I'm working on a project to put together a custom bike light from some LED flashers and a battery pack. I'm looking at two different options, one which comes with a built-in switch, and another where I would be wiring up my own switch.

If I build my own switching mechanism, I will purchase waterproof switches. If I buy the system with the built-in switch (it's actually cheaper than making my own, but there are other considerations), then it will not be water proof.

With the pre-built switch I would need a way to waterproof the whole thing; with the self-built I would need a way to waterproof the contacts on the backside, where the wires get attached. In either case, I must protect the battery from the elements. Most suggestions I have seen are to put the battery inside of a water bottle, which is then easy to put on the bike, and simple to add/remove.

What are some low-cost ($20 max, preferably cheaper) ways to water-proof switches and contacts? I'm not looking for a specific product, so much as a method for protecting them. Is it best to build some sort of cover? Use some sort of shrink-wrap? I've seen suggestions online to just put the finger of a latex glove over the switch, and seal that off. Would that be sufficien protection?

Note: this application will be using 12v DC and a max of 3 amps (likely more like 1.5A).


2 Answers 2


For waterproofness you can't beat.

  • Magnet + reed switch

  • Magnet + Hall sensor.

Reed switch has advantage of zero off power and no lower voltage limit.

Hall sensor can be more robust.
Reed can be extremely robust if mounted well (floating in soft packing)

These BU520xx hall sensors from ROHM cost about $1.20/1 from Digikey BUT have under 10 uA off state current drain worst case.
2.4 - 3.3 V operation.
If current does not matter you can get 6 MA when off parts for about 1/2 that cost.

If you want single cell operation at 55 uA typical for a horrendous $9.72/1 you can have this marvel.
NVL AFE series. Operation down to 0.9V.


Reed-relais and external magnet are often used in bike lights.

Use the sensor / magnet from a broken cable-bound bike computer, which you should find lying around in the bike shop for free.

Or buy one in the 1$ range, like the Assemtech PPS 470 or similar.


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