# Calculate RL where's thersaurus an LDR in the circuit

Discussion in 'The Projects Forum' started by Lewismm99, Jan 21, 2016.

Jan 21, 2016
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2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
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hi 99,
You have not been given sufficient information about the components in that circuit to enable you to calculate the exact values.
The LDR as you may know is a Light Dependant Resistor, the LED operating current is not specified.

Is this Homework.?
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3. ### Lewismm99 Thread Starter New Member

Jan 21, 2016
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Nope it's an assignment for my college work!

4. ### ericgibbs AAC Fanatic!

Jan 29, 2010
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389
hi,
OK, Lets assume that the LED is a general purpose type, say 25mA maximum current.
Its usual to under run this maximum current in order to extend the life of the LED, so assume 20mA, also assume a 2V drop across the LED.
Also assume that the voltage drop across the transitor [ collector to emitter ] is 0V.

Can you calculate the value of Rload that allows a current of 20mA to flow.?
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5. ### Lewismm99 Thread Starter New Member

Jan 21, 2016
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I should be a to, I'm in college now I'll check with my tutor sometime soon and I'll let you know, thanks for the help!

6. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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Try a 470 ohms resistor for starters..

7. ### Lewismm99 Thread Starter New Member

Jan 21, 2016
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Do we know it's a 470? Or am just trying 470?

8. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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470 ohms will give approx 21mA led current, without knowing what the spec of the transistor and led, your guessing.