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How do I know my LDR value?
LDR (light dependent resistance) can be very easily tested by using a digital multimeter. We all know that the resistance of an LDR varies according to the light falling on it. At bright light, the LDR resistance will be around 500Ohms and at darkness the resistance will be around 200K.
What happens to a LED when a LDR is exposed to light?
When the light level is low the resistance of the LDR is high. This prevents current from flowing to the base of the transistors. Consequently the LED does not light. However, when light shines onto the LDR its resistance falls and current flows into the base of the first transistor and then the second transistor.
What is the output of the LDR?
The LDR gives out an analog voltage when connected to VCC (5V), which varies in magnitude in direct proportion to the input light intensity on it. That is, the greater the intensity of light, the greater the corresponding voltage from the LDR will be.
How do you set up an LDR?
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Where does the current go in an LDR circuit?
Therefore, no current will go from the collector to the emitter of the transistor. All the current will instead pass through the LDR and the potentiometer. When it’s light, the LDR has low resistance.
Why is the LDR not turned on in a transistor?
When it’s dark, the LDR has high resistance. This makes the voltage at the base of the transistor too low to turn the transistor ON. Therefore, no current will go from the collector to the emitter of the transistor. All the current will instead pass through the LDR and the potentiometer.
How to calculate emitter current times emitter resistor?
Rounding that is emitter current times emitter resistor: IERE = (1mA) (470) = 0.47V. In addition, we need to overcome the VBE = 0.7V. Thus, we need a VBB > (0.47 + 0.7)V or >1.17V. If emitter current deviates, this number will change compared with the fixed base supply VBB,causing a correction to base current IB and emitter current IE.
When do you use NPN transistor in LDR circuit?
Well, if you want to use NPN transistor as in the 1st shematics but you want the LED to turn ON when it is dark (as in the 2nd schematics), you could just exchange the LDR and R2. When it is dark, the LDR’s resistance would be large compared to the R2 so the NPN transistor would be ON.