OK, here's a clean a sheet of paper. I need a way to LOG a series of on on/off pulses, so that I have a record of time a circuit is "on" versus total time. Even better, I'd like to also log a DC voltage along with that. Essentially I need a strip chart recorder.

Here's the scenario: I've gotten a nice, thermostatically controlled cooler circuit working nicely. It gives me temperature control to within about 1°C by toggling a TEC on and off every few minutes. Now I need to start collecting data using my cooler, looking at how it responds to things going on inside of it. (Yes, it's being used as a calorimiter.) An ideal solution would be a strip chart recorder to record the sawtoothing temperature and also the square wave of voltage applied to the TEC. But I don't have a dual pen strip chart recorder and need to rig up a poor man's alternative. I DO have old computers lying around that could be put to the task, but they're not good with DC voltages. Sound yes, DC no.

Any cheap, simple ideas? Controlled stopwatches?

I should add that an observer with a clock and a multimeter is also a perfect solution. They could record temperature and on/off times by hand. But we're talking about experiments that will take hours, maybe 6-12 hours or more, so I need an automated solution.

 

If your PC is a desktop model, you can get plug-in interface boards. And even if it's not, you can get USB-based interfaces--example found randomly on the net, and I have no financial interest:

http://www.dataq.com/products/startkit/di148.htm

Actually that looks like a pretty good system. $25, and you get recording software with it.

 

I was hoping for something even cheaper, but I've been wanting data acquisition for a long time. Might be time to bite the (small caliber) bullet. Really would save a lot of time in the long run. I'll see if I can find a Mac version that cheap.

 

If you dont mind mucking about with it a bit you could remove the decoupling caps from your soundcard input and use the two ADC's that are at its heart ...
Google - sound card oscilloscope -

Plenty of software and examples ... mostly free.
(probably not as good as an interface board mind you, it depends on your budget)
Al

 

If you dont mind mucking about with it a bit you could remove the decoupling caps from your soundcard input and use the two ADC's that are at its heart ...
Google - sound card oscilloscope -

Plenty of software and examples ... mostly free.
(probably not as good as an interface board mind you, it depends on your budget)
Al

Interesting! As much as it scares me to remove the safety of the decoupling caps, it's intriguing to think this might give me a DC voltmeter. Could be the perfect poor man's solution. Identifying and bypassing the caps might be a challenge. These things are pretty small these days.

 

bypassing just involvs a small wite loop, usually un-insulated and arranged not to toutch anything.
You will probably find it easier to atach to the track rather than the cap.
I would recomend a cheep plug in sound card ... dont do this on the mother board.
if you measure the DC resistance, with a mulitmeter, from the input pin back through any serease componants the offending cap probably the first to block the DC completly and settle to an infinite reading.

Alternatives...
Picotech do several small ADC products designed for low end data aquisition.
http://www.picotech.com/data-acquisition.html

And back to the sound card ....
Most have game controler ports which use a timed pulse to determine a resistance.
Its not fast ot particulaly accurate but given your application may just do what you need without butchering or buying anything. Heres a primer ... (There ar many more)
http://pinouts.ru/Inputs/GameportPC_pinout.shtml

Al

 

25 dollars is pretty cheap, what is your time worth? An arduino could also be used, but again is in the 25 dollar range.

By using a voltage controlled oscillator you can go direct to your sound card and record signal as a wave. Decoding would involve some software. The VCO could be based on a 555 circuit

 

If your PC has a old style printer port there are (what?) 10-14 bytes you could use with a simple A/D converter. Same basic idea.

 

Another follow-up.

Well, I decided to go large and got myself a LabJack U3-HV for $114. It's an awesome device with many, many capabililties: http://labjack.com/u3
It appears well built and I'm very impressed after playing with it now for a few weeks.

I was attracted by the high versatility of the device, its programming options and the support level they offer, including Mac support. With their help I've been able to write some Visual Basic macros to build an interface to the device within Excel. So now it'll be easy to watch ~8 voltages all at once and log them all into a virtual stripchart.

That's all I need for now but it's barely touching the capabilities of this thing. One thing I plan to exploit is its ability to output controlled PWM. All in all, having this thing at my disposal will completely change what I need - and don't need - to build in the future. I just wish now that I would have bought it sooner.