Possibly most families except the misfortunate have a bicycle. The “premium” China bicycle speedometers sells at $5 to $7 on Ebay, Amazon. Initial plan to develop a DIY speedometer with Arduino and TM1637 LED display dismissed as the unit costs lesser than half than DIY. Here is China bicycle speedometer review with infographics explaining functioning parts, pdf manual and how it really performs in real life. Manual has not much technical information (i.e. it is not a data sheet). Here is a PDF of china-bicycle-speedometer-manual, you can download and check.
China Bicycle Speedometer Review
The package will have the meter, battery, mounting base, a receiver (which is intended to be installed over the shock absorbers/front wheel column) and a “magnet” (which is intended to be installed on any spoke/fork). There are cables, ties etc supplied. The sensor and “magnet” can have plastic film coverings for packaging, remove them after installation. When you’ll switch on the unit, a four digit will blink.
Open your accompanying manual, that digit needs to be adjusted according to your radius of wheel. Please note that front wheel always travel more than the rare wheel. In old days, automobiles had speedometer at rear wheels. I do not know how it became common in front wheels for automobiles like cars, may be to falsely show higher mileage.
How it is? At $5 – $7 what you expect? It is somewhat matter of luck, your unit can give unexpected good result. You’ll notice on all online shops (where rating available) ratings are either 1 star or 5 star. There is no quality control and possibly there are inferior companies who manufacture look alike. My one beautifully performs and can compete with costly wired mechanical speedometer. My father’s one is just a crap. My luck usually goes with cheap China DIY electronics stuffs. But luck with Nokia 5110 display was not hugely great. With that Nokia 5110 display I came to know that China collects used electronic stuffs, desolder the SMD components from PCB, then manufacture from those used things without much quality control. Our standard “good companies” now also follow the same principle. You’ll notice recycle icons here and there on regular branded stuffs as if the package can be recycled.
The cheaper version of these speedometers (which are under $7) does NOT come with a battery. Battery has written 23A 12V (replaces) MN21 / MS21, V23GA, L1028. Do not forget to check whether there is backlight. The display light does not remain continuously on. You have to press a button to turn it on briefly. If the wheel stops spinning, the unit goes to “sleep” mode.