Brilliant Auto-off Feature For A Bike Light


If you’re going to use your bicycle as transportation at night you really must have a head and tail light in hopes that the crazy drivers don’t hit you. For good reason, these lights don’t turn themselves off. But [Miceuz] kept forgetting to shut it down upon arrival and always ended up with dead batteries. His quest for an auto-off feature that actually worked ended in a brilliant and simple add-on circuit.

He first thought about using an accelerometer, but couldn’t find one that fit the bill without also adding a microcontroller. He came up with an even simpler circuit, which can be seen at the base of the black plastic housing. It’s a bit of copper clad board with a small spring attached. The spring completes an RC timer circuit which drives a MOSFET. When that circuit is charged, the MOSFET connects power to the bike light. When the cap runs out the MOSFET threshold cuts power and everything turns off. Since the spring jiggles while he rides it provides the momentary connection necessary to charge the capacitor. Stay stationary for about 30 seconds and the auto-off kicks in.

30 thoughts on “Brilliant Auto-off Feature For A Bike Light

  1. This feature is known as a “stand light” and is used for dynamo powered lights. 30 seconds is far too short for sitting at a traffic light. I know, a cyclist at a stop light, crazy talk!

    Otherwise it is a good hack.

  2. The spring should still vibrate enough if he’s still physically touching the bike, even while stationary. For that matter, someone bumping the bike rack will probably turn it on. :p

    1. Exactly what I thought, Rasz. A dozen different kinds of sensors and switches would fit inside the seat and work better (and safer) than an improvised vibration switch with a thirty second timer. A flex sensor in the seat. Or, hell, one of the makeshift tilt switches HaD recently featured calibrated to activate when the bike is straight up and down, as opposed to leaning on its kickstand. A thermistor in the seat connected so it flips a transistor when his butt warms it up.

        1. There’s no practical difference between “zero power when off” and “less power than the batteries would lose even if left alone,” though a tilt switch would fulfill your zero power requirement.

  3. >He first thought about using an accelerometer, but couldn’t find one that fit the bill without also adding a microcontroller.

    A Bosch BMA250 has an SPI / I2C interface and can be initialized to operate in stand-alone mode. I really like the BMA2XX series but the LGA12 package (2x2mm footprint, no-lead with 250µm pitch) is a pain.

  4. I like it :) The spring switch is a nice touch.
    I ended up doing things a little differently and cheated and used a demo timer circuit from an old keyboard and a boat pushbutton under the seat. I did get pretty lucky that everything just kind of worked and thankfully there were even wire jumpers to cut instead of traces. It was one of those lucky messes that rarely happen :) My buddy still has it and loves it all these years later.

  5. At about 2002 I had an after market wireless door lock control system in my car. It worked by locking the doors once no signal was received.
    The transmitter in the keyring remote worked exactly this way. A long, flimsy spring that charged a small capacitor.

    This hack is actually way older and I’d call it one of the primitives of electrical engineering. Still, getting that idea yourself is worth something :)

  6. Go with fastened magnetically powered bike lights.

    They fasten to the chassis and the wheel and every revolution adds a current spike to the electronics which makes the lamp light up. If you stop the bicycle the power is automatically cut since you’re not pedalling.

    They are bright, requires no batteries and are near impossible to break.

    AFAIK it’s a patented Danish invention, I’ve had two kinds and they’re amazing :)

    1. Those are expensive (in the shop) and not as damage-proof, and if you have the kind that blink you have the disadvantage that in some countries blinking bike lights are not allowed.

  7. Dirty ,water admitting power switch or jiggle and ride w/sealed insides. Only with a winter layover will the spring stop working due to corrosion. Spider’s webs maybe.
    Worth using in a scaled up multi-watt led and lithium power pack. If you want the best safety and notice without the put-off of a led-strobe light, this is the way to go. Join the rest of the flock with the same lighting. The street ain’t a disco.

  8. To +1 stupid improvements: use an accelerometer hooked up to a rasberry pi, when it detects movement it turns the led on for 30 seconds!

    The simplicity and practicality (for its application) of the hack is its elegance.

  9. The circuit I was looking for with minimum component.excellent work, how can I add an ambient light sensor to vary the brightness of light according to the light level ?
    Any hlep/ suggestions welcome

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