NOTES:
1. R2 limits current spikes to Q1.
2. R3 limits current flowing through Q1c-e.
3. Bulb resistance measured when cold.
4. Total Darkness* current measurements:
5. Q2c-e / bulb I=172.3mA
6. Q1c-e / Q2b I=9.8mA
7. Q1b I=55.3uA
8. Total circuit leakage current in daylight and twilight: I=62.1uA.
9. Try substituting other NPN transistors for Q1 & Q2; just about any should work fine.

 


THEORY OF OPERATION:
In daylight, value of CDS cell is 200 to 2000 ohms, depending on amount of light striking it. Current flows from the input, through R1, through the CDS cell to ground. In darkness, the resistance of the CDS cell is very high, on the order of 2 meg-ohms in total darkness. Therefore current flows through R1 to the base of Q1 turning it on. With Q1 "on," current flows through R2 through Q1c-e into the base of Q2 turning it on.

Q2 is a power transistor more than capable enough to handle the 170mA flowing through its c-e junction once it is turned on. Current flows from the input through Q2c-e through the light bulb to ground, turning on the light bulb.

 


NOTES ON FIELD OPERATION:
I built this circuit to act as a night-light for my cottage. It runs from a 3.4 amp hour NiMH battery built from surplus cell phone batteries. The battery is charged during the day by a small solar array supplying a maximum 200mA charge. Rough daytime charge average is probably around 1 amp hour.

At night, the total circuit drain is about 182mA, which should keep the light going for about 5.5 hours after sunset.

With a larger solar panel array, or supplemental charging from a generator during the day, the nightlight should stay on until dawn even with a 75% charge (winter) or 60% charge (summer).

 



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