Potentiometer current question

I have a 100K potentiometer connected up across the supply of a picaxe 08m, ready to give a voltage to a picaxe input pin via the wiper. I am loath to switch it on because I cannot work out if I need to add another resistor between the wiper and input pin to limit current to the picaxe.

The potentiometer will later be replaced by a temt6000 light sensor board from sparkfun (https://www.sparkfun.com/products/8688) which uses a 10K resistor on its emitter, will that need a current limiting resistor?

Potentiometer and/or light sensor will share a supply with picaxe, very short (2") wires between picaxe on axe08 proto board, and sensor/potentiometer. Both devices potentially pass 0-5V to the picaxe.

Thanks,

Richard

Yes I would put a resistor between the potentiometer wiper and the Picaxe, mostly just in case you accidentally set the pin to an output. Same goes when you have that light sensor hooked up.

There is no need for current limiting resistors when the voltage applied to an input is between 0V and the PICAXE's V+ level.

Adding one will however help to prevent damage should the PICAXE input pin inadvertently be used as an output.

That's good to know, thanks both!

100k is too large a resistive value for the ADC inputs. Can you use a 10k instead?

I wa thinking the same thing after reading the manual, and a few posts on here. I'm sure I have a 10k pot somewhere.

Long ago I did some tests and calculations with an LDR as opposed to a potentiometer.
See here: http://www.picaxeforum.co.uk/showthread.php?9265-Info-on-selection-of-series-Resistors-with-LDR
Higher resistance values for the LDR and series resistor to maximise analog input contrary to "good" practice/recommendations for source impedance may still give good accuracy.

The ADC input is a single sample and hold circuit, which scans all the ADC pins. The input voltage has to charge up a capacitor through the input impedance, which is then read by the ADC circuit. It may be that the accuracy depends on the frequency and amplitude variability of the input signal and the number of channels scanned. So a static value on only one channel might be read more accurately than a varying voltage with many channels in use.