production runs, and we see evidence of cost-cutting to further reduce the unit price. The PCB is manually assembled and soldered, which may be an economical solution for the manufacturers. 

The design also lends itself well to reassembly and repair; so should your racket stop working, it may be an interesting project to figure out what component has stopped working and then replace it. Common components that can fail are: the FE transistor, the battery, the plug contact mechanism, and the soldered connections. However, given the low cost of the device, many people opt to replace the complete racket when it stops working. 

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Thank you for going through T Work’s very first product teardown. The purpose of these teardowns is to help promote a culture of making by lowering the knowledge barrier required to design and create your own ideas and inventions. By studying existing solutions, the designer can draw inspiration for their own design, and also avoid ‘reinventing the wheel’, which can help save significant design time and resources. We want this series to be a collaborative effort between T Works and our community and so we would love to hear from you! What are some products you would like to see taken apart? What sections can we add to our teardowns that would add more value to the maker community? Send us your comments, questions, and suggestions to teardown@tworks.in!