PAGE 137 OF 150 · PROJECTS
Build a Torch with LEDs
Several LEDs in a small case give a handy light.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Design
Use 3 AA cells for 4.5 V, a slide switch and two white LEDs, each with its own resistor.
Numbers
- White LED about 3.2 V at 20 mA.
- R = (4.5 − 3.2) ÷ 0.02 = 65 ohms, so use 68 ohms for about 19 mA.
- Two LEDs draw about 38 mA, so 2,000 mAh gives about 50 hours, or around 35 hours after a safety margin.
Build tips
Mount the LEDs in a reflector or cut-down plastic cup to focus light, fit the switch in the positive lead and keep the batteries in a holder for easy replacement.
REMEMBER
Each parallel LED needs its own resistor.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
Explore LED clocks, stopwatches and electronics at cosycom.com.