LDR as Digital POT ?

[asking]

Hello,

I have been searching for 50K Digital POT but it seems to be costly 5-7$ + Shipping to India. I have idea how it would be if i use PWM based LED light and use LDR as Digital Resistor ? Can i use it ? any draw backs ?

Please advice...

Thanks

 

[Borber]

You must not use PWM else you will amplitude modulate resistance. Use DC.

 

[crutschow]

The LDR resistance is very non-linear with light so you would have to calibrate the PWM duty-cycle to the LDR resistance.

 

[FvM]

The LDR resistance is very non-linear with light so you would have to calibrate the PWM duty-cycle to the LDR resistance.

Don't want to argue if it's very non-linear or just non-linear, but there also temperature dependency and a certain memory effect. So for higher accuracy requirements, a multi-channel LDR with a compensation element for feedback is strongly suggested.

 

[asking]

Ok thanks. So i can use Smooth-out (Caps) PWM signal to FED to led.

Regarding Multi-Channel LDR with Compensation ? can you explain me more ?

 

[jpsganesh]

https://www.edaboard.com/threads/304251/#post1316628

Simple way to construct a LED and LDR in a house with heat shrink sleeve pack it nicely.

I used this technique to control my Fan speed Phase angle control with PWM (without zero crossing feedback)

Ref:

**broken link removed**

 

[neddie]

You cant use caps to smooth your PWM signal , you will just peak detect. Use an RC circuit to give you the PWM -> DC.
50% duty cycle = 50% of peak PWM value. Should be a linear relationship.
LDR won't be linear , but you could always create a look-up table to translate PWM / resistance required.

 

[crutschow]

Don't want to argue if it's very non-linear or just non-linear,........

Then why did you bring it up? I realize very non-linear is a none specific term but there certainly is a large difference between the slight non-linearity of, say a digital pot's output, and the larger non-linearity of a logarithmic type response which is characteristic of an LDR.

 

[FvM]

Then why did you bring it up?

E.g. to clarify that it's not logarithmic.

 

[crutschow]

E.g. to clarify that it's not logarithmic.

My first post never mentioned that it was logarithmic. And it certainly is not linear. For that it would be a straight line on a log-log plot.