Introduction: Inductive Bike Lights

Be seen on your bike with no batteries involved!

This instructable shows how to build an inductive back bike light.

When magnetic flux passes a coil of wire it induces a current to flow within that wire. This is the principle of all motors and generators. This principle is shown here:

Here we use a coil from an old relay, along with a strong magnet to power a couple of LEDs and make a flashing bike light.

Thes kind of bike lights are available commercially, such as those here:

Others have done instructables which I have used to help me with this design such as:

And via Hack-a-day:

Step 1: Parts Required

You will need:

  • Old relay - you need a 24V DC coil one to get the right output voltage. My local hackspace had been donated a load of old relays which were easy to take apart.
  • Neodynium magnets - I already had a lot of 5mm x 2mm round magnets, so I used those. Lots of other people have had luck with strong magnets from hard drives.
  • 3mm Acrylic around A4 sized piece. Colour to suit your bike.
  • Small piece (5cm x 5cm) of polypropylene sheet.
  • 3mm Nuts and Bolts - 20mm long bolts. Mix of normal and lock-nuts
  • Cable ties - 4mm wide, around 300mm long.
  • LEDs - Red, high brightness type.
  • Small piece of strip-board
  • Reflector - such as from an old torch.
  • Wire
  • Araldite/quick set epoxy glue
  • Super glue

Tools required:

  • Glue gun and glue sticks
  • Laser cutter (or access to a laser cutter)
  • Soldering iron and solder
  • Screwdriver
  • Small long nosed pliers
  • Wire cutter

Step 2: Get the Coil From the Relay

Ensure you have a 24V (or higher) DC relay. I tested 12V relay coilc but they did not give enough output voltage with the magnet/speed ratio. I also tried 110V AC and 240V AC coils and they worked, but not quite as well for some reason.

The first thing to do is to dismantle the relay and get the coil out in one piece.

The relays I had were quite old ones with no plastic cover, but I also tried with some plastic covered, newer style relays and show the photos from that.

They just required one nut to be unscrewed and the coil came off the switching unit.

You want a coil with the circular metal shaft through the middle accessible, as the magnet will pass this to induce a current into the coil.


Be very careful with the thin wires on the coils - these are very easy to break. If at all possible keep the solder tags from the base of the relay.

Step 3: Prototype Design

To start with I rigged up the coil to two LEDs, with the LEDs connected in two different orientations (ie the +ve of one connected to the -ve the other and visa versa).

I used a hard drive magnet and moved it in front of the coils and the two LEDs lit up with pulses as the magnet went past. This proved that the concept would work.

I then needed three brackets:

  • One to hold the magnets to the bike wheel
  • One to hold the coil to the bike frame, close to the bike wheel
  • One to hold the reflector

I designed an adjustable bracket to hold the coil which coul be used to attach to most bikes.

I prototyped all of this in 3mm plywood, but the final units will be made in acrylic.

My design is attached here are a .dxf and a pdf.

Step 4: Laser Cut the Pieces

Using a laser cutter, cut out the deign file into 3mm acrylic.

Ensure the pieces are all cut out cleanly.

I split the design into three parts:

  • The Coil holder
  • The LED unit
  • The Magnet holder

Cut the 2 smaller pieces for the magnet holder in the polypropylene.

Step 5: Put Together the Coil Holder

I use T-nut holders to hold the pieces together. You could glue them if you like.

This unit holds the specific coil from my relays. You will have to adjust this for your design.

The coil is held in place with two clamps. The photos show this in more detail.

I used two cable ties which fit through slots to hold this onto the bike.

There is an adjustable slot to hold the coil just close enough to the magnet holder on the wheel (needs to be around 2-5mm away).

Step 6: Build the Magnet Holder

The circular piece of the magnet holder is glued to the longer piece. Wait for this to dry.

I used three sets of two magnets in a line.

I put a blob of super glue onto each magnet and stuck them into the holes. This holds them in place for sealing with epoxy. If you dont do this you will find that the magents affect each other and flip out of place due to the magnetic field.

Mix up some epoxy (Araldite) and cover the magnets. The epoxy will seal the magnets into the holder and stop them flying out.

Leave to dry - prefereably overnight.

The polyproplyene spoke holders are held in place with some M3 nuts and bolts. These will hold the unit onto the wheel.

Step 7: Build the LED Unit

The LED unit will depend upon the reflector you find, so this is just for your infomration.

You could use an old ready-made reflector for this.

My designed used T-nut holders to clamp the reflector in place.

Cable ties are used to hold the reflector onto the bike.

There are two sets of holes so the cable ties can be used in different directions, for fitting in different places on the bike.

Step 8: Wire Up LEDs

We now solar and wire up the LEDs.

A piece of two core/twisted cable is used to wire from the coil to the LEDs.

Thee are two LEDs (I tried also with four LEDs, which is just a repeated lot of LEDs). One must be put positive to one wire and negative to the other, the other LED goes in the opposite direction.

The output from the coil will be AC, so wiring up two LEDs in opposite directons gives us two flashes as the magnet passes the coil.

Solder onto a small piece of vero/strip board. Cut the vero board to fit into the reflector.

I sealed this with hot-melt glue gun, which makes it water proof. Silicon sealant could also be used.

I tested the LEDs with a power supply at this point to check that they still work.

Step 9: Fix It to Your Bike, Adjust and Test.

First attach the magnet holder to the wheel, at a decent radius from the centre.

Use the cable ties to hold the coil holder onto the back frame, aligned to where the magnet holder goes past.

Adjust the two nuts/bolts on the edge of the coil holder so that the coil is as close as possible to the passing magnets, but not hitting. You might need to have the coil at a light angle.

Tighten up the nuts/bolts - use a lock nut here so it does not become loose over time.

Attach the LED unit to the back of the bike some where visible.

Use cable ties to hold the wire to the bike.

Give the wheel a spin and see the flashing lights.

Use two or more magnet holders to get more flashes per rotation.

Warning: This light only works when the wheel is spinning so switches off when you are stopped. This means you must use andother form of lighting on your bike.

Step 10: Additional Ideas

I'd really like to build a unit which has a capacitor to store a bit of power and ensure a steady light so you can be seen when stopped at lights.

I tired this with a 1F capacitor, low voltage drop (schottky) diode and some resistors to have an 'always on' LED.

This did not work with the coils I used. It worked fine with the power supply, but not with the small 'blips' of power from the magnet passing the coil.

This could have been due to lack of voltage available from the coil.

Anyway - experimentation is fun!

Have a play and let me know if I'm doing something wrong.....