Emergency Light circuit

The idea for this circuit came when I was repairing my aunt’s emergency light. Bulb based emergency lighting has a low efficiency and ultrabright l.e.d.s provide greater efficiency and more illumination, with a much extended battery life. It was therefore decided to change from a 12V light bulb to a pack of l.e.d.s. To get constant light, a constant voltage technique was adopted, using the circuit in Fig.2. For more power saving, step-down switching is used in preference to step-up. This function is provided by IC1 in conjunction with diode D3, inductor L1 and capacitors C3 and C4.

Emergency Light circuit

The resulting output is a stable 5V and powers the 12 ultrabright yellow l.e.d.s D4 to D15. The l.e.d.s are grouped in pairs, each pair buffered by a resistor, R4 to R9. Diode D3 is a high-efficiency type – do not change it to a 1N4000 series type as they are less efficient in this role.
The network consisting of R1 and D1 is a power-on indicator. The network around VR1, TR1, R2, R3 and D2 is a low-battery indicator. Preset VR1 should be adjusted until l.e.d. D2 turns on when the battery voltage is low. Inductor L1 should be a toroid type in order to reduce radiated r.f. interference caused by the switching regulator IC1. It should be glued to the circuit board to prevent it from vibrating. A small heatsink should be fitted to IC1. The circuit efficiency was found to be around 80%, i.e. about 10% better than with the step-up switching technique. The circuit operates for about four times longer than the bulb-based one in respect of battery life.