Diy Christmas lights

Introduction

Make a simple battery powered Christmas light with seven (7) leds.

Theory

Use leds in series, as parallel leds might light on uneven brightness.

Usually the longer led is the + lead.

LEDs and wiring

• Red and yellow led: 2 V
• Green and blue led: 2.9 V
• White: 3.3 V

Seven blue leds makes $7\times 2.9V=20.3V$, and white blue leds drops the voltage by $7\times 3.3V=23.1V$. If we use simple AA batteries, we will need $14\times 1.5=21V$ or $16\times 1.5=24V$. Quite many. . .

However, if we wire them alternately, it makes $3\times 2.9V=8.7V$ and $4\times 2.9V=11.6V$ for blue leds and $3\times 3.3V=9,9V$ and $4\times 3.3V=13,2V$ the white leds. Thus, we need only six (6) or eight (8) batteries; or 7 and 9 in the case of white leds.

Photodiode and transistors

We can use a photodiode and transistor to turn a led on. The example given by EviMadScientist uses

• a battery,
• an LTR-4206E phototransistor (in a black package: blocks visible light; sensitive to IR)
• a 2N3904 transistor,
• and a 1 k resistor.

A simple photodiode circuit by EvilMadScientist

References

• https://www.evilmadscientist.com/2007/a-simple-and-cheap-dark-detecting-led-circuit/
• CdS (“LDR”) phototransistor circuits

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