Page 25 - Electronics Starter Kit_EN

4.5 Circuit Explanation

In this circuit we made a voltage divider using a 100 KΩ resistor and a Photoresistor (LDR).

We can use voltage divider law to calculate the output voltage in the daylight and in the dark.

= · ( )
+ 100 Ω

In the dark: = 5 · ( 600 Ω ) = 5 · 0. 86
600 Ω + 100 Ω

= 4. 3

In the daylight: = 5 · ( 6 Ω ) = 5 · 0. 057
6 Ω + 100 Ω
= 285

In the daylight, voltage on the base of the transistor will be less than 0.7V so the transistor will
be in the Cut-Off mode, and the LED will be off. But in the dark, voltage on the base will be
higher than 0.7V so the transistor will be in the Saturation mode, and the LED will light up.

Note:
To calculate R fixed which is 100KΩ, we need to achieve 3 conditions:
● 0. 7 ≤ in the dark

● 0. 7 > in the light

● The current of V out should be more than 20 A in the dark because it is the base current. If
µ
we consider the gain of the transistor “150”
○ = · β = 20 · 150

○ = 3

So we need at least 20 on the base, to make the LED light up a little bit in the dark.
We are going to use the voltage divider law to achieve the first 2 conditions.

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