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[focalpoint]

I'm interested in making a Chronulator using antique radio panel meters.



That one goes up to 300 uA, though. Could the Arduino drive it to full scale, or would I need to modify the internals of the meter for that you think?

[jboone]

If your meter operated at 300 microamperes, it'd be no problem. But the photo you posted shows a meter that requires 300 milliamps -- 1,000 times more current. That meter would drain a pair of AA batteries in less than a day. So I'd suggest modifying your meter. Meters I've seen have a shunt resistor that sets the range. Removing or changing that resistor should give you different current ranges. See this page for more details on how this works:

http://www.allaboutcircuits.com/vol_6/chpt_3/11.html

And please don't hesitate to ask if you have questions.

- Jared

[focalpoint]

Ah yes, wow, I should've checked that more carefully.

I bought a different set of panel meters:



that are 0-150uA and should be much more suitable. Smaller, too, they'll fit in the space I'm interested in with better clearance.

I saw there was a link for a web site that helped calculate resistor sizing, but it was a bit over my head.

[kdbrown]

Hi. I'm the one responsible for the current source calculator. It was more of an exercise for me than a you-must-use-these-values decree. It determines the super-ideal situation (although using real component values). The calculator has two modes, the first shows what to expect adjustment range-wise from a given set of components. The second mode follows a design path to get an ideal set of values. As long as what the first mode gives you includes your desired full-scale current and that full-scale current is reasonably in the center of the range, the circuit should work, and it should give you a reasonable adjustment range.
To determine the ideal situation in the second mode, by entering "150 u" in the "nominal current" box and hitting return (link) you will get the following: R1=1 kΩ (the pot), R2=4.42 kΩ 1%, R3=23.7 kΩ 1% and R4=1kΩ 5%. The 1% resistors can be ordered from Digi-Key or Mouser, etc. For example a 4.42 kΩ 1% 1/4 W resistor at Digi-Key is 4.42KXBK-ND and you would have to order 5 of them (you need two, one for each meter circuit) for a grand total of $0.56 (US). Similarly the 23.7 k parts. R4 is not too critical, the 82 k that Jared supplies might be fine, but the potentiometer is the problem. I'm not sure what the product line that he uses for his pot is but he supplies a 5 kΩ device.
If you can't get a 1 k pot in the same PCB-mount style you can consider an off-board pot, just wire up the three wires as if it were soldered into the board.
Hope this helps...let me know if it did or did not!

[focalpoint]

Ah, I understand a bit more know. It would have probably been more clear, but I don't have an incredibly strong electronics background - just enough to be dangerous, I guess you could say.

I'm ordering the kit sans-meters now; will keep you guys posted on the build. Thanks for the resistor values!