#2494
LectronFan
Moderator

Improved circuit 11 with NPN transistor :
Circuit 11 with NPN transistor
Improved circuit 11 with PNP transistor :
Circuit 11 with PNP transistor
Note that we also use a resistor of 47 Ohms to reduce the current flowing through the lamp.

Here are some measurements I made :
Ic = 46,6mA (Collector current = current flowing through the lamp)
Ib = 0,8mA (Base current = current flowing the 5,6K resistor)
Ildr = 1,76mA (current flowing through the LDR when it is bright light)
The voltage across the 47Ohm resistor is about 2,25V, so the lamp gets about 6,75V at max.

All these currents are now safe for our transistor and components.

Feel free to experiment :
Interchange the LDR with the 10K resistor : now the lamp only lights when the LDR is completely darkened.