I purchased this product in May 2023. Today is Nov 2024 and I have found a major design flaw in this product.

My toothbrush is just not stopping!

The stop function on the brush is just not working, this made me realize there is a fault within the PCB.

I opened the case from the bottom and used a screwdriver to pull the entire assembly out, the toothbrush was still vibrating, I pulled the battery out and only then it went off.

I connected my multimeter and quickly found the issue with the transistor, this NPN transistor’s collector and emitter were shorted, motor is connected to collector so even without a base current on transistor (provided by turning on switch) motor was working as it had a path to ground via emitter, collector to emitter current flow is only supposed to work on switch operation but as there was a short, the flow was on by default.

But why transistor failed? there could be several reasons, sudden surge current? continued operation for 1.5 year( lol! i used 2mins a day, that would total to be 1095 mins or 18.25hrs, any decent transistor is not supposed to fail at these operational hours), a possible reason could be cheap parts. There was no number/marking on the transistor just the letter O was there, no idea what manufacturer or transistor series that is.

I am an electronics hobbyist, and with the right tools, I was able to track down the issue. Other users with this same issue might just remove the battery and throw out the device or contact customer care if it is still within warranty.

Solving the issue

I used a hot air gun and removed the transistor. I replaced the transistor with a BC547.

Soldered Image Attached.

This issue could be easily avoided by having a quality part instead of a knockoff or at least should have a defense mechanism like a dedicated power button or additional circuity to allow user to at least turn off without accessing the PCB, user can then call customer care to get a resolution.

Impacts

There is a potential danger that someone might have this issue while the device is left charging or they just kept it in the washroom and transistor could fail leaving the device vibrating until the batteries die also it will become hot as it’s a Li-ion battery.

This is not a well-thought-out product from a user perspective, I don’t think Caresmith hired engineers to design this product, must have walked into a factory in china and secured the cheapest deal for the Indian market, Like every other tech startup in india which claims it does tech but all it does is design logo for the product - marketing companies disguised as tech companies.

Yes, I bought it as it suits my budget but there should be a basic minimum of thought and design that needs to go into any product before mass producing it.