# Transistors and Resistance

Discussion in 'General Electronics Chat' started by RhysGM, Nov 25, 2012.

1. ### RhysGM Thread Starter New Member

Nov 4, 2012
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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/Electronic-Components/Light-Dependent-Photoresistor-58-0134

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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You need to add this components

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3. ### MrChips Moderator

Oct 2, 2009
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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.

4. ### RhysGM Thread Starter New Member

Nov 4, 2012
10
0
I put in a 1m ohm and the circuit remained on

5. ### RhysGM Thread Starter New Member

Nov 4, 2012
10
0
Does R2 and R1 need to be the same?

I'm actually trying to replicate this project.

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.

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
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R1 don't need to be 10K it can be from 470R to 10K.

7. ### Audioguru New Member

Dec 20, 2007
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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).

8. ### RhysGM Thread Starter New Member

Nov 4, 2012
10
0
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?

9. ### Audioguru New Member

Dec 20, 2007
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896
Simply use ohm's Law:

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