Miniproject - LED Brightener with PNP & Sensors

Thread Starter

outro

Joined Mar 7, 2017
43
Simulator: http://everycircuit.com/circuit/5839454009032704

Current situtation above.

Problem: LED is on only when resistance is under 50k.

Question: Which variables, or other things should I note when creating circuits like this?
How should I solve the problem so the LED is off @ maximum resistance, otherwise 1% - 100% on?

In other words, I have a sensor (here: potentiometer), which varies in its resistance. I would like to indicate the value with the LED.

Seems easy enough, but in practice, it requires some understanding I'm missing ☺
 

Alec_t

Joined Sep 17, 2013
14,280
Try using a Darlington transistor arrangement, so that less base current flows through the variable resistance.
 

Thread Starter

outro

Joined Mar 7, 2017
43
Try using a Darlington transistor arrangement, so that less base current flows through the variable resistance.
Thank you for your answer.
I tried a few ways of implementing the Darling transistor for the PNP transistor setup, but it didn't work. Did I miss something?

It works well amplifying the NPN -version though
It seems to make the NPN version on/off instead of varying

http://everycircuit.com/circuit/5839454009032704
 

Thread Starter

outro

Joined Mar 7, 2017
43
After many failed attempts, I come here for help.
I was able to get it to work in the simulator, but in real life it doesn't work.
So either
A) wiring is wrong
B) components arent compabilile

Any help?

See images, I tried to make them clear
My pot is only 100k, so I gave it an extra 200k boost

http://everycircuit.com/circuit/5839454009032704

pnp: 2N3906
npn: 2N7000

Other components available for me / I have: https://electronics.netlify.com/

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Moderators note : removed excessive pictures
 

Alec_t

Joined Sep 17, 2013
14,280
For future reference: try to keep wires as short as possible on a breadboard, to avoid interference pickup and unintentional oscillation.
 
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