For this last programme [next week's challenge was filmed earlier - editor's note] we decide to build a burglar alarm for a reef!
The idea is to suspend an insulated wire above the water in front of a reef. If a boat comes too near and touches this ‘tripwire’ its wet bow will electrically connect the wire to the sea. An electrical circuit will detect this ‘boat made circuit’ and trigger an alarm - that’s the plan. The apparatus has to be powered by the sun and from the waves to be self-sufficient and ecologically sound!
The Rough Science team all get together to plan. Mike, Kathy and Ellen are to make a string of flags and floats which will form the fence on which to hang the ‘tripwire’. I will work on converting a household smoke detector so that it can be triggered by the tripwire and boat. I also am really looking forward to making a wave powered generator - something I’ve wanted to do for years but never (for some reason!) wanted to do in the wet and cold of Brighton beach. Now I can do it out in the heat and sunshine of the Zanzibar sea!
I set up a little demo to show Kate how electricity can be generated from a coil of wire and a magnet – I show her the shake-a-gen. Talk to her about how I will modify this to make the wave powered generator. This works by using waves (or swell) to rock magnets in and out of wire coils so generating pulses of electricity. There is a long metal ruler in the Rough Science kit which I decided to use for the flexible support for the moving magnets. It also has the convenience that the magnets stick to it.
I make up two coils for the wave generator of about 1000 turns each and mount them so that the rocking magnets fall into one and then the other coil. I then make up two rectifiers and capacitor storage devices from the radio parts that will smooth the pulsed power so that (like a battery) it will power the smoke detector electronics properly. It will also allow me to add the voltages together to get enough to run the smoke alarm.
The next big step was to see how I could modify the smoke detector so that it would work for us. Initially I had imagined only using the bleeper from the alarm. However, ‘test’ button circuitry built-in to the smoke detector turned out to be ideal for our purpose. That was good luck. The test button on the smoke detector is triggered by a high resistance circuit so all I had to do was simply to re-wire the two wires from the switch to ones going into (1) the sea and (2) the tripwire. All the equipment was put in plastic boxes to protect it from the sea water.
The other wonderful thing was that smoke alarms are designed to work on batteries for years and so work with the widest possible voltage range to get the most out of the battery life. I found that it worked really well on the voltage we got from the wave power generator! By gently rocking the apparatus by hand I got 4–8V. As the smoke detector uses a 9V battery this was perfect! We were given little solar powered garden lights which I took apart for the solar cells. These produced a very similar voltage to the wave power generator. By using a diode from the radio it was an easy thing to make them switch automatically when and if the wave power fails.
Tests made using a bath of sea water (to act as the sea) and a wire suspended over it (the tripwire) showed that we could trigger the alarm when a wet piece of wood (representing the hull of the boat) ‘connected’ them – it was looking good.
Mike, Kathy and Ellen’s flags and floats looked fabulous. We packed everything on board the boat and sailed away with the gear. Once on location we started to set-up the flags one-by-one; streaming off from the boat. As we laid them out we attached the tripwire. The sea current was amazingly strong and at one point it broke the string / rope! It took the combined strength and wit of all of us to get that damned equipment up and running, it was amazingly difficult.
For our first test we needed to show that the wave / sun power was working and indeed it was as the smoke alarm bleeped when it was wired to do so (by deliberately shorting out the tripwire circuit). By putting a hat over the solar cells we could also see that the swell was enough to power the apparatus by the wave generator alone – so it would work at night.
Now came the nail-biting part where we would connect the equipment to the tripwire. This was done (the other connection made via a metal rod into the sea). The circuit failed! There was a short – somehow one of the insulators had got wet and was shorting the line to the sea. After much searching by swimming round the equipment, Kathy found the problem and we were soon back up running again. It was all working well.
Now we needed the sailors to move the boat directly towards the wire to see if the boat would trigger the circuit – we waited - all our combined efforts, perhaps 4 or 5 hours of constant back breaking work in the midday Zanzibar sun to get to this point.
The boat sailed along, turned towards the wire and slowly drew up towards it, it came near and bleep, bleep, bleep – it worked! The look on our faces tells it all - it was a terrific Rough Science group success!