This was a quick project to build a voltmeter to display the DC Supply Bus Voltage for the Shack.
Since most of my transceivers and ancillary equipment operate from a 12 Volts DC supply, I wanted to see how much it varied with reasonable resolution.
I didn't want a digital display voltmeter because fast variations would be difficult to see, particularly when using SSB on HF.
Saturday, 21 June 2008 13:58
The unit was built on Veroboard and the following two images show the layout. In hindsight, the trimpot could have been spaced one set of pins further from the rest of the components, leaving more space for the tantalum capacitor.
I scanned the original meter's scale into my computer at 600dpi and then edited it using PaintShop Pro to create a new scale for the meter. I then printed it out using a bubble jet printer onto good quality low grain paper. Normal printer paper works almost as well. I then used spray paper adhesive to glue the scale to the back of the original meter scale.
Note the graduated colour shading. I think it turned out better than I'd expected, although the picture doesn't show it. As can be seen, each division on the meter scale is 0.1 volt and you can easily discern 0.05 Volts of meter movement.
Initially I used an analog multimeter but I found that the needle hardly moved on the 15 Volt Scale and that fluctuations of around 0.5 to 1 Volt were needed before I would notice them.
I decided upon a voltmeter based upon an original article from Electronics Today International (Project 159) by Simon Campbell and Roger Harrison. It is simple to build and allows me to observe variations in voltage of up to 0.05 Volts DC.
I replaced the original LM723CH of the original circuit with a more readily and cheaply obtainable LM78L05 three terminal regulator. This reduced the board size and circuit complexity.
Also, I didn't have a 2N3819 in my junkbox, so I used a J310 instead, which worked fine. The pinouts are different so compensate for that in your layout on the veroboard.
The circuit is shown below:
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Expanded Scale 10 - 15 VDC Voltmeter