[sdiy] synth V meter

[Paddock, Toby]

If you want to use a 0-x meter to read like +/-10V, could you connect it's
negative to -10V? (and scale it with resistors like Roy described)

Or if you have a center zero meter and want to measure 0-10V, connect it's
negative to +5V?

I agree that a 0-50uA is probably the best common meter to look for. Are
there any amateur radio swap meets in your area? You could get a few cheap
used meters and hope one is what you need. Plus some of them have
interesting scales. 

Last month I made a large demonstration meter using an RC servo and scrapped
overhead projector housing. The glass that usually faces up that you write
on was the meter face. 2 555s for a CV to servo driver. 

On TV a few nights ago they were showing kitchen gadgets and there was a
blender with an analog tachometer. Sorry to ramble on, I just like talking
about meters I guess.

Later,
Toby Paddock


-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Fernando de
Izuzquiza
Sent: Monday, May 24, 2004 10:09 AM
To: synth-diy at dropmix.xs4all.nl
Cc: rtellason at blazenet.net
Subject: Re: [sdiy] synth V meter

Roy,

Thanks a million it helps a lot. I'll try to digest all the data, but 
it's very clear, thank you for your time.

The bad part is that there is not very much meters to choose from here.
I can't find meters with 0 in the middle, by instance... I thought that 
it can be done with a 0-x meter...
I have to buy a different meter for 0 in the middle? I mean, is the 
only way?

OK, I won't need to monitor bipolar signals so often, but I want a 
monitor where I can see, to start with, that the examinated signal IS 
bipolar, or see what's it's excursion in V...
I want this meter to monitor signals when something is not working as 
expected or just to see it's shape (without using a scope)... and I'd 
like to have it integrated in the synth, instead of buying an expensive 
analog meter (wich AFAIK will not read bipolar signals). I already have 
to buy a better digital one for SDIY and it's not cheap.

Your way of using just trimmers and the meter is great because is 
simple,
may be I have to use a more complex circuit to read +/- voltages on the 
SAME meter?

Fernando




---------  mensaje original --------
Enviado por : Roy J. Tellason <rtellason at blazenet.net>
Enviado a: Fernando de Izuzquiza <fdi at ran.es>
Asunto: Re: [sdiy] synth V meter
Fecha: 24-05-04 15:13

 > On Monday 24 May 2004 10:45 am, Fernando de Izuzquiza wrote:
 > Hi list,
 >
 > I'd like to have a meter to see voltages on the synth (a la AKS)
 > My idea is to have 3 fixed scales switchable: +/-20V, +/-10V and +/-5V
 >(+-10V will be the most used, but the others can be useful too)
 > It would be nice to have another switch so you can use the whole scale
 > to measure positive voltages only with more definition. At least on 
the
 > +/-10V scale.
 >
 > Can someone point me to an schematic please? I know is not a difficult
 > circuit but I just can't design it.

What you want there is a resistance in series with the meter movement 
(called
a"multiplier"),  and it would be a different resistance for each range. 
  I
don't understand why you want bipolar ranges,  but if I am reading that 
right
then you want meters with a zero in the middle,  which probably won't 
be all
that useful for your 0-10V meter -- maybe you'll end up purchasing two
different types?

 > I'm talking about a needle meter implementation, so I also need to 
know
 > what kind of meter to buy for this purpose (i.e.  "0 to 100mV 
meter",
 > or "1 to 300mA meter"... etc)

The requirements (besides what I wrote above) will depend on a few 
things.
More sensitive meters are generally somewhat more expensive,  bigger 
ones are
more expensive,  etc.  My best guess is that either 0-1mA or 0-50uA will
probably be best for you,  the first one being somewhat less sensitive 
and
also putting slightly more of a load on the circtuit you're measuring.  
How
much of a problem that will be you'll have to determine.

 > I will design and print the scale on paper and substitute the one that
 > comes with the meter. In this case 4 scales should be printed: -20 to
 > +20, -10 to +10, -5 to +5 and 0 to 10V. Or at least -10 to +10 and 0 
to
 > 10V, wich may be much clearer...

Good luck with that,  I've never done that part.

What you need to do is to determine the _internal resistance_ of the 
meter
movement you've chosen,  if it's not specified.  An easy circuit to do 
this
is to put a couple of multiturn trimpots and a meter together,  one in 
series
and one in parallel with the meter movement.  Say 50K or 100K trimpots.

Set them both at maximum to start with!

Apply some low DC voltage,  say 10 volts or whatever.  Adjust the series
trimpot until you get a full-scale reading.  Then adjust the parallel 
trimpot
until the meter reads half of what it did.  The resistance of that 
trimpot is
then equal to the meter's internal resistance.

Don't try and measure it directly,  ohmmeters mostly put out *way* too 
much
current on their lowest ohms scales.

The multiplier resistance you need for a given circuit is figured as 
follows:

Say you want an 0-10V scale.  And say that the meter movement you have 
chosen
is an 0-1mA movement.  That means that 1 mA needs to flow to get a 
full-scale
indication,  which from 10V means we need 10K ohms of resistance 
altogether.
To be really accurate we subtract the meter's internal resistance from 
this
so that we're looking at 10K ohms total.  The "sensitivity" of that 
meter
movement can also be described as"1000 ohms/volt".  Similarly,  a 50 uA
meter can be described as having a sensitivity of 20K ohms/volt,  pretty
common for a lot of analog meters.  The internal resistance of a meter 
can
often be ignored,  if you're getting up there in voltage,  it becomes 
more
critical as you get closer to the meter's values itself.

Hope this helps...