Resistor Help

It is hard to see what you have in the bag but whatever it is, it is made to run on 3 volts DC. This is not as simple as a resistor. You need a diode or rectifier to convert the AC from your transformer to DC, plus a capacitor probably to smooth the ripple. Two suggestions: take pictures of what is in the bag and describe what you want to do with them. We have a $2.24 investment we don't want to mess up.

Actually, it was on sale for $1.12.

Here are a few extra pictures.  Yes, I picked up the lights from Walmart.  Still, if they can work, I would like to use them.  The original string was made up of 20 lights in a series.  I cut / separated one light as shown.  There are two wires from & return to the power source and two wires to continue the series.

I would like to separate the lights into single units and use throughout my layout.  I know I need a resistor but I don't know what size resistor.  The total string of lights operates on two AA batteries.  I can set the voltage to 10-12 volts. 

Thanks

@hokie71 posted:

... We have a $2.24 investment we don't want to mess up.

@Attorneyvan posted:

Actually, it was on sale for $1.12.

It's a quest.

I enjoy threads like this - IMO, it's really fun to repurpose an inexpensive non-train item into something that's useful on the layout.

This thread might be a good start. Most also recommend adding a diode to protect the led from the peak inverse AC wave.

https://ogrforum.com/...-and-excel-worksheet

I forgot to mention that in figuring the DC voltage equivalent to use for the spreadsheet in the thread above, based on your AC transformer voltage, there are a few calculations:

1) For AC and transformers, we refer to RMS voltage. For example, 120-volt RMS AC really has a 170-volt peak sine wave roughly.  RMS voltage is (V peak / 0.707) or the square root of 2) so, for a 12-volt setting on your transformer, the peak of the sine wave is actually 17 volts.

2) a common formula to figure the DC equivalent of a half wave rectified voltage is (V peak / pi) so the DC equivalent of a 12-volt rms, or 17-volt peak AC half rectified wave would be 17/ 3.14 or roughly 5.4 VDC.

Here is a good summary of half wave rectifiers (one diode) like we are discussing:

https://www.electrical4u.com/half-wave-rectifiers/

Do you have a voltmeter to check this out on your bench before you solder things together?

Discussion follow up:  Thanks all for your help.  I purchased 1.5 resistors.  With them, I may use one of the led lights or a string.  Note the lighted loading dock.

Looks super- nice building and effect!  You win the best $1.12 LED investment award. What power supply did you decide to use?