EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: IO390 on September 05, 2014, 01:19:38 pm
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I posted some time ago about my college project (here: https://www.eevblog.com/forum/chat/would-you-buy-this/ (https://www.eevblog.com/forum/chat/would-you-buy-this/)). It's simple as anything with an opamp current source. (Schematic the first attachment. (Also bear in mind that a lot of what I'm saying might seem a little retarded as I'm almost entirely self-taught))
I've actually been using it on the bench and it's handy, but since it's still the holidays I decided I may as well further the design a little. Here's what I wanted to address:
- The ranges are fine, although I was lazy and the 100R range puts 10mA through whatever you're measuring, and the 10R range uses 100mA. This seemed a wee bit too high to me.
- Autoranging. Naturally, I got a bit fed up with twiddling the switch at the same time as trying to hold the probes on something. I figured I could just do it all using digital, but where's the fun in that? Besides, I much prefer messing about with a pencil and paper as opposed to sitting on my PC.
Here's what I came up with (second attachment and pictures). As I said earlier, a lot of it might seem a bit dumb but I'm new to this :) . It's also my first analogue design of any real complexity, and also my first time using more than one logic IC.
It's relatively simple. To begin with, I addressed the 10mA and 100mA range issues by using the 1K range (1mA) and amplifying it by 10x or 100x using LT1101 instrumentation amps (way overkill but I had them in the drawer, so it was more convenient than fiddling about with precision resistors). There's also probably more analogue switches than any sane person would use, but I couldn't think of any other way of doing it.
The ranging is done by two comparators and a D flip-flop counter. Of course, with this method, when the next counter goes high, the previous one is also high. I used XOR gates so that I only get one output at a time.
The measurement is done with a moving coil meter, so I used a voltage follower to drive it. The input to the voltage follower is what the comparators use. When the voltage goes above 1V (the meter is set to 1V FSD) the whole thing resets to the 1M range, and when the voltage goes below 100mV, the circuit counts down until the voltage is back within the two thresholds. If that makes any sense...
The clock comes from a 555 and is at about 100Hz. This means that the meter cycles through all the ranges within 60 milliseconds. It's quicker than my VC-99 so I'm happy.
So yeah it works great. I built it as a birds nest. IE you have a power rail, ground rail and ground for the opamps. Just solder the components on wherever you want and connect it all up with enameled wire.
The LED furthest away from the main circuit is the 1M one, and the one furthest inboard is the 10R one. I've attached a few images of the circuit running, too. The wires running off are the makeshift probes. When I find a box to put it in I'll connect up proper probes etc...
Any and all feedback is appreciated.
1.8R:
http://imgur.com/nEnExVc (http://imgur.com/nEnExVc)
330R
http://imgur.com/S15T8ZQ (http://imgur.com/S15T8ZQ)
560K
http://imgur.com/QknrpHl (http://imgur.com/QknrpHl)
The rest:
http://imgur.com/B0qlrWQ (http://imgur.com/B0qlrWQ)
http://imgur.com/1R2eB8m (http://imgur.com/1R2eB8m)
http://imgur.com/tEiQgqt (http://imgur.com/tEiQgqt)
http://imgur.com/BgWkphH (http://imgur.com/BgWkphH)
http://imgur.com/B5qyG2U (http://imgur.com/B5qyG2U)
http://imgur.com/RBmXANB (http://imgur.com/RBmXANB)
http://imgur.com/3x7V64q (http://imgur.com/3x7V64q)
Thanks for looking!