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Salvage It! BY JULIAN EDGAR A $5 2-channel vibration sensor Want to make a vibration sensor for just a few dollars? All you need is an old cassette deck and a couple of loudspeakers! T HIS 2-CHANNEL vibration sensor costs almost nothing to make but is sensitive enough to detect a cat walking past on a wooden floor! To make it, you’ll need a discarded (but still working) cassette deck that has VU meters (these can be either analog or digital) plus a couple of loudspeakers, which can be easily salvaged from an old stereo TV. If you can’t score that lot for under $5.00, you’re not really trying. High-gain preamplifiers The unit takes advantage of the fact that a cassette deck uses two high-gain preamplifier stages that work with very small signals. Normally, these signals are read off the tape by the heads but what we do here is feed in new signals which are derived from coils of wire moving in a magnetic field. And since loudspeakers have very strong magnets, coils with lots of windings and very small internal clearances, they make ideal sensors for our vibration detector. If the speaker basket (or frame) is firmly attached to the ground and a vibration occurs, the basket and the cone will tend to move at different rates. For example, if there is a sudden movement upwards, the inertia of the cone means that it gets left behind for a moment. As a result, the magnet will move in relation to the coil (which is attached to the cone) and a small voltage will be generated. This voltage is amplified and displayed on the cassette deck’s VU meters. The greater the needle deflec- It looks like a $1000 instrument but costs less than $5 to make. This 2-channel vibration detector is actually based on a slightly modified cassette deck and uses conventional loudspeakers as vibration sensors. It’s sensitive enough to detect a cat walking past on a wooden floor. 94  Silicon Chip tion, the greater the amount of vertical vibration that has occurred. Building it At its simplest, the vibration detector will take only a few minutes to make. First, make sure that the power cord is disconnected from the mains supply and then take the cover off the cassette deck. Now trace the leads (they’ll be shielded) that connect the PC board to the heads. There will be six conductors in all – a common, play and record signal feed for each head. Cut these wires and feed them out of the case. That done, replace the lid, power-up the deck, press the “Play” button and then connect a speaker across the wires for one channel, trying various combinations until you find a pair which causes a VU meter to strongly react to any speaker movement. Now do the same for the other channel. You may need to extend these leads and in our case, we used the RCA cables that came with the deck. While we were at it, we also stripped the cassette deck of the surplus parts. For example, the complete tape mechanism was removed. Why? Well, the DC motor, drive belts and springs can find a use in another project, as can the tape counter. There’s no need to leave them inside the “unit”. Of course, if you take this approach, you’ll need to activate the same switches that pressing the “Play” button normally does. For example, if a single switch is closed when “Play” is pressed, the wires leading to that switch will need to be connected together. On the other hand, you may find that when the cassette mechanism is removed, the unit is effectively always in “Play” mode. Note too that different speakers will give different sensitivities. We tried a siliconchip.com.au Rat It Before You Chuck It! We chose to remove the internal bits and pieces that were no longer needed – the cassette mechanism, tape transport buttons, access door and so on. This allows these parts to be used in other projects and gives a much lighter unit. Whenever you throw away an old TV (or VCR or washing machine or dishwasher or printer) do you always think that surely there must be some good salvageable components inside? Well, this column is for you! (And it’s also for people without a lot of dough.) Each month we’ll use bits and pieces sourced from discards, sometimes in mini-projects and other times as an ideas smorgasbord. And you can contribute as well. If you have a use for specific parts which can easily be salvaged from goods commonly being thrown away, we’d love to hear from you. Perhaps you use the pressure switch from a washing machine to control a pump. Or maybe you salvage the high-quality bearings from VCR heads. Or perhaps you’ve found how the guts of a cassette player can be easily turned into a metal detector. (Well, we made the last one up but you get the idea . . .) If you have some practical ideas, write in and tell us! here, we elected to keep the original scales. Changing the sensitivity The loudspeakers detect vibration and generate a small voltage as the magnet moves past the voice-coil in each unit. The larger the speakers, the more sensitive the instrument. variety and found that the larger the speaker, the more sensitive the unit became. The speakers shown here (100mm units salvaged from a stereo TV) were used in the author’s unit and give a noticeable reading when anyone walks anywhere in the house (a two-storeyson-stilts Queenslander). And that even includes walking on the concrete pad under the house! As I type this, the unit is on my desk; with each normal force keystroke, the display meters are reading just under full-scale! New faceplate To make the unit look less like a siliconchip.com.au cassette deck and more like a vibration detector, you can make a new faceplate. If the original faceplate is removable (most are), take it off and scan it into your PC. You can then use image manipulation software to construct the new visuals, putting on whatever labels you want. That done, print it out at full-size on heavy stock, gloss paper and affix it to the original faceplate. The label can then be protected using clear contact adhesive film or a couple of strips of broad adhesive tape. Another option is to replace the scale behind the VU meters. You can make the scale read anything you like but in the case of the unit shown If the sensitivity of the unit is too great, simply reduce the size of the speaker. Adding weights to the cone also alters the response. When exploring the use of different speakers, be aware that a typical house is full of background vibrations. The compressor in the fridge can cause sufficient vibration to swamp other signals, while a PC cooling fan can also cause clearly visible room vibration. So to be able to watch earth vibrations caused by (say) visitors walking up to your front door, you’ll need to remotely mount the speaker sensor away from this house-borne noise – but note that the sensitivity will be reduced if the cable is too long. Logging the output Finally, if you want to feed the output signal into a logging system or drive an external display, line-level output voltages will be available on the normal RCA outputs of the deck. SC October 2005  95