Production of a Small Portable LED Emergency Desk Lamp

The author made an LED emergency desk lamp using white light-emitting diodes. The lamp has the advantages of small size, simple circuit, and high luminous intensity. Its brightness is close to that of an 11W fluorescent desk lamp. When powered by 4 fully charged 1.8Ah nickel-hydrogen batteries, it can work continuously for 3 hours.

1. Working Principle

The circuit is shown in the figure below. When the switch S is turned to the left, the 4.8V DC voltage outputs a stable DC voltage of l2V (measured 11.8V) after passing through the DC/DC boost module NR5Sl2/1OOA, and then powers LED1-LEDl5 after current limiting by R1-R5. Each LED passes a current of 20mA.

LEDl6 is used to indicate the discharge status of the battery pack. The discharge termination voltage of the NiMH battery is IV. If the NiMH battery voltage is lower than the discharge termination voltage and continues to discharge, the products on the internal plates of the battery will not be easy to recover during normal charging, thus affecting the battery life. When the voltage across the battery pack is not less than 4.1V, the output voltage of the DC/DC boost module is not less than 10.9V, the voltage regulator diodes VD1 and VD2 are broken down, and LEDl6 is lit, indicating that the battery pack does not need to be charged; conversely, when the voltage across the battery pack is lower than 4.1V, LEDl6 goes out and the battery pack needs to be charged.

When S is turned to the right, the lighting circuit stops working. If a 9V DC power supply is inserted into JP at this time, the battery pack will be charged. The charging rate is 1/12C, which is a slow charging. Therefore, there is no need to worry about the problem of "overcharging". Each charging takes 12 hours. LEDl7 is the charging indicator light.

2. Component selection

LED1-LED15 use square, 45 white light emitting diodes. Compared with round ones, square ones have advantages such as less light decay, longer life and easier installation. VD1 and VD2 use 0.25W voltage stabilizing diodes. LED16 and LED17 use ordinary red light emitting diodes.

The DC/DC boost module NR5Sl2/1OOA has an input voltage of 5V, an output voltage of 12V, and a maximum output current of 100mA. Other models can also be selected, but the output voltage must be stable and the output current must not be less than 100mA. S uses a single-pole double-throw switch. The battery pack is composed of 4 1.8Ah nickel-metal hydride batteries connected in series. Nickel-metal hydride and nickel-cadmium batteries of different capacities can also be used, but it will affect the duration of lighting. JP uses a small round hole socket. The charging transformer can use a commercially available 9V/5W DC power supply or a repeater power supply.

3. Production and use

The whole circuit can be installed on a universal board, and 15 white LEDs are scattered on the universal board to expand the illumination area. The output current of the battery pack is several hundred milliamperes, so the battery pack and the circuit board should be connected with thick wires.

When using, pull S to the left end, and all white LEDs will emit dazzling white light. During use, if you feel that the light is getting dim, you should observe whether LEDl6 is off. If LEDl6 is off, you should immediately pull S to the right end, stop using it, and plug in the DC power plug for charging.