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.

Operation.

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.

With the transistor switched off the collapsing field in the collector winding produces a voltage pulse greater than the supply in this case~25V, open circuit voltage. Once a load is applied in this case the LED the voltage will be clamped at the LED on voltage.

Adjusting the base resistance affects the collector current and brightness and the current drain which affects the longevity of the light, the base resistance is set to 10K giving approximately 3 minutes of light.

The transistors types used were 2N2222A and ZTX450 (as used in the One-Volt LED).

The transformer is made from a Ferrite Ring around which is wound 20 turns of enamelled copper wire.

The oscillation frequency with a stable supply is ~33kHz but as the supply voltage will be constantly reducing as the capacitor discharges the oscillator frequency will vary until the supply drops to ~0.4V at which point the oscillation will stop and the LED will be extinguished.

Operation

Shaking the circuit rapidly will charge up the capacitor.

On initial use the capacitor needs to be charged up to => 0V4 but once it has been used the discharge will stop at <=0V4 as at this voltage the circuit will cease to function reducing any discharge. This will make next use quicker to charge to >0.4V

Press the button and the LED will illuminate.

The light pipes are replaceable and can be customised in length and colour.