# How to calculate resistor value in transistor light detector circuit? Thanks in advance.......

Discussion in 'The Projects Forum' started by Thinn, Feb 10, 2015.

1. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1
Hello fri,

Is there anybody explain me how to calculate resistor value in this circuit? I'm really new in circuit theory. When Vin input is 14V , Vout need to be zero voltage and when Vin input is around 5V, Vout need to be 12V.

Thanks and regards,
Thinn

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2. ### Thinn Thread Starter New Member

Feb 4, 2015
21
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Here is the .png version for my circuit.

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3. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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put a zener diode of 4.3V in series with R1, make R1 a 1K.

4. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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What is the coil resistance of the 12V relay?

ak

5. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1

How's about other resistor? Please can you explain me with an example equation. Thanks!

6. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1
coil resistance???? Do you means R4 or something else???? I'm really new in circuit. That's why I don't know what you mean.

7. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1
Please, anybody guide me to how to calculate resistor value in this circuit..........Thanks in advance.

8. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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The transistor will turn on when it gets 0.7V across the B/E junction, so if you want it to turn off below 5v, you have to get rid of 4.3v,easiest way is a zener diode,otherwise you're using resistor dividers, just divide the base current into 4.3v and it will give you the value of R1.

If you dont understand that i suggest you take up knitting instead!

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9. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1

Thanks for your help! I think I got why I need to use zener diode. But I still don't know how to calculate other resistor value.

10. ### ErnieM AAC Fanatic!

Apr 24, 2011
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By inspection, R3=0, R4= infinite.

No answer is possible for R1 and R2 without knowing the resistance of the relay coil. How much current is that dang thing supposed to draw?

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11. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1
Thanks ErnieM
Now, I got the realy coil resistance 400 ohm. Also current draw is nearly 2A. Please let me know the calculation.

12. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1

Realy coil resistance is 400 ohm.

13. ### ScottWang Moderator

Aug 23, 2012
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12V relay 400 Ohms.
I=V/R=12V/400 Ohms=30mA.
Where is the 2A came from?

I think you only using one bjt is enough to drive the relay, moving the relay to replace R2, and take the R3, R4 and second bjt away.

If the relay need 30mA, then we can offer it some more as 40mA, so if we want to make the bjt to get into the saturation region then the gain should be 10.
Ib=Ic/gain=40mA/10=4mA
R1=(Vin-Vbe)/Ib=(14V-0.7V)/4mA=3325 ohms, use 3.3K.

Last edited: Feb 13, 2015
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14. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1
Thank you very much for your help.
2A is came from load current (LED). I'm really new in ckt, that's why may I ask more question If I take out second bjt, Is the relay is on when Vin is around 5V?

15. ### ScottWang Moderator

Aug 23, 2012
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You just follow the last formula to calculate the R1 is ok.

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16. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1
Thanks! I'll try it again and back to you

17. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1
Hello Scott Wang, I was testing my ckt again using your formula, relay is making a sound continuously when the light is on. I don't know what wrong with relay.
Load draw 2A. system voltage is 12V. R1 is 3.3K ohm. I used 2n2222 transistor. Zener diode is 4.7V. Please guide me is there something wrong in my ckt?

18. ### ScottWang Moderator

Aug 23, 2012
5,478
864
Are you used one or two bjts?
Sounds like the current of relay is not enough, it needs reduce the resistor of base.
Could you attach the circuit, and label the voltage and the values of resistor?

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19. ### Thinn Thread Starter New Member

Feb 4, 2015
21
1
Here it is. I used 2 Bjts because I don't know how to hookup the relay in ckt. That's why I can't make new ckt yet. Also I put zener diode series with R1.

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20. ### ScottWang Moderator

Aug 23, 2012
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The below have two kinds of circuits, used one bjt and two bjts.
If you have different voltage to input then you need to using different resistor, 3.3K for 14V and 1.075K for 5V, you can try 1k and 1.2K.
The first you can measure the current of relay how much current is it drawn.

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