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Tim Stinchcombe

DC Voltmeter

Not long after I bought my first batch of modules in 2001, I bought this meter movement - I guess like many others, to me the idea of a large meter on a synthesizer seems pretty cool. It was mounted in an awful hacked-out-in-the-kitchen piece of aluminium for a long time, which didn't do it justice, and all I had was some resistors in series with the coil, so it was lousy at measuring voltages. Finally I got around to sticking a high-impedance op amp in front of it, illuminating it with some LEDs, and putting it in a Schaeffer panel: it is definitely more 'showy' than practical, but I'm pretty pleased about the way it turned out. (Though at 20hp wide, it does tend to take up a lot of real estate too!)

The meter movement I've used is a 76 x 58mm, 50-0-50μA one made by Sifam, available from RS in the UK, part no. 196-8418, along with the mounting bezel, part no.196-8733, but I dare say many other places sell them too. Compared to the (almost give-away) cost of semiconductors these days, the movement does not come cheap - once VAT is added you're well over £30: hence I put a simple little 'protection circuit' in it in the form of a few transistors and LEDs, which divert the current from the coil through the LEDs if the current is getting on the large size. It's fairly crude but hopefully effective. The schematic is here, and along with a brief set of notes and BoM I wrote for a few people who were interested in the detail.

The following photo shows a general side-view of the unit: there are a couple of bolts on the meter, both for holding it together and as a mounting possibility I guess, and they make handy places to bolt a little bit of stripboard on to (the blue thing at back is just a set-square, holding it up for the photo):

Another shot of the back, showing the 8 LEDs (lit in the photo) mounted above the meter:

It is pretty hard to get a good photo showing how it looks with the backlight LEDs doing their thing - this was about as good as I could manage, and even so, it looks like the overload LEDs are on, but they aren't (they are picking up a bit of light bleedthrough from the backlight LEDs, as they are mounted too close - oh well, one can't think of everything...)

Here is another snap of it lit up, mounted in a rack:

It also had a minor role in Michael Zacherl's video, taken at the Synth DIY meet in Cambridge, UK, July 2008.

[Page last updated: 14 Dec 2009]