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[on:]

Hi,
    I am planning to try multiplexing additional analogue inputs through a decade counter and an opamp in a summing configuration -

Roughly

1) Decade counter outputs are connected to pots
2) center tap of the pot is connected to the opamp summing circuit
3) only one counter output is high at a time, so (hopefully) the summing circuit will output its value

Lots of assumptions on my part namely
Assumption 1: low outputs are effectivley grounded
Assumption 2: grounding one or more of the inputs does not impair the summing circuit

I know there are dedicated parts available I am just hoping to build something from the parts I have today and this seems viable.
Speed is not important in the given application, accuracy within several % is also fine.

Any comments or well know alternatives using generic components ?

Thanks

Duane B

rcarduino.blogspot.com

[Reply #1 on:]

A decade counter is not what you mean. What you mean is a ring counter, one that only outputs one high with the others low.
The main problem is that the pots with zero on both ends will act as variable pull down loads to the active pot. Making the value on them affect the pot you are trying to measure.
By putting a diode in seriese with the wiper you can prevent this but the voltage drop of the diode brings its own problems with range or a dead spot at each end.

[Reply #2 on:]

Sounds fine to me. I assume you mean something like a 4017 where each op goes high in turn.

You may be able to avoid op amp summing and just uses resistors to sum, as long as they are much higher value than the pots, but significantly lower than the input impedance of the analog input.

Time to get the breadboard and multimeter out!

[Reply #3 on:]

Hi,

Yes I do mean a 4017.

It might just be simpler to go with the 4017 and diodes, a dead spot will actually come in handy giving me a guaranteed 'off' region for each setting.

Will have a go this evening,

Thanks both

Duane.