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Bicycle gadget speed detector

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I am making a bicycle gadget with led lights to light up the front and warn the back. However, I also want to add a sensor or something to detect the speed of the bicycle and the buzzer will buzz when the bicycle is travelling at a certain speed. What should I use to measure the speed and what do I need to trigger the buzzer if the speed is over a certain speed?
 

The classic method would be to shine light from an LED though the wheel spokes to a sensor. As each spoke interrupts the light source it will generate a pulse rate proportional to road speed (if you aren't doing 'wheelies!'). You can use the pulses to operate a charge pump, giving a voltage rising as you go faster or you can count the pulses per second and calculate an exact speed from that.

Brian.
 

Hello, Thank You for replying so quickly, but is there any way to detect the speed of the bicycle without the need of the wheel, but just a device on it's own?
 

HiE said:
...but is there any way to detect the speed of the bicycle without the need of the wheel, but just a device on it's own?
Click to expand...

Newton's first law says that a body at rest or in uniform motion in a straight line, are equivalent (both are inertial frames). In other words, a device without external information has no way to make out whether it is in uniform motion or at rest. Your mobile uses satellite connection to determine the velocity of you moving in the road.

So you need a velocity sensor. It must look outside to find some reference with respect to some stationary object. Most are messy and I think an air-speed detection may be something like what you are thinking. One advantage will be that you can unmount it rather easily. Just mount two pressure sensors back to back (one facing the front, the direction of motion and the other looking back) and measure the air speed.
 

Air speed detector? Sometimes when I ride my bike I feel the wind behind me, but I am not going backwards. Other times I feel the wind, but I am not moving.
 

Hi,

GPS?
Dynamo frequency?

Klaus
 

Audioguru said:
Sometimes when I ride my bike I feel the wind behind me, but I am not going backwards. Other times I feel the wind, but I am not moving.
Click to expand...

That is precisely the purpose for two back to back sensors. Perhaps it may be better to mount the second one in a perpendicular way? The first one measure the velocity of the air+cycle. The second one measures the velocity of air only. Another alternative is to install an ultrasonic receiver and transmitter in the direction of motion. The velocity can be measured measured by Doppler shift.

(this is not my idea; this is the way we measure the velocity of stars and galaxies)
 

Hello, would it be okay if I add a Hall effect sensor? How far away can my magnet be to get detected by the sensor?
 

What I have seen in 'common' motors the sensor is quite close to the magnet- about 1 mm or less. Depends of course on the strength of the magnet. You need to experiment.
 

Hi,

There are hall sensors with analog output...then you may adjust the threshold yourself in a wide range with external circuitry.
And there are hall sensors with digital output. Read datasheet about threshold value.

And there are magnets in different size and different field strength, for sure this determines distance too.

Klaus
 

An alternative to Hall sensors is a reluctance sensor, basically a pick-up coil with a magnetic core. It can detect the passage of nearby ferrous materials so you could position it close to any undulating rotating part of the bike, possibly even the chain. Their drawback is needing analog circuits to amplify the signal they produce but you can still get LM2907/LM2917 or equivalent frequency to voltage converters which would give you a voltage proportional to speed. The advantage of wheel speed is it tells you your actual road speed, if you measure anywhere else you may have to take into account gearing ratios.

Brian.
 

Hello, I would like to ask that if I use a reed switch, on bumby surfaces, will it cause the reed switch to close? I might want to use it to detect speed.
 

My bicycle speedometer uses a reed switch and it seems to work OK. I have a small magnet (~10mm) clipped to one spoke of the front wheel that operates a reed switch mounted on the forks. I have never noticed any vibration problems but you do have to accept that reeds do eventually wear out as they are mechanically operated devices. They should be good for a few years of normal useage though.

Brian.
 


Reed relays were originally used in telephone exchanges and are very robust- many have mercury wetted contact lips that live for very long time (I am only spreading hearsay).
 

Hi,

Maybe the mercury increases contact livetime, but mainly this is used to avoid contact bouncing.

Klaus
 

I don't think mercury reed switches have been manufactured for a long time except for very high power switching applications. In that instance they are frighteningly expensive and rather large to fit on a bike!

A small dry reed should be good for several million operations but it's surprisingly fast how the number of wheel rotations counts up. A short cycle ride to my local village and back would be just over 2,500 rotations.

Brian.
 

KlausST said:
Maybe the mercury increases contact livetime, but mainly this is used to avoid contact bouncing.
Click to expand...

Very likely; mercury has an exceptionally high surface tension. Around 500 whereas it is less than 80 for water.
 

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