Hi friends. i have recently made an LDR circuit that turns a LED on when dark. I used a joule thief approach with a transistor as to try an use 'dead'' AA batteries. everyone has those..but to be able to set the threshold to what intensity of light it turns on i added a potentiometer... now my first circuit looked like this:

As you can see the sensor was afterwards.
this worked very well. problem was the transistor was always on and batteries didnt even last a night as power flows even if light is off...

Then i decided to add the 'sensor' at the base of transistor(2n2222)
like so:

This also worked, but i struggled to set the threshold... then i made a small adjustment to the connection with the sensor like so:

This works as desired but the settings are waaaaay to sensitive on or off at spicific light, can't set the desired action anymore. am i missing capacitors or zeners or what.
My question is:
Is there a way to make it more proficient witout adding any other components like 555 which will just complicate things. If i have to add another or a few extra transistors then i can do that.But how can i make it better and better at adjusting light threshold? must i swop the ldr and potentiometer or what?

 

LDR's and Joule thief circuits are very simple- and for this reason very parameter sensitive.

If you want a more stable and specific trigger point - you will need to up the complexity of the circuit.

 

I would suggest connecting the LDR directly between the base and ground. You could also vary the 1K resistor to change the sensitivity. Perhaps 470Ω in series with a 1K pot.

I don’t know that this would work, but I think it is worth a try. A pot in series with the LDR might be another way yo control the sensitivity.

 

The other thing is, LDRs come in different values. You need to get one with the right resistance for the application.

Before adding an LDR to the circuit, you need to study the behaviour of the circuit using an oscilloscope. A Joule Thief circuit is an oscillator. Hence you need to observe the pulse frequency and width as you modify the circuit.

Only then should you add a variable resistor to the circuit to see its effect on the circuit.

 

I am guessing that the LDR in use is one whose resistance drops with exposure to light. And I would GUESS that the goal is to have the LED come on as the light level drops.
The really bad news is that the way that is working is shunting the drive to the base to switch off. And that consumes power. I don't see away around that with an LDR that drops resistance as the light gets stronger.

 

In order to simulate the effect of the LDR, use a resistor at the minimum value of the LDR in series with a variable resistor at the maximum value, for example 1 MΩ or higher (i.e. the LDR dark resistance).