I am helping to build a costume prop that has four LEDs lights on it (1 white and 3 amber).

This device is worn on the wrist, so space for power is limited- a 9V battery or something that will fit in that space will need to power the four lights.

The device would work on a 4-position rotary switch. Here is how it needs to function:

position 1: off
position 2: white light on and left yellow light on
position 3: white light on and center yellow light on
position 4: white light on and right yellow light on

My long-ago electrical experience tells me that this should be pretty simple, but I just don't recall enough to make this work correctly. I also wonder whether the LEDs needs any circuit help since one of our folks only managed to burn them out connecting directly to a 9V.


Thanks for the feedback WhatRoughBeast and Spehro Pefhany! I appreciate the LiPo suggestion and safety feedback.

I'll take a closer look at the circuit diagram- it looks helpful. I reckon I'll need to do calculations on the resistors now that I have LEDs in hand.

There are more specifics for this case if any of y'all are still following this...

The rotary switch is a 3-pole 4-position switch. The 3 poles (labeled A,B,C) are in center of the switch, with 12 posts (labeled 1-12) in a ring around them.

The LEDs are: white- Vf 4.1V max, If 25mA max, Vr 4V max, Pd 120mW max, Iv 4000mcd Typ amber- Vf 2.4V max, If 25mA max, Vr 5V max, Pd 100mW max, Iv 5500mcd Typ

Thanks again for the help!

  • \$\begingroup\$ Just as a suggestion, you might want to look into LiPo batteries. They have much more capacity than a 9V, and are relatively flat and thin, so you could tape them to the forearm under the costume. They are pricey, though, and need a special charger. \$\endgroup\$ Commented Jul 30, 2014 at 20:04
  • 2
    \$\begingroup\$ @WhatRoughBeast Any safety considerations there? \$\endgroup\$ Commented Jul 30, 2014 at 20:17
  • \$\begingroup\$ Safety? We don' need no steenking safety. Although, seriously speaking, I doubt it. The voltages are low enough that shock isn't a concern, although I guess that accidental shorts could cause burns. If you used the batteries to support the circuit, I wouldn't expect problems. Hmm. Come to think of it, any time you conceal circuitry under cloth there exists the possibility of setting it on fire. You paying attention, Technician? \$\endgroup\$ Commented Jul 30, 2014 at 20:44
  • \$\begingroup\$ @WhatRoughBeast "vent with flame" is not too common with LiPo - but common enough that it has a name :-(. \$\endgroup\$
    – Russell McMahon
    Commented Jul 31, 2014 at 9:20
  • \$\begingroup\$ As an aside, SparkFun now sells a current-limited LiPo cell intended for wearable use. \$\endgroup\$ Commented Dec 21, 2014 at 18:40

2 Answers 2


This should do it. SW1..SW3 are the contacts for positions 2,3,4.

R1 to R3 control the brightness of D1-D3 yellow LEDs, if you increase them also increase R4 proportionally.

R5 controls the brightness of the white LED.


simulate this circuit – Schematic created using CircuitLab

If you have a double pole rotary switch rather than a single pole, you can lose the transistor and R4 and just tie the second set of contacts to R5 to turn the white LED on whenever the switch is not in the 'off' position.


A simpler version, no transistors or extra resistors required. Assumes Standard value Forward Voltage Drops (Yellow 2.0Vf @ 20mA, White 3.3Vf @ 20mA).

(9Vs - 2Vf - 3.3Vf) / 0.02A = 185Ω

200Ω is next higher value 5% resistor, giving ~18.5mA draw through any chain.


simulate this circuit – Schematic created using CircuitLab


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.