The Dark Detector

/Basic concept

This blog will go through one of the useful applications of npn transistor & the voltage divider circuit. The npn transistor BC547 is configured to work as a switch.

So from the datasheet of BC547 transistor we can see that the maximum value of Vbe(on) is 0.7 V. That means if the base to emitter voltage is less than 0.7 V, no current will flow through collector to emitter of the transistor and it will remain off. When Vbe will be greater than 0.7 V, current will flow from collector-emitter, switching on the transistor. Now if a load is connected to the collector it will be switched on. Now in this circuit we implemented a voltage divider, the output of the voltage divider is connected to the base of the BC547. The voltage divider is the combination of a resistance and LDR.

/The voltage divider circuit
Voltage Divider Circuit
Voltage Divider Circuit


It is a very useful circuit which is implemented in different applications. The combination of R1 & R2 will affect Vout.Vout = Vin.(R2/R1+R2) so if we want to make 5V to 2.5V, R1 must be equal to R2. Here we take R1 = R2 = 1KΩ, and it gives perfect 2.5V.





/The Dark Detector Circuit
Dark Detector Circuit

Here the output of the voltage divider which is generally a combination of R1 & LDR is connected to the Base of the BC547 & a white LED is connected to the  Collector. The 220Ω resistance limits the current and prevents the LED from getting damaged.

When light falls on the LDR the resistance offered by LDR is very low. Let, R1 = 220 KΩ & R(LDR) = 4.6 KΩ. If Vin is 9V then voltage on the Base of BC547 = 0.184 V which is less than 0.7V so the BC547 is now off and the LED will not glow. Now when it is dark the resistance offered by LDR is very high in order of MegaΩ. Suppose practically R(LDR) = 150KΩ so Base voltage of the BC547 is 3.648V which is greater than 0.7V. Now current will flow from C-E of the BC547 & the LED will glow.

There are different sizes of LDR available in the market. Sometimes small LDRs don’t work properly in the circuit, so to increase the sensitivity connect two small LDRs in parallel or change the value of R1 which can be calculated from the Voltage Division formulæ.


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