5V 1W LED night light using two 2N2222A NPN BJTs

The 2N2222 can handle the current needs to switch a 5V 1W LED module that I have. The LED module produces over 250mA of current when it is connected directly to a power supply, and a 2N2222 NPN BJT can switch a maximum of 600mA of current.

5V 1W LED module night light circuit two 2N2222A NPN BJTs and LDRs learning electronics shorts 120
5V 1W LED module night light circuit two 2N2222A NPN BJTs and LDRs learning electronics shorts 120

Trying to switch a 2N2222 with light dependent resistor (LDR) voltage divider in order to make a night light circuit  present a challenge that must be overcome.

Don’t use a 2N3904 or similar NPN BJT in this circuit. Their max collector current (Ic) is only 200mA.  

A lot more base current is needed to saturate the 2N2222 NPN BJT than a LDR voltage divider can provide.

Therefore, I added another 2N2222 NPN BJT to the base of the switch 2N2222. The LDR voltage divider signal can turn the first 2N2222 NPN BJT on, which will allow a lot more current to flow into the seconds 2N2222 that switches the LED on and off.

The 330Ω collector resistor will limit current to a bit less than 15mA of current. Remember that the base to emitter of the second transistor will drop about 0.7V. Leaving about 4.3V across the resistor. Giving approx. 4.3V/330Ω =  0.013 (13mA) of current flowing through the second 2N2222 base to emitter, that is switching the LED on and off.

I had to use 2 LDRs in parallel on the low side of the voltage divider, to be able to pull down the voltage enough to turn the transistors off. I couldn’t use a 10k resistor on the high side, because it couldn’t provide enough current to saturate both 2N2222 NPN BJTs when it is dark.

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