LED emergency lighting

The emergency lighting introduced here can be powered by a rechargeable battery (such as a 3.6V Ni-cd battery) or a common battery (3.0V CR2032). When the AC power fails, the white LED (LED1) can automatically light up to keep you away from darkness. The quiescent current of the circuit is very small. In fact, the battery will only come in handy when the LED is lit.

Figure 1 shows this emergency lighting circuit. When sunlight falls on the phototransistor T1 (L14F1), T1 is turned on and T2 is turned off, making the output of N1 high. Gates (circuits) N2, N3 and N4 are connected to the output of gate (circuit) N1, and their outputs are still high, resulting in the white light LED not lighting up. In addition, the +5VDC (direct current) jack additional circuit shown in the dotted box in Figure 1 is used to charge the rechargeable battery.

At night, when there is no sunlight falling on the phototransistor T1, T1 is turned off and transistor T2 is turned on, which causes the output of gate N1 to be low level. As a result, the outputs of gates N2, N3 and N4 are also low level, and the white light LED is lit.