Looking for a USB interfaced ohm/resistance meter

Yes, I agree - what happens if you accidentally connect it to an AC source with 240v running through it? I've lost a multimeter's fuse that way working on a power supply...
 
I basically want to know if it's light or dark. Based on that my application will do certain activities.

Was thinking of using a photoresistor and an ADC circuit, perhaps with a PIC.

I was thinking if I can read the resistance I can auto calibrate using my code to elimninate noise and variable environmental conditions.
 
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Have you tried extech.com ?

They have some pretty awesome and specialized instrumentation.
 
What about using the voltmeter and a Wheatstone bridge?
 
Perhaps one of you electronics types would be interested in designing and making these for me? I need 100 units.
 
I'll take a closer look for you, biometrics, and revert. Should be easy enough.
 
kayvee said:
Here we go, this will work a treat.

http://www.phidgets.com/products.php?product_id=1127

There must be lots of other items like this around too. Maybe check out the robotics forums/links.

Edit: Okay you need another interface board with this :(
Click to expand...

Unfortunately, that also requires a phidgets acquisition board, which starts at $50, and goes up from there. (As your edit notes).

I'd suggest getting an Arduino/Launchpad/STM32Discovery, and wire a simple Light dependent resistor to it.

Arduino's can be had from netram.co.za, Launchpads are $4.30 from TI (free shipping from the US), and STM32Discovery are R110 from AvnetKopp. Options depending on how urgently you need it, vs how big a deal cost is :-) STM32 is MASSIVE overkill for this, the Launchpad would be fine (and cheapest), although it would take some time to arrive. You may find a local stockist, though, I've never looked.

The circuit itself would be pretty much trivial, then all you would need is some code to sample the Analogue to digital converter, and output it over the USB (effectively a serial) link.

Arduino should have lots of sample code you can use, and for a microprocessor n00b, it would probably be the easiest solution.
 
Here's my brief:

We want to read values from a photoresistor (LDR) into a PC over a USB connection.
The LDR should be of a size not more than 5mm in diameter and preferably smaller (like 3mm).
The LDR will be attached to an appliance from which we wish to measure a changing state in brightness levels.
The LDR needs to be attached to the circuit board by a thin wire with a connector on the end (preferable molex clip-in or similar).
The circuit board needs to be small enough to sit "in-line" on a USB cable (with shrink-wrap).
From the PC side of things, we want to be able to measure the change in resistance values (or voltages for that matter).
It doesn't have to be highly accurate as we want to detect an on/off state but we would prefer the raw analogue data so we can calibrate it.
The final product needs to be low-cost as we require 100 units initially.
 
It'd be fairly easy to do. You can go for something pretty low-end MCU-wise like an Atmega48 at around R12/each the last time I checked. This, coupled with a simple USB/USART chip and a couple of resistors and a LDR and of course the USB socket, and you're good to go. Excluding power, of course, but that's fairly simple as well as you can run it from the USB. For a batch of 100, you'll end up closer to R100 each than R300, and that's fairly easy. This excludes the labour of actually building the things, of course. For a simple setup like this, you can skip surface mount and just go for DIP packages. Should simplify construction. I'm not sure what availability is like for DIP USB/USART ICs though. They're pretty simple and generally come with a fairly forgiving pitch, so even if they're surface mount only, they shouldn't be too much of an issue.
 
Sigh, I really wish I had gone into electronic design after varsity, this is exactly the kind of stuff I want to be doing, and exactly what I studied :(

One thing to add, no need for analogue data - calibration can be done via a potentiometer. And generally LDRs can vary their resistance quite a bit, so if you only looking for light/dark, calibration should not ever be required.
 
Don't forget PCB origination costs, whichever technology you use.
 
Archer said:
Sigh, I really wish I had gone into electronic design after varsity, this is exactly the kind of stuff I want to be doing, and exactly what I studied :(
Click to expand...

It's never too late, Archer.
 
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