The schematic diagram shown right here is the automatic switching-on emergency light circuit which is controlled using IC. The most important capabilities of this circuit are: automatic switching-on of the light on main power failure and battery charger with overcharge protection.

When mains electrical power is absent, relay RL2 is in deenergised state, feeding DC source from battery to inverter section via its N/C contacts and switch S1. The inverter section comprises IC2 (NE555) that is applied in stable mode to generate sharp pulses / wave with frequency of 50 Hz to drive the power MOSFETs. The output of IC3 is fed to gate of MOSFET (T4) directly while it is applied to MOSFET (T3) gate just after inversion by transistor T2. Therefore the power amplifier designed close to MOSFETs T3 and T4 functions in push-pull mode.

The output across secondary of transformer X2 can simply drive a 230-volt, 20-watt fluorescent tube. In event light isn’t needed to become on during mains power failure, then just flip switch S1 to off position.

Battery overcharge preventer circuit is designed close to IC1 (LM308). Its non-inverting pin is held at a reference voltage of about 6.9 volts that is obtained implementing diode D5 (1N4148) and 6.2-volt zener D6. The inverting pin of IC1 is connected to the positive terminal of battery. Thus when mains electric supply is present, IC1 comparator output is high, unless battery voltage exceeds 6.9 volts. So transistor T1 is normally forward biased, which energises relay RL1. Within this state the battery stays on charge via N/O contacts of relay RL1 and current limiting resistor R2. When battery voltage exceeds 6.9 volts (overcharged condition), IC1 output goes low and relay RL1 gets deenergised, and thus stops more charging of battery.

MOSFETs T3 and T4 may be mounted on appropriate heat sinks to prevent overheating on the MOSFETs and keep the MOSFETs in good performance.

This automatic switching-on emergency light circuit taken from EFY magazine. The circuit is already tested and should be working properly. This circuit idea available in PDF document, download from the following link:

Automatic Emergency Light Project

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