In the event that the electricity from the mains supply is interrupted, the emergency light will immediately switch over to a light source that is powered by a rechargeable battery. The lamp will turn off once the power from the mains supply is restored. Portable emergency lights almost always make use of batteries that are rated for 6 volts and are maintenance-free and sealed. Because many inexpensive lights do not provide any notice when the battery is running low, there is a possibility that the battery will not work. The following diagram shows a battery guard circuit that can be used with emergency lights that do not have deep-discharge prevention circuitry. In the event that the voltage of the battery drops below the predetermined threshold, it will turn off the bulb (5V).

Block diagram of an emergency light The schematic representation of an emergency light may be seen up above. It is primarily composed of a power supply, a battery pack that can be recharged, an autonomous sensing circuit that can detect the presence (or absence) of a mains supply, and a charging circuit. In the event that there is a disruption in the power supply from the mains, the sensing circuit will connect the lamp (or an inverter for a compact fluorescent tube) to the battery. It will then break this connection once the power from the mains is restored. The battery is charged via the charger circuit whenever there is access to the AC mains supply. Battery guard circuit Battery guard circuit

The battery protection circuit is depicted in the following image. Two transistors, one Zener diode, and a few passive components are utilized in its construction. LED1 displays the current state of the battery voltage. When LED1 begins to glow, this signals that the battery voltage is lower than the safe voltage and that the connection between the bulb and the battery has been severed. Circuit operation When the battery has been charged to its maximum capacity, the voltage from the battery is transmitted to the base of transistor T2 through resistors R3, ZD1, and R4. Once transistor T2 begins conducting, the previous transistor, T1, is forced to turn off. The relay is left in its de-energized state, and the lamp is wired to the battery jumper J1 that is used to connect points A and B. Fig.3 wiring of battery guard circuit to emergency light circuit When the voltage of the battery drops below the secure threshold, the transistor T2 is turned off. As a consequence of this, T1 becomes conducting, and the relay that was used to disconnect the load from the battery becomes energized. Set the conduction voltage for transistor T2 using preset VR1 so that it conducts a voltage of 5V or higher. When the voltage is less than 5V, the transistor T2 will stop conducting, which will cause the transistor T1 to start conducting. As a consequence of this, the relay becomes energized, which disconnects the bulb from the battery. As a consequence of this, the battery no longer undergoes deep-discharging.

Construction and quality assurance Construct the battery protection circuit using any standard PCB and enclose it in a container after you are finished. With the use of screws and spacers, secure the device to the interior of the cabinet that houses the emergency lighting. As shown in the wiring diagram, connect the usually closed contacts and pole of the relay shown in the circuit in series with the on/off switch of the emergency light. This will allow the light to be turned on and off.