Monday, 3 February 2014

Related developments in 2013/14...

What's been happening?

i decided to try and apply the DIY cell 'technology' to something slightly more demanding than a flashing LED load circuit

over the past year, i've powered several LCD-based clock & weather-station units, using the same basic approach as the cells described here

the higher current-drive requirements of these devices has been achieved by increasing the water content - the electrode area has been reduced

normally, a galvanic cell arrangement such as these, using a few ml of water as a single electrolyte, would have polarised within a few months - needing the water to be changed and probably requiring the electrodes to be cleaned

these devices have been operating for nearly a year now, without attention

at the start of 2014, i wanted to move up to powering something more than just electronics, so i've started an experiment using the basic LED flasher circuit to energise a coil with just enough power to maintain a spring pendulum

it uses a dual transistor pulse circuit, based on the drive part of my original 3 transistor LED flasher; the coil is air-cored to prevent drag on the magnetic 'bob' of the spring pendulum;
the device is powered by 3 DIY cells in series providing a total of approximately 1V on continuous load (current draw is slightly less than 10uA)

Spring Pendulum drive circuit

it retains the energy capture and feedback from the coil field-collapse, via an LED, back to the supply (as featured in the earlier circuits) - and since the coil/magnet of the spring pendulum is similar to a 'Shake Flashlight' type arrangement, some kinetic energy is being converted back to electrical energy and returned to the supply, too

the system is contained within a clear plastic 'bell' cover to reduce effects from any ambient air movement

the LED flash period is approximately 20 seconds, the pendulum period is approximately 1 second (full-cycle):


(apologies for the creaky sound-effects - must be my knees!)

in the video, the DVM is displaying the supply voltage of the relaxation oscillator, which triggers at approx 1.2V and drops to approx 0.75V on pulsing the coil; the oscillator supply then re-charges from the battery supply
the components for the circuit aren't critical

C2, C3 and R1 form the timing of the relaxation oscillator - i selected values for C2 (10uF) and C3 (300uF) which produced sufficient pulse width to give an acceptable 'kick' to the pendulum, and then i selected R1 (150K) to give a repetition period of approx 20 seconds

a certain amount of feedback in the circuit triggers each pulse at a regular point in the pendulum movement (which can be either a vertical linear path, or an arc)

Q1 and Q2 are general-purpose, low-power, high-gain transistors with hFE >= 400

diode D1 is Schottky (for low reverse-leakage/forward-voltage), D2 is Germanium (to provide some leakage current)

C1 needs to be a suitable value to buffer the high impedance of the DIY cells - i have had the system operate with a value as low as 300uF, but 1000uF seems more reliable in the long term

the battery output voltage, on-load, is less than 2V, so low-voltage capacitors can be used, to reduce physical size 

LED1 is a hi-brite white type

the drive coil was hand-wound onto a card spool;

DC resistance is approx. 20 ohm
(a few hundred turns of multi-strand insulated copper wire, 7/0.09mm, air-core);
approx 30mm diameter, 10mm high, 10mm diameter air-gap
the battery consists of 3 DIY cells, 1 strip each of 15mm-wide copper and 5mm-wide zinc, approx 35mm submerged in about 2mL of 1:1 honey:tap-water solution, well-mixed; reasonably-well sealed with a plastic cap

the copper foil is bent into a 'U' shape to partly overlap the zinc strip on either side; a piece of sponge/foam is used to keep the two electrodes separate at the bottom of each cell