Hey Geektimes!
I will tell you how you can use a regular bike computer as a cadence sensor.
There are more questions on this topic than answers on the Internet.
And even before you start, I will say that, if possible, it is worth buying a ready-made bike computer with a cadence sensor than to fence such a collective farm, as described in the article. Personally, I was bored and there was an extra bike computer lying around.
First, what is cadence? cadence is the cadence, the number of revolutions per minute.
Why know your cadence? Cadence is important to know for what is the optimal cadence - it is 80 to 110 rpm. This is a fairly high pedaling speed - this allows you to ride in a smaller gear, with less effort, as a result, there is less pressure on the joints, as well as less strain on the muscles. This is especially important for beginners - it can be enough strength to pedal with much more effort than it should be and ruin your health.
In my case, to create a cadence sensor, I used an abandoned bike computer with a broken reed switch (the very thing that is attached to the bike fork)
old bike computer
I had to restore the same buggy reed switch - I just cut it off from the wire leading to the bike computer and soldered the ordinary reed switch I bought instead. If the sensor in your bike computer works, then, of course, you don’t need to do anything. (but may require extending the sensor wire)
old sensor - reed switch
new sensor - a conventional industrial reed switch glued to a piece of plywood and filled with hot glue from a glue gun
installing the sensor on the pen and the magnet on the crank
Now it remains just to stretch the wire from the sensor to the computer and install it on the steering wheel.
In addition to the “cadence sensor” (white), I also have a regular bike computer installed.
One question still remains - how to set up the bike computer so that the readings from it can be adequately read as readings from the cadence sensor?
Very simple. It is enough to enter the circumference of the tire equal to 1666. (In any bike computer there is a wheel setting).
Specifically, in this bike computer - BCP-22, you can enter any wheel circumference, starting from 4999 mm, up to 1 mm.
Now the readings of the bike computer, multiplied by 10, will be equal to your cadence. For example, 7.5 mph equals a cadence of 75 rpm. Thank you for your attention.
Cadence for cyclists is the number of revolutions per minute that a cyclist makes while riding.
The optimal cadence depends both on the cyclist's own feelings and his age, weight, physiological characteristics and training, the type of bike and pedals, in particular, the style and speed of riding, the slope, the nature of the surface, and so on.
Of course, the pedaling speed must be changed depending on the circumstances: at the start (to warm up the muscles) and at low temperatures (to avoid damage to them), it should be quite high. But on average, a comfortable cadence of a cyclist is from 80 to 100 rpm - these are the numbers that beginners advise as a reference, at this speed the cyclist produces maximum power with minimal energy costs.
However, many experienced cyclists in the process of active riding come to their optimal figure on their own, and even deliberately reduce the cadence for stronger pumping of the muscles and adjusting to the desired pulse.
If you are an amateur and ride rarely, it is not necessary to ride with the ideal 80+ rpm. Slightly lower values are acceptable, for example, 70.
In order to measure cadence, you need to detect 1 minute and count the number of revolutions made (which is laborious and dangerous). Or you can purchase a special one that will take into account not only the distance traveled, time and speed of the journey, but also this important indicator. The cadence sensor consists of two parts: the cadence sensor, which is mounted on the bike frame, and the magnet on the crank.
Why is optimal cadence so important?
A beginner will ask: what difference does it make if you pedal slowly or quickly? And there really is a difference. Try to ride on flat terrain at the first speed - you will hardly move from a place, and you will spend a lot of energy - the number of revolutions per minute will be very large. And vice versa, try to drive up the mountain on a big star - if you succeed, then with great difficulty, it will simply be impossible to pedal.
Low cadence - dead knees!
First of all, cadence is important because of the harmful stress on the knee joints that occurs when pedaling too slowly. The knees are the most painful part of the body in cycling, as they have a long and unnatural load for a person.
With a low cadence, you need to constantly press on the pedals, and because of this, the muscles get clogged up very quickly - it is extremely difficult to ride. Moreover, slow pedaling with hard pedaling not only contributes to knee pain and muscle stiffness, but can also cause serious diseases - arthritis and arthrosis, which can be "earned" by improper cycling.
This is especially important for beginners, who, due to inexperience, can easily “kill” their knees: it seems to them that by pedaling with great force they are pumping their legs, but in fact they clog muscles and slowly kill joints!
If this problem is not relevant with rare “rides” for short distances around the city, then for those who decide to engage in cycling tourism or participate in marathons, this is extremely important. Do not lower the cadence below 60 rpm.
Pedal... fast?
It would seem that everything is simple: pedal as quickly as possible, and there will be no such problems. But not everything is so simple: with a fast rotation, an irrational use of energy occurs, since the forces expended on such a large number of revolutions do not give a proportional speed. And if you don’t use it, the legs slip off every now and then, which can cause a fall or just a strong blow to the foot with the pedal - not nice.
In cycling circles, maintaining a high cadence is considered to be a sign of a professional athlete, as pedaling quickly and efficiently at the same time is really difficult.
How to increase cadence?
If you somehow measured the cadence and realized that it was low, there is only one solution: choose a smaller gear in such areas and pedal faster. At first it will be unusual: it seems that you are spinning “idle” and you don’t feel the usual tension in your muscles. But once you get used to it, both your knees and the bike will thank you with their durability. Another option is to instill in yourself the habit of pedaling not constantly, but in small batches - this way the load on the joints will be less.
If you cannot achieve the recommended number of revolutions (from 80 to 100), you can try contact pedals or (as a more budget option) special straps for ordinary pedals - touklips. This will not only relieve your knees, but also increase efficiency.
Also, do not forget about the pedaling technique - press the pedal at the top and pull it up in the back, and not just push the pedal with the whole foot for a whole turn.
This video shows how fast the pros can pedal:
The ability to choose the right cadence is extremely important for both beginners responsible for the health of their joints and experienced cyclists. This is one of those indicators that a cyclist simply needs to track in order to record his athletic growth.
Summer is over, the cycling season is closing, it's time to take stock. I bring to your attention a review of a slightly advanced wireless computer with a cadence sensor and a heart rate monitor
But unfortunately he got into a small accident, broke his arm and the cycle computer being monitored. After licking the wounds, I had to choose a new one.
The choice fell on a relatively inexpensive bike computer from AliExpress. I bribed the presence of a cadence sensor and a heart rate monitor.
What is cadence
Cadence - from the English cadens (ˈkeɪdəns) - a word denoting in a general sense rhythm, pace, in cycling matters it means pedaling frequency, the number of revolutions per minute.
A very important indicator for those who do not want to kill their joints
The order is made, I paid $ 18.47 with delivery (Attention!!! now the price has changed!!!), the parcel came to non-rubber quickly, in 10 days. The packaging was standard - a package, and inside - a box a little tired from the road.
photo box
Equipment: 
scan of installation manual
The installation did not take very much time - the speed sensor magnet is placed on the rear wheel spoke, the cadence and speed sensors are attached with flagella, the cadence sensor magnet is attached with flagella to the left pedal crank.
photo of installed elements
speed sensor magnet 
The speed and cadence sensor is two sensors connected to each other. Sensor power - CR2032 
cadence sensor magnet 
The bike computer itself is easy to install on the steering wheel in any position. Bike computer power - CR2032 battery
Handlebar bike computer
platform for bike computer, fastened with flagella 
Backlight (turned on by long pressing button B) 
The cardio sensor is a chest belt. To activate, slightly moisten the rubber base of the sensor with water. The cardio sensor is powered by a CR2032 battery.
photo of the cardio sensor on the carcass
Scanned instructions
The bike computer has two control buttons 
Button A switches the setting parameters and pages, button B changes the parameter.
When you turn on the computer, the initial settings are made - time, wheel size, temperature units, age, weight, gender - for counting calories, the desired heart rate zone, and so on.
1 page - set the circumference of the wheel of bicycle No. 1.
2 pages - set env. 2 led
3 page - choose km/h or miles/h.
4 pages - age
5 pages - choose kg or Lb
6 pages - weight
7 page - set the desired range of heart rate zones (HRM) first Hi then Low.
8 pages - select 24 hours or 12 hours will show the clock.
Page 9 - Setting the current time.
10 pages - set after how many km there will be a reminder about the maintenance of bike No. 1
11 page - set after how many km there will be a reminder about the maintenance of led No. 2
Page 12 - Choose Celsius or Fahrenheit
Page 13 - Home screen, setup completed.
The screen has three zones. 
The upper segment is fixed and shows the cadence, the middle segment is also fixed and shows the speed, the lower one can be adjusted and has three menus: "BIKE", "CAD" and "HRM".
I'll tell you more about setting the view mode. Button B switches between the menu "BIKE", "CAD" and "HRM"
There are 10 values in the "BIKE" menu:
CLOCK - shows the clock.
ODO (TOTODO-1) - distance traveled led 1
TOT (TOTALODO) - the distance traveled in general
DST (TRIP DISTANCE) - completed per trip
MXS (MAXIMUM SPEED) - maximum speed
AVS (AVEGAGE SPEED) - average speed
TM (TRIP TIME) - trip time
TEMP (TEMPERATURE) - current temperature
CAL (CALORIES BURNED) - calories burned
FAT (FAT BURNT) - fat consumed
SCAN - cycles through some of these functions.
In the CAD menu (cadence) 4 values
CAD (CURRIEN CADENCE) - current cadence
MAX (MAXIMUM CADENCE) - maximum cadence
AVG (AVERAGE CADENCE) - average cadence
SCAN - cycles through values
There are 6 values in the HRM menu
HRM (HR PULSE) - current heart rate
MAX (MAXIMUM HR) - maximum heart rate value
MIN (MINIMUM HR) - minimum heart rate
AVG (AVERAGE HR) - average heart rate
ZONE(H***;L***) - parameters of the given training range of heart rate zones, first the highest value and then the lowest value.
SCAN - iterates over values.
A few more "tricks":
* Reset settings by long pressing buttons A and B
* If you need to reset the HRM values except for ZONE, you need to press the A (right) button for a long time in the HRM menu,
* ZONE must be reset separately in the ZONE menu, long press A.
* From the interesting - switching led1 - led2 is a quick press of 2 buttons.
* If you long press A in CLOCK mode, you can set the clock without resetting.
*TRIP DST, AVS and TRIP TIME can be reset to 0 by long pressing A in DST mode.
* If you long press A in ODO mode, then you can first set the circumference of 1 bike, then the second, then ODO 1 of the bike (5 values in the ****.* format, that is, the maximum is 9999.9 km) and then ODO 2 is large.
* Long press B turns on the backlight
Traveled with him for two months. Everything works and functions. There was a nuance with the cardio sensor - there was a marriage, the cardio sensor killed the battery to zero in half an hour of driving. I contacted the seller, asked about the cardiosensor. Sold promised to send a working cardio sensor. After some time, a serviceable cardiac sensor arrived, all questions about it were removed.
CONCLUSIONS
I am satisfied with the purchase, the seller surprised me with his decency, the bike computer is functioning. Well made, well made, looks great
important to me pros:
- wireless version (I don't like wires and flagella)
-cardiosensor for monitoring the training zone and a cadence sensor
- auto-on when the wheel is turned
- backlight
- temperature
- no interference from flashlight and other electronic sources
- informing the sensor battery low
- informing about the need to service the bike after a certain distance traveled
Minuses:
- Moisten the transducer with water or ECG gel before training
- only one window is customizable
- slightly complicated menu
The item was purchased with my own money. I wish you all the best and happiness!!! Take care of yourself!!!
In short, so as not to "kill" the knees. For more details, read on.
The cadence sensor, or cadence sensor, is an integral part of cycling. Any cyclist will attest. True, some of them will say that you can calculate it yourself: mark a minute and go - count.
This opinion is partly based on the fact that the price of cadence sensors used to be part of bike computers, and the bike computer was (and still is) quite expensive. True, separate cadence sensors have long been produced, which are not so expensive.
Why you need a cadence sensor
Optimal cadence is an opportunity to strengthen and maintain health. Cadence is developed individually over time, but if you are not yet at the “pro” stage, then you can find basic recommendations on the network: from 80 to 100 rpm. These figures are considered optimal in terms of stress on the joints and energy expenditure.
Low cadence is bad for the knees! If you dare to test this at first speed on a flat road, then pay attention to how often you will pedal: the load on the muscles is prohibitive, energy costs are enormous! There is a risk of falling off the bike, not getting off.
For those who get on the bike from time to time, it is generally not recommended to kill the readings on the cadence sensor below 60 rpm, so as not to provoke muscle clogging, diseases such as arthritis, arthrosis and related ones.
Is it possible to reduce the rotation of the pedals
Strictly over time. Professional athletes achieve optimal heart rate and energy expenditure by reducing the cadence.
Max cadence
The cadence sensor may go off scale, but this will not work when you first sit on the bike. Yes, and here, too, there are pitfalls: you can banally break. If, nevertheless, you do not plan to participate in the Olympic Games, then you should not play with the indicator: focus on averages and take care of your knees.
Which cadence sensor to buy?
Although technically a cadence sensor is a simple device, it is worth trusting brands that have already proven themselves in the circle of professional athletes.