I saw these alternating Christmas lights:

Christmas lights with all lights on

Christmas lights with odd-numbered lights onChristmas lights with even-numbered lights on

The top image has all lights on, the bottom left image has odd-numbered lights on, and the bottom right image has even-numbered lights on.

The LEDS are joined by two enameled wires, and three AA batteries power the lot.

How does the controller turn the odd-numbered lights on and off?


1 Answer 1


These kinds of lights have LEDs connected in an alternating polarity fashion. Take look at the following circuit.


simulate this circuit – Schematic created using CircuitLab

Lights with this kind of configuration are cheaper to make and adequately serve their purpose.

The controller is able to power the LEDs in the following two ways to create the effect:

  1. P1 (HIGH), P2 (LOW)... all even LEDs are on.
  2. P1 (LOW), P2 (HIGH)... all odd LEDs are on.
  3. AC ... all LEDs on (actually just alternating too fast to see)
  • 2
    \$\begingroup\$ Actually, it's probably AC all the time, and the controller just plays with the duty cycle in order to create a cross-fade effect. \$\endgroup\$
    – Dave Tweed
    Commented Feb 7, 2019 at 17:07
  • \$\begingroup\$ Cheaper to make than what? \$\endgroup\$ Commented Feb 9, 2019 at 0:59
  • \$\begingroup\$ @Misunderstood, cheaper than using something like addressable leds for the same effect. \$\endgroup\$ Commented Feb 9, 2019 at 4:17
  • \$\begingroup\$ I got hold of the lights and the power supply plugs directly to the power outlet. The sign says "U_rated=4.5V~ U_out=4.5V~" and "Combination in waves: sequential, slo-glo, chasing/flash, slow fade, twinkle/flash, steady on." I hooked a multimeter on AC voltage and I measure between 0 and 2 volts most of the time, except some 0 voltage at some points when the lights are on, so I assume those have DC. \$\endgroup\$
    – emonigma
    Commented Feb 14, 2019 at 23:15
  • \$\begingroup\$ @DaveTweed How can I test your explanation with a multimeter? \$\endgroup\$
    – emonigma
    Commented Feb 19, 2019 at 22:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.