Hello, I have a transistor (BC547B) that is being used as a switch, connected to the base I have an 9v battery and an LDR with a maximum resistance of 1m ohms. However this is still not enough not to switch off the transistor. Can someone tell me how I can work out what resistors I need, so the transistor is on in the light and off in the dark?

Links for the components I'm using;

http://www.rapidonline.com/Electron...468/?sid=b45ead51-5d50-4c3d-9f34-1e06a4f70d0c

http://www.rapidonline.com/Electronic-Components/Light-Dependent-Photoresistor-58-0134


Thanks for any help you can give.

 

You need to add this components

 

Put a resistor from the base to ground. Experiment with the value of the resistance starting with a high value such as 470kΩ and working your way down.

Edit: Jony130 beat me to it. That circuit should help.

 

Put a resistor from the base to ground. Experiment with the value of the resistance starting with a high value such as 470kΩ and working your way down.

Edit: Jony130 beat me to it. That circuit should help.

I put in a 1m ohm and the circuit remained on

 

You need to add this components

Does R2 and R1 need to be the same?

I'm actually trying to replicate this project.

www.madlab.org/schematics/StopThief.pdf

I think I bought the wrong LDR there were 2 one with a max of 20mΩ and another with 1mΩ.


I'm not sure what the 2 10k resistors are doing in your diagram and I think they are similar to the LDR and the 47k and preset in my project, however I'm not sure how to balance the equation.

 

R1 don't need to be 10K it can be from 470R to 10K.

 

You said 1m ohm and 2m ohm which are milli-ohms (0.001 ohms and 0.002 ohms).
You should have said 1M and 2M which are megohms (1 million ohms and 2 million ohms).

 

You said 1m ohm and 2m ohm which are milli-ohms (0.001 ohms and 0.002 ohms).
You should have said 1M and 2M which are megohms (1 million ohms and 2 million ohms).

Thank you for the correction. Do you know of a formula that I can use to work out what how much resistance I need to use?

 

Do you know of a formula that I can use to work out what how much resistance I need to use?

Simply use ohm's Law: