LDR based switch - a switch to turn off at high resistance

[jinxx]

LDR based switch

I'm trying to make a transistor-based switch that will turn on when the resistance from the LDR is larger than a certain resistance and will turn off when the resistance is less. I've made switches using 2n3904's that turn on when the resistance is low, I just need that for the other way around.

Also, it has to be as efficient as possible as it will be used in a solar application.

Thank-you in advance for your help
-Jinxx

 

[pranam77]

LDR based switch

You may try the opposite type of transistor you are using now. Mean if you are usiing NPN type, youmay change it to PNP and vice versa. It may help suggesting you something more if you post your present schematic here. Good luck

 

[jinxx]

Re: LDR based switch

Right now it's basically set up as a darington pair with the output off of the second transistor's emitter and the base of the first transistor has the LDR connected to the collectors of both transistors.. I know that probably won't do anything except amplify the incoming signal (so it can detect the presence of light), however I don't know how I could set it up to do the opposite...

 

[pranam77]

Re: LDR based switch

Right now it's basically set up as a darington pair with the output off of the second transistor's emitter and the base of the first transistor has the LDR connected to the collectors of both transistors.. I know that probably won't do anything except amplify the incoming signal (so it can detect the presence of light), however I don't know how I could set it up to do the opposite...

As per your description, i guess the collector of the second transistor is connected to the Vcc and the output is taken from the emitter. To reverse the output, connect the second transistors emitter to Ground and take the output from the same transistors collector. Good luck.

 

[jinxx]

Re: LDR based switch

Now it works perfectly! The LDR is active on transistor 1 and the collector of transistor 1 is connected to vcc and the output from the emitter of transistor 1 goes to the base of transistor 2 and the collector of transistor 2 is the output and the emitter of transistor 2 is connected to vdd and the setup works as well as I had hoped.

Thank-you
-jinxx