Automated switching of various resistor values into circuit

[M. Kibat]

Started sifting through and organizing a 'grab bag' of caps. Found a number with 'mystery markings', so I threw together a quick, simple circuit and program to read out RCTIME values to help ID them. Various resistor values (1k, 10k, 100k, 1m) are manually switched into the circuit, the program is reset and the RCTIME result displayed via debug. Nothing fancy here, but it's been useful.

What I'm wondering -- how best can I automate the switching process so the Stamp can autorange to find the best resistor value?

Looked at digital potentiometers, but they don't have the range of values or accuracy I need.

How about replacing the manual pushbutton switches currently attached to each reistor with a transitor switch? Each transistor would be controlled by a pin on the Stamp, so the program would simply poll each one, charging the cap, then turning on that resitors transistor to discharge it and measure RCTIME. The program would select the pin with the highest value returned and display that one.

Since this is a one-time project (I don't expect to run out of caps again anytime soon!) I don't want an over-engineered, highly accurate result. Just something quick-n-dirty.

Thoughts?

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M. Kibat
Bloomington, MN
kibatme@visi.com

[Bean]

Something like a CD4066 quad bilateral switch might do the job.
Bean.

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·

[M. Kibat]

Bean (Hitt Consulting) said...
Something like a CD4066 quad bilateral switch might do the job.
Bean.


Also found ISL43141 (http://www.intersil.com/data/fn/fn6032.pdf). Would like to duplicate the functionality with discrete components on-hand rather than waiting for a mail-order to arrive, but may still order a couple chips just to play with.

Thanks for the quick response!

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M. Kibat
Bloomington, MN
kibatme@visi.com

[Beau Schwabe (Parallax)]

With using the option "A" schematic in the RCTIME example, the circuit below might be what you are looking for.
Simply make the "Range Select" pin a logic "0" on the range you want to use.

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

[Bean]

Beau,
· That is brilliant. Nice and simple. Kinda like me ;)
Bean.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"SX-Video·Module" Now available from Parallax for only $28.95

http://www.parallax.com/detail.asp?product_id=30012

"SX-Video OSD module" Now available from Parallax for only·$49.95
http://www.parallax.com/detail.asp?product_id=30015

Product web site: www.sxvm.com (http://www.sxvm.com)

"Ability may get you to the top, but it takes character to keep you there."
·

[Paul Baker]

Thats great Beau, I was thinking along the idea of using IO pins but was strugging on the "some pulling up, others pulling down" issue, the diodes take care of that.

<edit> ack, the whole input/output issue is also important, just shows I was much further behind the thinking process than I thought </edit>

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·1+1=10

Post Edited (Paul Baker) : 1/12/2006 5:45:47 PM GMT

[M. Kibat]

Sweet.· Hits all my design points!· Can't wait to get home and try it.·

With all the hardware time you've saved me I can focus on the software and maybe calc / display the exact value in uf on an LCD display!

PLUS I can use the same approach to sift through the photocell grab bag I got -- capacitors in place of the resitors and the photocell in place of the capacitor.· A little program change and I'm off and running!...

THANKS a ton!

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M. Kibat
Bloomington, MN
kibatme@visi.com