Expanded Scale Voltmeter

[commanda]

This is the circuit diagram of the voltmeter I just built.

circuit in pdf format

I built it on a little round pcb & bolted it directly to the back of a MU45 1mA meter. I photo-shopped the meter face to read 10 to 15 volts.

Most of the circuits I have played with tend to become non-linear at the lower end of the scale. This circuit is absolutely linear. Using an LM358, the current drain is approx 1.2 - 2.2 mA. The ICL8062 gives 1.2 volts. The dual op-amp could be replaced with a TSM103, which has the reference voltage built in. You could then do away with U2, R6, C3. U2 could be any reference source; zener diodes, even an LED. But a precision reference is more stable.

The box in the bottom left explains the basic math. For my implementation, the voltage at point B is 1.2 volts. So the voltage at point A needs to be 1.2 volts when the battery is 10 volts. There will then be zero volts across R1 & R2, so no current will flow in the meter. When the battery voltage is 15 volts, the voltage at point A will now be 1.8 volts, so there will be 0.6 volts across R1 & R2. If R2 is adjusted so R1 + R2 = 600 ohms, then the current through R1 & R2 (and hence the meter) will be 1mA.

As built photos tomorrow. Questions welcome.

The LM358 & ICL8062 were on hand, hence their use.

Amanda

[Old F]

Amanda

Top notch  work as always

Short sweet and to the point with small parts count.

Have you ever tried or heard of ground plane construction some times called dead bug ?

Where you  places an IC with the pins sticking up in the air looks like a dead bug on a copper clad  circuit board  and the board becomes the ground point  for the circuit. Not pretty  but it works.

Not every  body  is set up for PCB work. I have been playing with the bit and bobs  and photo  etching  boards for years heck when I was in school we had these thing called tubes  :  )    Where are you getting your meters  and parts ?

Your projects are great for folks who want to get there feet wet in electronics construction and have something use full when there done.  

Keep up the great work !

Old F

[commanda]

Never heard of ground plane construction, but I have used Xmas tree construction. And a variation where you use plain old cardboard, punch holes through for the component leads, and use the pigtails on the underside of the cardboard to make connections.

I built the prototype of this circuit on veroboard. The pcb I made using the "join the dots with a texta" method.  Did the layout with Protel autotrax, printed it, layed it over the blank pcb, center-punched the holes & drilled them. Them drew the tracks by hand with a CDR marker.

Parts I get from Jaycar or Farnell. I'm in Australia.

Amanda

[jacquesm]

that is one hell of a clever way to make a pcb, I'll steal that trick if you don't mind !

I'd hate to do anything a little larger like that, but for small projects it's probably the fastest way of doing it that I have ever heard of.

ground plane construction (above 'plain' pcb material or copper) is very common in HF prototyping. You use a long strip of pcb, solder on little vertical slats to separate the various stages and pass signals & power from one stage to the next using pass-through caps (soldered into the slats) and ferrite beads. Works wonders, but it has a few drawbacks (microphony and mechanical instabiliy) which make it pretty hard to build something two times and get exactly the same performance.

It's quick though, and much easier than designing a pcb for an HF design.

[commanda]

And here's the promised photo's.

Amanda