Force Induced Sabre Torch

Magnetic induction LED torch

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This project will describe the process of making a magnetic induction LED hand sabre torch primarily made from poly pipe. The simplicity of the design meaning that assembly is push fit requiring no screws or glue, no primary cells are required reducing the cost of ownership as power is generated by movement.

Feel the force and no batteries required.

Physical Design

The physical design needs to be such that the torch can be easily held in your hand with sufficient length to allow the magnet to travel up and down the inside of the tube and a visible extension to fit the electronics and the light rod.

The torch is constructed in three main sections from 22mm poly pipe.

The generator is made up of a 135mm length of pipe.

This contains the magnets with bumpers and stoppers at each end, containing it within the tube whilst creating a soft impact at each end to prevent damage and dull the sound when shaking. Around the outside are wound the coils.

The convertor is made up of a 100mm length of pipe.

This contains the electronics mounted on strip board which will be used both to store the energy generated and drive the LED.

The light pipe being made of epoxy resin, coloured or clear in various suitable lengths as required.

Force_Powered_Torch_Sabre physical design can be found on TinkerCad

The individual elements are shown full scale and the fully assembled unit is shown at 50% of full scale to fit on the same workspace.

AC to DC Conversion

The alternating voltage is fed into a bridge rectified and this is made up of four diodes.

A diode only effectively conducts in one direction once the turn on voltage is reached.

But as we are generating an induced voltage by effectively electro mechanical means the voltage levels are relatively low.

Schottky diodes rather than general purpose rectifier diodes will be used as they have a lower turn on voltage giving us a little more of the valuable energy to harvest.

How it works.

The coil will generate a alternating voltage with the polarity dictated by the direction of travel of the magnet in the coil.

Assuming a positive voltage in one direction of travel for the magnet this makes point A more positive than point B and current flows through diodes D1 and D4 charging the capacitor.

With the magnet travelling in the opposite direction a negative voltage is developed across A & B with B being more positive that A current flows through diodes D2 and D3 charging the capacitor.

Both voltages generated by the coil for the two passes develop a voltage across the capacitor in the same direction and thereby for each transition of the magnet down the tube a positive voltage pulse is applied to the capacitor.

Each voltage pulse progressively charging the capacitor.~2.5V after a few minutes of shaking.

The diodes prevent the capacitor discharging back into the coil and the open switch prevents the capacitor discharging until the switch is pressed.`

Blocking Oscillator

The DC voltage stored in the capacitor is used to power a Blocking oscillator commonly referred to as a Joule Thief. 

The voltage generation provides less than 3.1V and is therefore insufficient to drive a typical white LED directly.

(Although lower voltage white LED's at 2V exist, non were available at the time of creating this project.)

Therefore, a means to boost the voltage is required accomplished by the Blocking Oscillator.

Its called a Blocking oscillator because the transistors spends more time blocked (switch off), than on.


When power is applied a bias is supplied via the base winding in series with base resistor starting to switch on the transistor which in turn creates a current in the collector winding developing a magnetic field this couples to the base winding which attempts to switch on the base even more.

This process continues until the transistor is fully saturated meaning no more increase in the current and hence the magnetic field resulting in a reduction in the base voltage turning the transistor off. With no current the magnetic field in the collector winding collapses and creates an opposing voltage spike. Current cannot flow through the transistor as it is switch off and therefore the path is through the LED which switches on until the energy in the spike is depleted.

Then the cycle repeats....

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  • 1 × White LED 3.1 V White LED 5mm Through Hole
  • 1 × Supercapacitor 1F Supercapacitor EDLC ±20% Tolerance, UM 2.7V dc
  • 4 × 1N5817 Schottky Diode
  • 1 × 10k Resistor
  • 1 × ZTX450 NPN Transistor

View all 13 components

  • 1
    Generator Construction

    Cut a 135mm length of pipe with a suitable saw (hand saw and mitre block or circular saw).

    Smooth the edges at both ends with sandpaper to remove any burrs.

    Sand the outer edge at both ends to a rounded profile, this will make inserting in to the straight coupler easier.

    Cut two 21mm diameter bumpers 5mm thick from silicone sheet, high density foam or polystyrene, optionally use 2 tap washers.

    If cutting the bumpers use a small length of 22mm pipe as a template press this over the material to be cut and insert a marker or sharp blade into the inner circumference of the pipe and mark or cut the material.

    The bumpers will be inserted in to the 135mm pipe one at each end.

    Cut two 21mm diameter blanking plates ~1mm thick from aluminium, PCB board or nylon washers.

    If cutting the blanking plate use a small length of 22mm pipe as a template press this over the material to be cut and insert a marker into the inner circumference of the pipe and mark the material. Then cut with a saw to shape and file the edges smooth.

    The blanking plates will be inserted in to one end of each of the straight couplers.

    The magnet will be inserted into the 135mm pipe and should be able to freely slide up and down the tube with ease.

    In this case 6 * (18mm diameter x 3mm thick) neodymium magnets are stacked together, attracting faces together. Stacking magnets together with increase the field intensity however there is a "law of diminishing returns".

    Optimum Stack = Diameter/Thickness (18mm/3mm) = 6

    Meaning that beyond this the field strength does not keep increasing at the same rate and further additions to the length will provide small increases in performance.

    A single 18mm x 18mm magnet could be substituted.

    Assemble the generator pipe in the following manner.

    In to one end of each of the straight couplers insert a blanking plate and into one end of the 135mm pipe insert a bumper, push the straight coupler onto the pipe ensuring the blanking plate is pressed against the inner ridge of the coupler and therefore held in place by the pipe. A little lubricant (oil/grease), will aid in pushing on the straight couplers as the can be a tight fit but do not apply too much lubricant as you do not want then coming off when shaken.

    Insert the magnet.

    Attached the other straight coupler on the free end as per the previous straight coupler.

    The other integral part of the generator is the coil.

    Start the coil at the edge of one straight coupler ~150mm is free and uncoiled being used to connect to the electronics and this is taped to the pipe . Wrap the wire ensuring the coils are tight against the pipe and against each other with no gaps continue wrapping until you meet the other straight coupler edge and apply a little tape to prevent unravelling. Apply a single layer of tape along the length of the pipe over the coils this will prevent the next layer slipping between the gaps in the previous layer.

    Continue this process until you have six layers of coils.

    Wrapping tape around this final layer will add protection in addition to preventing unravelling.

    I used clear tape as it allowed the coil to be visible and enhance decoration.

    Ensure you have ~150mm of free wire at the end to connect to the electronic along with the other length.

    In summary ~420 coils per layer and 6 layers for a total of ~2520 turns measuring ~70R.

    The voltage generated being ~10.4V peak to peak*

    *More turns, a stronger magnet or greater acceleration in the movement of the magnet will generate a greater voltage. However, there will be a trade off with an increase in the weight and size.

  • 2
    Electronics Housing

    Cut a 100mm length of pipe with a suitable saw (hand saw and mitre block or circular saw). Smooth the edges at both ends with sandpaper to remove any burrs. Sand the outer edge at both ends to a rounded profile, this will make inserting in to the straight coupler easier.

    This tube will be used to house the electronics to which the switch will be fitted and the light pipe will be inserted.

    The switch will be fitted ~35mm from one end.

    Measure and mark this distance and drill any holes required for the switch.

    Due to the limited space the switch was chosen to have minimal intrusion into the pipe to prevent fouling the electronics.

    It is held in place by the springiness of the solder tabs and diameter of the solder gripping the hole.

    A small drop of glue could be used but exercise cause as too much could gum up the switch.

    Light Pipe

    The lights pipe was made from a liquid resin epoxy mix.

    The process is described in one of my previous Instructables Neopixel Candelabra

    You can make light pipes of various lengths using this process adding translucent dyes for a variety of colours..

    As a torch the length is ~30mm

    As a sabre the length is ~ 210mm or longer.

    To maximise light coupling a 5mm diameter hole 5mm deep is drilled into the centre of one end.

    Slightly round the edge of the light pipe to make insertion into the poly pipe easier.

    Additionally, polishing is applied to the exposed end.

    Use progressively finer sanding paper to minimise scratches and make polishing for a smooth finish easier.

    Use an epoxy resin polish to give a smooth finish.

    In this case we want the maximum light to escape from the end for focused lighting.

    For a sabre end lightly roughen the surface with fine sandpaper and/or wire wool and round the end by grinding and sanding. The roughened surface will allow the light to escape along its length rather than mainly the end.

  • 3
    Circuit Assembly

    The circuit (AC to DC converter and Booster), is assembled on Stripboard of sufficient size to fit within a 18mm wide by 100mm long tube.

    Most of the components on the board are polarised (with the exception of the switch, resistor and generator coil), ensure that the components are correctly orientated and check for open and short circuits prior to applying power.

    In order to fit the toroidal transformer into the tube it sits at the end of the board with the LED passing through the centre of the ring.

    For the transformer start with 86cm of wire and fold it in half with ~25mm held beyond the ring proceed to wrap the long piece around the ring keeping the pairs parallel and not crossing until ~25mm is left free. Cut off a small piece from the folded centre, remove the insulation and tin the ends with solder. One pair of serially connected wires will forms the primary and the other pair will form the secondary. Cross the secondary wires over and with one of the wires twist this with one of the primary wires these will be connected to the supply for the oscillator.
    One free end is connected to the base resistor and the other free end to the collector.

    The switch should not protrude through the pipe too far, so as not to foul the wiring or create a short circuit.

    The two wires from the generator coil are attached to the circuit by drilling a small 1mm hole 5mm behind the switch and threading through this hole and the pipe then soldered to the inputs of the bridge rectifier the hole is covered with tape to protect the wire from damage.

    Once the circuit is complete it is slid into the tube and capped with a light pipe.

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