Circuit to reverse meter needle direction?

[Ian MacGregor]

Hi,
I've been thinking about a mod to Gyraf's SSL to make the meter move to left with GR instead of to the right. Other than turning the meter upside down (haha) does anyone have any ideas?

So far, I've thought about biasing the meter to always be FSD with no GR, and then using a NPN transistor to short across the meter. The transistor would be driven by the op-amp that drives the meter in the original Gyraf circuit. For higher GR the op amp output voltage goes higher, making Vce of the transistor lower (which makes the voltage across the meter lower as well becuase Vce is in parallel with the meter).

With no GR, the meter is fed the 1ma for FSD. At full GR, the transistor is pretty much saturated and the meter is shorted out by the transistor. I guess the hard part might be for all the values besides full GR and no GR.

Does this make sense?? If it does, then I will do some calculations and test it out. I just wanted to make sure it wasn't a stupid idea before I spend too much time on it.

...or I could just turn the meters upside down... hehe

Thanks!!

Ian

 

[Steve Jones]

I contacted the guys at Sifam about 6 months ago, and they said that thay have a couple of models that rest to the right and deflect left. I never found out the model numbers though.

Al Smart uses these meters on his Smart SSL, and I have one on my ADR Vocal Stresser. I would love to know who stocks them, but I am guessing no-one in Australia.... Perhaps you might have more luck over there!

 

[Ian MacGregor]

Ahh yes.. I guess that would be the best route! I am assuming that a reverse direction model would be even more expensive and hard to get ahold of than the regular Sifam meters.

Ok, so my original question stands, but does anyone know a US or even UK source for the right to left Sifam meters??

Ian

 

[Steffen]

Ian, a simple opamp inverter plus dc offset for full scale zero should do the job. something like this:

the transistor thing may work but I found this solution easier ....
steff

 

[gyraf]

Yes, that would work - but it could even be done passively by offsetting the +-side of the meter with a bit of current (some resistors and a trimmer) and then sending the meter signal to the -side

You'll have to experiment with the resistor values.

A good thing when experimenting would be to protect the meter with two antiparallel 1N4001 diodes across it (in order to limit voltage across it to some 0.6V)

Jakob E.

 

[VacuumVoodoo]

hi,
this circuit will ofcourse work but bear in mind that the meter movement has different "ballistic dynamics" in both directions. In the GR indicator application it's of secondary importance.

 

[PRR]

Either the op-amp with inversion and offset, or a reference voltage plus reversing the meter terminals.

Or, for "cheap" meters (ones where you won't cry if you ruin one): open it up, find the zero-adjust lever, and crank it about 90 or 180 degrees clockwise. Now the needle sits at the righthand peg. Reverse the terminal connections, and it deflects to the left peg.

As you see, the cost of a "backward" meter "should" be the same as a "normal" meter. Everything is the same, just assembled different. However since backward meters are super-rare, you won't find them in-stock anywhere, you have to disrupt the assembly line to get some made just for you, so it isn't practical unless you order enough to make the meter-company happy.

Minor drawback is that now you can't zero the meter with the external trimmer, you have to pop the cover and fiddle the spring-lever directly.

Major drawback is that you may bust-off the spring-lever by turning it far beyond where it is supposed to be.

Better meters have two springs, front and back. To shift the needle 90 degrees, you either have to crank the front spring 180 deg (which may degreade linearity, though in GR use that may not matter), or take the movement out so you can get at both spring levers and turn them both 90 deg.