Inertial LED brake light for bicycle

wayneh

Joined Sep 9, 2010
17,496
My inclination would be to just use a ready-built accelerometer IC. I'm a fan of buy over make, if the right thing is commercially available.

You know, you could experiment with an iPod or iPhone. Strap it to your bike and ride around with an app that gives access to the accelerometers. (I have fun with this free one, but I'm not sure it would suit your need.) Before long I think you'd get a good feel for the physics and numbers involved. Would be good for testing your prototype, too.

BTW, be careful to keep your eyes on the road, too. ;)
 

jinbtown

Joined Aug 11, 2013
5
Didn't read every post here, but for low cost and simplicity Why not just use a mercury switch? Mounted at the correct angle, it would be 0-power draw and some fine tuning would allow you to get the angle to the point where only down hills would pose a problem. A small weighted gyroscopic mount could solve your hill problem and it would only light upon more rapid deceleration.

EDIT: could also salvage one for free from an old thermostat!!
 

BillO

Joined Nov 24, 2008
999
Why not just use a mercury switch?
Mercury is fairly low viscosity (believe it or not) and would tend to jump around with small vibrations from the road. This might be cheaper, but my feeling is it might not work too well. Certainly worth a shot though.
 

Wendy

Joined Mar 24, 2008
23,415
Looks like a hall effect switch next to a magnet on a spring. If the inertia of the magnet brings it close to the hall effect...
 

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
Looks like a hall effect switch next to a magnet on a spring. If the inertia of the magnet brings it close to the hall effect...
Correct, except that the Hall sensor is linear, instead of bipolar.

It's pretty much as I described it in post #19. With a 5V power supply, I get a 1V output differential from the Hall sensor between when the magnet is against the back stop and when the magnet is against the Hall sensor. Just looking at it with a voltmeter, it does seem fairly linear. That should be sufficient for the PIC to work with.
 

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
I took the assembly with me this morning while my wife drove the car, and it didn't work very well. The spring is too stiff to respond to gentle braking, and since I can't very well adjust the spring, I'll have to add more mass. I wish I had a flat spring in order to confine the movement to one axis. I'll try to think of something while I am stripping wallpaper. :(
 

Shagas

Joined May 13, 2013
804
I think this can be done with a 1 axis accelerometer. If you use an accelerometer then deceleration when going up a hill won't cause an led output because the accelerometer isn't really decelerating . It's just losing energy.
You can always write up some software which will filter out false outputs when you go over speed bumps etc.
 

LDC3

Joined Apr 27, 2013
924
I took the assembly with me this morning while my wife drove the car, and it didn't work very well. The spring is too stiff to respond to gentle braking, and since I can't very well adjust the spring, I'll have to add more mass. I wish I had a flat spring in order to confine the movement to one axis. I'll try to think of something while I am stripping wallpaper. :(
You can use a piece of plastic cut from a plastic container (probably the lid). You could use 2 or more strips if there is not enough stiffness.
 

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
At what deceleration rate do you think brake lights should come on?

Please state your answer in terms of MPH per second. Because this may be hard to think about, attached is a table that shows some various deceleration rates in terms of slowing from 60 MPH to a stop in a certain number of seconds. This table is obviously for automobile traffic, but the deceleration rate should be consistent for bicycles as well.

Thanks.
 

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wayneh

Joined Sep 9, 2010
17,496
My gut tells me that anything more than about -2mph/s is "braking". That's a 5 second stop from 10mph. It's gentle, but you certainly wouldn't stop that fast just coasting.

That's a total guess though. A few rides on the bike would resolve the issue.
 

THE_RB

Joined Feb 11, 2008
5,438
You can use a piece of plastic cut from a plastic container (probably the lid). You could use 2 or more strips if there is not enough stiffness.
Why not just use a mercury tilt switch and adjust the tilt angle so it turns on at the correct braking g force?
 
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