Bicycle Dynamo Brake Lights

Discussion in 'General Electronics Chat' started by IGill, Jan 6, 2013.

  1. IGill

    Thread Starter New Member

    Jan 5, 2013
    I am a keen cyclist and have a front light which is powered by a dynamo hub. There is an output from the front light which can be used to power a rear light.

    There are some fancy lights on the market which include brake lights. A brake light can be very useful if you are in a large group of cyclists.

    When you slow down, the dynamo produces less power. The rear light detects this power drop and activates the brake light.

    I am intending to build my own back light which will include a brake light but cannot figure out a way to activate the brake lights when the power drops.

    If I use a capacitor which will be charged up to the supply voltage, then when I slow down the voltage in the capacitor will be greater than the supply voltage. I want to use this voltage difference to activate a transistor to power the break light LEDs. But I do not know how to do it.

    Do I need to use some form of comparator (about which I know nothing), or is there a another option?

    Any advice would be appreciated.
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    You could simply use a mercury switch (encapsulated in glass). The switch place in a slight tilt (higher to the front of bike) so the mercury will flow up hill as you decelerate and turn on your circuit. It won't be perfect but an easy start.

    Now, with REACH and RoHS legislation in the EU, how do you find a mercury switch?
    PackratKing likes this.
  3. IGill

    Thread Starter New Member

    Jan 5, 2013

    Thanks for that. I know that you can get non mercury tilt switches which would do as you suggest.

    The problem is that it would probably be activated by the general bumpiness of the road and not just slowing down. It may end up looking like a typical rear flashing light.
  4. Brownout

    Well-Known Member

    Jan 10, 2012
    an opamp differentiator circuit should detect when the charging voltage is rising or falling. If you're doing any filtering or battery charging, make sure the differentiator is not coupled to the filtered vaules. Putting the circuit upstream will probably do the trick.
    IGill likes this.
  5. IGill

    Thread Starter New Member

    Jan 5, 2013
    I have been trying to read up on comparitors. Will the following do the job?

    | -(comparitor)
    | +(comparitor)

    (I can't upload an image but the image is at

    If I am understanding correctly, the capacitor will be charged at the same voltage as the supply and therefore the comparitor will not switch. When the supply voltage drops the capacitor will supply a greater voltage to the + terminal and trigger the comparitor until the voltages equalize.

    I may be completely misunderstanding this.
    Last edited: Jan 6, 2013
  6. MrChips


    Oct 2, 2009
    Or you can get brake levers with a built-in switch. My ebike has them so that when I brake it sends a signal to the controller to cut motor power.
  7. IGill

    Thread Starter New Member

    Jan 5, 2013
    Thanks for the suggestion but I don't really want to change my brake levers, they are relatively new, expensive and also change the gears (shimano 105).

    I like the elegance of a switchless and wireless (other than the power) system and want to replicate it.