Are Such things existing ?

[DavenPaget]

First and foremost , is there such thing as a digital resistor , whose resistance can be digitally controlled by a MCP ?
Background info :
Let's say somebody wants to key in exact voltage on a LM317 digitally just by hitting a soft-touch button , would be interested to know how it all pans out 

[alparent]

Digital pot maybe?

[earthtonesaudio]

http://www.coolcircuit.com/circuit/programmable_supply/index.html

Your ABCD inputs would connect to an up/down counter which in turn would be controlled by the pushbutton(s).

This gives you 16 precise, somewhat arbitrary, discrete voltages from which to choose.

[DavenPaget]

http://www.coolcircuit.com/circuit/programmable_supply/index.html

Your ABCD inputs would connect to an up/down counter which in turn would be controlled by the pushbutton(s).

This gives you 16 precise, somewhat arbitrary, discrete voltages from which to choose.

Not that ! But something like , those programmable power supplies whose voltage(s) goes up when you press the momentary switches .

[xfc]

as stated above
a digital pot at the ref of the LM317 should do the job

Digital pot maybe?

[DavenPaget]

as stated above
a digital pot at the ref of the LM317 should do the job

Digital pot maybe?

Ah yes ! I forgot digital pots are in IC-form , i nearly thought digital pots were still , pots ( mechanical form) 

[Gurner]

Another alternative (if your required voltage is less than 5V... some of the more recent PICs have a 32 step DAC inside - you can feed the top of the dac either VCC or a fixed voltage reference derived internally...this allows you  a large array of precise voltages you can get the PIC to output - just follow it up with a voltage follower. Keeps it all internal to the PIC...lower parts count blah blag.

Another alternative is a simple dedicated DAC IC (vs a digital pot) ...they have a bufffered output voltage.

[DavenPaget]

Another alternative (if your required voltage is less than 5V... some of the more recent PICs have a 32 step DAC inside - you can feed the top of the dac either VCC or a fixed voltage reference derived internally...this allows you  a large array of precise voltages you can get the PIC to output - just follow it up with a voltage follower. Keeps it all internal to the PIC...lower parts count blah blag.

Another alternative is a simple dedicated DAC IC (vs a digital pot) ...they have a bufffered output voltage.

Hmm , do the AVR's have a similar DAC ? ( i don't really like PIC's  )

[nexekho]

Another alternative (if your required voltage is less than 5V... some of the more recent PICs have a 32 step DAC inside - you can feed the top of the dac either VCC or a fixed voltage reference derived internally...this allows you  a large array of precise voltages you can get the PIC to output - just follow it up with a voltage follower. Keeps it all internal to the PIC...lower parts count blah blag.

Another alternative is a simple dedicated DAC IC (vs a digital pot) ...they have a bufffered output voltage.

Hmm , do the AVR's have a similar DAC ? ( i don't really like PIC's   )

No, but you can use one of the square wave PWM outputs with a filter to get 8-bit analogue output, or two with resistors to get a rough 16-bit.
Up to now I've been using a RC lowpass but I'm thinking of switching to one of these:
http://interface.khm.de/index.php/lab/experiments/arduino-dds-sinewave-generator/
I can't find the inductors though; biggest ones I can get are 1mH.

[alparent]

if 1uH is the same as 1mF the Tayda as 4.7uH for 9 cents.

http://www.taydaelectronics.com/servlet/the-484/4.7uH-Inductor/Detail

[nexekho]

if 1uH is the same as 1mF the Tayda as 4.7uH for 9 cents.

http://www.taydaelectronics.com/servlet/the-484/4.7uH-Inductor/Detail

uH = microHenries
mH = milliHenries

It looks like you can get them on eBay but as I don't have a meter that can measure inductance I wouldn't buy from there.

[DavenPaget]

1mH ? no problem . Element14 / Newark / Farnell sells those .

[nexekho]

1mH ? no problem . Element14 / Newark / Farnell sells those .

No, no - it needs 4.7mH, I can only get 1mH.

[earthtonesaudio]

There are pushbutton-controlled digipots:
http://www.maxim-ic.com/app-notes/index.mvp/id/168

[DavenPaget]

There are pushbutton-controlled digipots:
http://www.maxim-ic.com/app-notes/index.mvp/id/168

Yes ! Thanks for your help . 

[DavenPaget]

1mH ? no problem . Element14 / Newark / Farnell sells those .

No, no - it needs 4.7mH, I can only get 1mH.

I did a search , threw up quite a hunk of results .
http://sg.element14.com/jsp/search/browse.jsp?N=2108+203354&Ntk=gensearch&Ntt=4.7mh&Ntx=mode+matchallpartial