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Salvage It! BY JULIAN EDGAR A voltmeter for almost nothing Want a really cheap voltmeter? Here’s how to adapt a VU meter that’s been scrounged from an old audio cassette deck. R ECENTLY, IN THIS column, we’ve covered both a very cheap leadacid battery charger (made from a plugpack and a resistor) and a variable output switchmode 12V power supply (made from a phone charger). In both cases, it’s useful to also have a meter displaying voltage. In the case of the battery charger, a voltmeter lets you monitor the battery’s voltage as it charges, while in the power supply, it lets you monitor the output voltage. There’s a heap of other uses for a voltmeter as well – especially when you realise that this meter will cost you next to nothing. And it’s easy to customise the scale and the voltage range over which the meter works. Want some more possible uses? Well, in a model railway layout, you could use the meter to display the voltage being fed to the lighting – but instead of having “volts” marked on the scale, you could have “dusk”, “night” and “day” ranges marked. Fig.1: a VU meter is a very sensitive instrument. It can be adapted to measure a wide variety of voltages by installing 100kW a variable resistor in series with the supply voltage. This allows you to easily adjust the Full Scale Deflection to match the peak voltage you need to measure. 46  Silicon Chip Another use is in battery-powered equipment. Because the meter draws very little power, it could be used in many applications to permanently display the battery voltage. That way, you’ll always know if you’ve got a battery that’s nearing the end of it charge (or its life). The Components You’ll need only two components, as well as access to a computer, just about any image manipulation program, a scanner and a printer. You’ll also temporarily need a plugpack, a 10kW pot and a multimeter and to do the calibration, you’ll temporarily.) The two electronic components required are: (1) an analog VU meter from a discarded audio cassette deck; and (2) a 100kW multi-turn trimpot, used here as a variable resistor. Old cassette decks with large illuminated VU meters turn up all the time in garage sales, at the tip and during kerbside collections. It is extremely rare for the VU meters to be dead, so you can be fairly safe in collecting any old cassette deck for this purpose. Try to obtain a deck that has two separate meters (one for each stereo channel), rather than one that has them combined into a single display. VU meters are typically moving coil voltmeters that have a very high sensitivity. This means that it takes very little voltage to move the needle across the full scale – typically, just 0.3–0.4V. Their coil resistance is very high, being around 600-700W. Plug these figures into Ohm’s Law (ie, I = V/R) and you’ll find that the meter current is only about 0.5 Here a multi-turn trimpot has been used as the series variable resistor. This makes calibrating the meter easy. milliamps (0.5mA) for full-scale deflection! Building It If you need to measure a voltage that rises only to about 0.4V, all you need do is connect the meter straight across the supply. However, it’s much more likely that you’ll want to measure a peak voltage of 5V, 12V or even 24V. Fortunately, it’s very easy to decrease the sensitivity of the meter – just wire a variable resistor in series with the meter, as shown in Fig.1. But how do you make the scale match the readings you want the meter to show? Most VU meters use a non-linear movement – that is, the needle moves less for a given voltage increment at the top end of the scale than it does towards the bottom. This means that you can either position the markings closer together as you move up the meter scale or you can space the markings evenly and jump further between numbers. We chose to do the latter. You can use your PC and printer to make the new scale. You could scan in the original scale and modify it but in practice, it’s best to start with the siliconchip.com.au scale from another instrument. The meters shown here use a scale that was originally scanned in from an old speedo. Once you have the arc and the increments, you can use the software to delete the numbers and any other markings you don’t want. That done, measure the width of the scale on the meter and then size the on-screen version to match this and print it out – ie, without any numbers on it. Next, carefully remove the original scale (most meters are held together with tape, with the scale glued in place) and temporarily place your “un-numbered” scale behind the pointer. To provide a variable calibration voltage, use the approach shown in Fig.2. This involves wiring a 10kW potentiometer across the output of a discarded plugpack. By adjusting the pot, you can vary the calibration voltage from 0V up to the maximum voltage provided by the plugpack. The multimeter is included so that you can measure the voltage being fed to the VU meter. So how do you perform the calibration? First, adjust the 100kW trimpot so that the most commonly read maximum voltage is towards the end of the scale. After you’ve set this, don’t touch the trimpot again. Next, alter the input voltage, measure it with the multimeter and work out what each of the other graduations on the modified meter scale should show. Be sure to round off the voltages to the nearest whole volt. For example, on one of the meters shown here, the scale goes: 4, 7, 11, 14, 18, 25, 34V. Once you’ve figured it all out, use On the right is one of a pair of VU meters removed from an old cassette deck. At left is its modified brother, recalibrated and rescaled as a voltmeter reading up to 34V. Fig.2: wiring a 10kW pot across a plugpack provides a variable voltage source, allowing you to calibrate the scale. The multimeter shows what the readings on the new meter scale should be. your graphics software to put the numbers on the scale, along with any other writing you want. Finally, print it out on gloss paper, cut it to shape and stick it in place. Using It Using the meter is as simple as connecting it (and its series 100kW trimpot) across the voltage source that you want to monitor. Note that if the meter needs to be used at night, it’s easy to illuminate the scale. In fact, the cassette deck that you scrounged the meter from probably also had a suitable bulb in it. Make sure that it has the correct voltage rating for your SC application though. Rat It Before You Chuck It! Hmm, “JE instruments” . . . gee, that sounds like a good brand. When you make your own scales, you can put anything you like on them! siliconchip.com.au 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 have a use for the highquality 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! June 2005  47