PS: Get one with a fuse that's easy to change otherwise you'll spend a lot of time wondering why the ampere readings are weird. Oh, wait, there's no such thing at any price.
Suggest you look at a Fluke 27. I picked one up (used) for around $50 US through eBay. When you open the battery compartment (two 1/4-turn latches), the fuses are accessible. They're expensive to replace due to their construction, but they're easily accessible. A number of other Fluke meters have easily-accessible fuses, but they're considerably more expensive. The 27 is also built like a brick, autoranging, and should do everything you'd need to do with an Arduino.
Not sure how much you'd actually use the current measurement with Arduino projects, but it's there.
As polymorph requested, here's a (partial) schematic--I couldn't see a way to represent the sensor itself in Eagle without defining a whole new part. It's connected as follows (red/black are +/- in, green/white are +/- out, according to the markings on the sensor):
Red to the output of the 7810
Green to pin 3 of the LM358
Black and white tied together, and to ground
I'll check resistances among the sensor terminals and report back. I'll also try the differential amplifier circuit out of the datasheet--looks straightforward enough.
OK, it's been a while since I last posted--between getting parts and traveling, I haven't had much chance to work on this in the last month. I got a 358, an assortment of resistors and capacitors, a 7810, and a 7805, and assembled it all on a breadboard this afternoon. I used a 56K resistor rather than a 50K, but otherwise as directed here. I powered the 358 from the 7805, and the sensor from the 7810. I tied the sensor input and output grounds together.
Likely as a result of tying the input and output grounds together, I got some unexpected results. Where previously the output of the sensor was 0 V with 0 psi applied, it became 946 mV. Instead of increasing 1 mV/psi, it increased about 0.5 mV/psi.
The output of the 358 was steady at 3.57V. I suspected this was as close to Vcc as its output would go, so I tried powering it from the 7810 instead. The results from that confused me. With 0 psi and an input voltage of 946 mV, the output was 1.834V. With 20 psi and an input voltage of 959 mV, the output was 1.826V. At that point I stopped the test.
I understand that this wouldn't meet any kind of certification--this is for an experimental application, so that isn't a problem.
Unfortunately, I do not have a data sheet for the sensor; what I know of its characteristics has been determined by trial and error. It came with an electronic module that was designed to be used with an aircraft engine instrumentation system. When I applied power to the module, it was feeding 10 volts to the "in" terminals on the sensor. Given that, I wired the "in" terminals on the sensor to a bench power supply set for 10 volts, applied air pressure in 20 psi increments, and measured the output. In doing so, I was looking for stable readings in voltage, resistance, or current; what I got was a stable and repeatable 1 mV/psi result.
My goals are to light an LED when the pressure falls below a certain level, and also output the pressure over a serial line (a fallback would be to just manage the LED, but that should be achieveable without involving a microcontroller, I'd think). I don't need lab-grade accuracy, but +/- 5% would be good. The application I'll be using it in will be powered by a 14V DC bus, which I'm planning to regulate down to 5V for the Teensy, and to 10V for the sensor, using a 7805 and 7810 respectively.
OK, op amp it is, and I'll look up the LM358. Thanks for the input!
(later) I've looked at the data sheet for the LM358, and it looks like the gain is set by the ratio of R2 to R1. If I'm reading and understanding it correctly, this means that the 50k/10k combination would result in a gain of 6 (1 + R2/R1), amplifying 0-100 mV to 0-600 mV. Is this correct?
I'm working on a project that needs to read a four-wire pressure sensor. The sensor takes 10 V DC in on two wires, and outputs 0-100 mV DC, 1 mV/psi. I need to read that voltage on an Arduino (actually a Teensy), convert it to a PSI measurement, light an LED if it's too low, and output the information over a serial line. I'm OK on the software side of it, at least conceptually, but questioning how to read the voltage.
My best guess here is that, to get decent resolution, I need to convert 0-100 mV to 0-5 V. My next guess is that I'd use an op-amp to do this. From there, my guesses are running out--I've not had occasion to play with op-amps before, and I'm not even sure that they'd be appropriate here. What would be the best (simplest, lowest parts count) way to accomplish this?
Looks like Rigol's just released (or about to release) the DS1000Z series, starting with the DS1074Z--4 channels, 70 MHz, and a package about the size of the DS1052E. Screen is 7" dia., which is larger than the 1052 but smaller than the DS2000 series. Starts at $585 US. For another $200 or so, a 2-channel function generator can be built in. Product page: http://www.rigolna.com/products/digital-oscilloscopes/ds1000Z/ds1074z/ Video overview:
It sounds like it doesn't have all the features available that the DS2000 series does, but the function generator could tip the scales a bit (as could the extra 2 channels). I wouldn't be surprised if it were hacked in fairly short order as well, but that's another matter...
I've not personally tested the increased bandwidth or added features, but the scope's status page states they're active. I can just confirm that the scope seems to work well for what little I've used it for so far.
Looks nice. I just received my Rigol DS2072, and converted it to a fully-optioned DS2202. Seems very nice, but it's my first scope, so what do I know? The I2C and SPI trigger/decode seem like they could be very useful for Arduino work. We're probably both rather far afield of what OP is looking for, though.
If you're looking at the Rigol DS2000 series, the DS2072 can be upgraded to 200 MHz bandwidth using just a software key. A web search for "rigol ds2000 hack" will bring up quite a bit of relevant information, and there's a long thread on eevblog.com on the subject.
I've used seeed for one batch of boards, and had no problems--got the boards reasonably quickly (I don't remember exactly how long, but I don't remember being frustrated by the wait), and they worked perfectly. Just one data point.