I'm working on a traffic light costume for my daughter to wear on Halloween. I'm hoping to go the extra mile and have the lights actually follow a typical traffic light sequence.
I'm eyeing 4 inch diameter "vase base plate lights" for the costume. These lights have 15 white LEDs and are normally powered via USB or with 3 AA batteries. I would post an Amazon link but am not sure if that's allowed.
Google tells me a fresh AA battery is 1.65 V, so three in series would be approximately 5V. I'm wondering if the 5V output on the Arduino Uno is suitable to power these lights, and if any resistors would be needed in the circuit.
I included an image of how I imagine the circuit will go. What is the recommended approach for merging the three wires that will go to the ground pin?
An Arduino output pin can safely source or sink 20mA. How much current do the LEDs require? I'd bet significantly more than 20mA. I think that you will need an external powerr source and transistor drivers for the LEDs, but without knowing ths LED current requirement I can't be sure.
There is nothing wrong with posting links to the ad from which you bought something. A data sheet that shows the product specifications is preferred, though.
When you visit Amazon type in the words "charger bar flashlight" which should give you some items like this Amazon.com which produce 5V. I use a charger bar flashlight as a backup to power a GPS unit or to recharge my regular headlamp. The GPS unit is a power drain, though I do manage to get over 2 hours of continuous operation of the GPS on the flashlight charger bar.
Any ways there are breakouts that will let you tap into the 5V output of the charger bar. That way you can power the lights and the micro controller off of one source, it is portable, and has the safety feature of being a flashlight if needed.
Many USB battery pack rechargers depend on the device being charged to draw x amount of current, I find the flashlight charger bars tend to not have that "issue."
If you use the word tactical such as "tactical charger bar flashlight" you'll find some beefy models. Do beware of the ones that offer wild mAh values. You want heavy batteries for the mAh, the heavier the battery, the more lithium, the more power.
If you want you can build your own portable power supply that will supply the electronics and the leds with power. I use 2 18650's, 2300mAh batteries in series with a switching regulator set at 7.5 volts (Amazon.com), and a 7805 (for 5V) to power a ESP32, a TFMini, and 9 of the ALITOVE 100pcs WS2812B Addressable 5050 smart RGB LED Pixel light on Black Heat Sink PCB Board for Arduino 5V DC Amazon.com in a ER CHEN 44mm Lens + 50mm Reflector Collimator Base Housing + Fixed Bracket for 100W LED Light Lamp Amazon.com. I got over 9 hours of continuous operation with the setup.
I would post an Amazon link but am not sure if that's allowed.
Yes that's allowed and it might be helpful. But, the specifications from 3rd-party Amazon suppliers are often "limited".
So no... They can't be directly driven from the Arduino, and it might be "tricky". You'll need something like a MOSFET driver circuit to supply the current, and maybe a higher voltage power supply, and maybe a current limiting resistor.
LEDs are "current driven". Typically, you supply/control/limit (at sufficient voltage capability) the current and the voltage "falls into place". That's the opposite of how "most things" work. i.e. With a regular light bulb you supply the voltage (with sufficient current capability) and the current "falls into place".
Often, little battery operated flashlights (and maybe this thing) rely on the internal resistance of the batteries to limit the current. If you use a power supply you can burn-up the LEDs.
If you are just driving one or two little LEDs from the Arduino you simply add a current limiting resistor. With multiple LEDs (or high power LEDs) you need a MOSFET or transistor to supply enough current, then something to limit the current.
When I find a electronics part on Amazon that I'll want/need a data sheet for, I do a search to see if the part has a datasheet before purchase. A lesson learned the hard way.
As previously alluded to, I doubt there will be any specification sheets available.
It has a button (circled in picture) to turn it on or off. If it is not possible to power the light from the Arduino, could I go with 3 AA batteries for power and somehow have the Arduino use the button wiring to open or close the circuit?
Further to the last post, could I load the three LED lights with AA batteries and wire the on/off switches to a multi-channel relay like the following?
Well, my first feeling is that those lights are horrifically expensive for what they are (about US$10 each), but I could be wrong. (I did come across this in search!)
An Arduino does not have a "5V output". It has I/O pins which can drive about 20 mA into a single LED, or perhaps 10 mA each into 2. Clearly the units you cite have 15 LEDs and would draw something of the order of 150 mA. You could probably use a TPIC6B595 to control them and yes, use batteries in each and wire to the switch.
That relay board would be ridiculously cumbersome. And draw a ridiculous amount of power itself!
We would probably suggest getting some WS2812 modules like these:
Or if you want to get really serious, the four-ring version of this:
to be powered by one of those "power banks" (with due care - these arrays can use some serious power but minimal if limited to one colour and one array) and controlled by an Arduino Nano or Pro Mini for compactness.
Note that the LEDs in these arrays are individually addressable - you could have not just traffic lights, but some really wild animated patterns!
(I did come across 
