I'm very new to learning electronics and have a project in mind. I have a battery powered light switch with 2x12 led strips that I want to adapt for light socket use. It runs off of 3 AAA batteries. The resistor pictured is Red, Black, Silver, Gold, .2 Ohms if my math is correct.enter image description here My problem is, I am not sure how to find the forward voltage for the leds without a voltmeter. I am thinking of using a DC power supply from another led light bulb to drop the voltage as well as rectify it.

I know this is probably not the best project to take on for a first but eh it's worth a shot, any help is greatly appreciated!

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    \$\begingroup\$ If you are "very new," do not do anything with mains without supervision. There is a very real risk of injury or death. \$\endgroup\$
    – LMS
    Mar 27, 2018 at 21:13
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    \$\begingroup\$ How did you manage to cast a shadow on the resistor? We can't make out the colour code. Trying to power a device intended for 3 V on a 120 V supply is asking for trouble due to inadequate stregth and insulation of the case. Get a mains to DC wall wart instead. \$\endgroup\$
    – Transistor
    Mar 27, 2018 at 21:15
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    \$\begingroup\$ This is one of those cheap devices that runs the LEDs in parallel from a common dropping resistor. Just keep the resistor and feed it 4.5VDC from a wall-wart power adapter. \$\endgroup\$
    – Dave Tweed
    Mar 27, 2018 at 21:16
  • \$\begingroup\$ just use an old phone charger's two wires to replace the battery; the voltages will be close enough not to need measurement. \$\endgroup\$
    – dandavis
    Mar 27, 2018 at 21:51
  • \$\begingroup\$ Re, "...without a voltmeter" We live in an amazing age. You can buy a cheap multimeter on Amazon for less than $10. When I was 11 years old, I had to assemble mine from a kit because I could not afford to buy one that worked right out of the box. \$\endgroup\$ Mar 28, 2018 at 0:35

1 Answer 1


The absolutely universal way you power a low-voltage DC project from mains voltage is to use common-as-dirt "wall wart" type power adapter.

Anything that touches mains power must meet strict design restrictions and slowly grind its way through the process of being tested at a UL testing lab and listed (or competitor: CSA, TUV etc. CE is not a testing lab.) But these rules only apply to the wall-wart itself. The fun product downstream gets to be entirely low-voltage DC, which means the low-voltage rules apply to it instead of the mains-voltage rules. The result is vastly improved safety, simpler design requirements and quicker testing.

And that is why you should use wall warts too.

Similarly, if you are doing household wiring inside a wall, mains wiring must comply with the tough Class I rules, whereas low voltage wiring can use simpler rules. "Put the wall-wart in the basement and run intercom wire to the load" is a strategy that's been in use since "Princess phones" had lighted dials.


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