Standard fluorescent lamps and their smaller versions called compact fluorescent lamps (CFLs) radiate light in all directions (360°) and tend to increase the room temperature. In emergency lights using these lamps, the battery lasts only a few hours due to the power loss during conversion of DC into AC. These limitations can be overcome by using ultra-bright white LEDs. Here is a multipurpose lamp using white LEDs that can also be modified to act as an emergency-cum-bedroom light. Its main features are long and continuous operation, very low power consumption, selectable light angle, very long life and negligible heat radiation.
Multipurpose lamp circuit
Fig. 1 shows the circuit of multipurpose white LED light circuit. The circuit is very simple and uses a battery charger unit built around IC LM317 (IC1) and a combination of white LEDs. Resistor R3 (4.7-ohm, 2W) limits the current through the battery. The radiation angles selected for white LEDs are 60° and 20°. Three columns of LED clusters (A, B and C) are made on separate transparent acrylic sheets, with each sheet having a total of twelve LEDs affixed to it.
The left (A) and right (C) columns use 20° LEDs, while the middle column (B) uses 60° LEDs. All the twelve LEDs of each column are connected in series to separate 15-ohm current-equalisation resistors (R8 through R19) as shown in Fig. 2, and to current-limiter resistors R7 (10-ohm, 1W) and R6 (5-ohm, 1W) as shown in Fig. 1. The entire unit is powered by a 6V, 4Ah maintenance-free rechargeable battery.
The continuous lighting life is around 7 hours in torchlight mode and around 14 hours in table lamp mode, depending on the battery capacity and quality. For the torch mode, only the left and right LED columns are used. These LEDs beam light up to 6 metres. In table lamp (spread light) mode, only the middle column of LEDs is used.
You can select between the table lamp and torch modes by using rotary switch S1, which is a single-pole, 3-way switch. When the pole of switch S1 is set at position 1, the C column of 60° LEDs lights up and the system acts as a table lamp. When the pole of switch S1 is set at position 3, columns A and C light up and the system acts as a torch. Both the table lamp and the torch modes remain off, when the pole of switch S1 is at position 2.
When mains is switched on, LED2 glows. To charge the battery, flip switch S2 to ‘on’ position. To check the status of the battery, flip switch S3 to ‘on’ position. This will give an indication of battery charge. If low-battery indicator LED1 turns off, the battery needs to be charged.
Fig. 3 shows the circuit of emergency lamp with brightness control, which is derived from Fig. 1 with slight modification in the combination of LEDs. Built around four multichip (MC) LEDs, it is very compact and simple, and can work in two modes, namely, bedroom lamp and emergency lamp.
In bedroom lamp mode, only one blue LED glows. This LED is mounted at the top in upside down position to avoid direct viewing of the blue light. The arrangement gives a pleasant, well-spread light.
Emergency lamp mode
In emergency lamp mode, 8mm, 80° bright-white multichip LEDs give 80° spread light, which is sufficient for indoor uses. Circular PCBs for multichip LEDs have four internal junctions each. Solder LED17 through LED20 in the first PCB, LED21 through LED24 in the second PCB, LED25 through LED28 in the third PCB and LED29 through LED32 in the fourth PCB, with a spacing of 3 to 4 cm between two adjacent LEDs. Finally, house all the four circular PCBs in a compact cabinet along with the reflector such that light can spread out in the room.
Each multichip LED gives a power of 32 candles. Therefore use of four 8mm multichip LEDs will give a total power of 128 candles.
In emergency lamp mode (selected through rotary switch S5), all the four multichip LEDs (including LED17 through LED32) glow. The DC power source is a 6V, 4Ah chargeable battery, with charging circuit built around popular IC LM317 (IC2). Resistor R21 (2.2-ohm, 1W) acts as the current limiter for the battery.
You can control the candle power (brightness) of LEDs as per your requirements. Transistor SL100 (T1) and its associated components form the candle controller (brightness controller). The base biasing voltage of the transistor is stabilised by resistor R24 and diodes N3 and N4 (1N4001). This constant voltage is given to the base of the transistor through a potentiometer VR1 (4.7k lin.). By adjusting the potentiometer, you can control the intensity of the multichip LEDs. No heat-sink is required for the transistor.
The article was first published in November 2004 and has recently been updated.