Some AAA have a 1.7V open circuit voltage. 3* 1.7 is 5.1 V. So I think a 5V USB supply will work as long as it can provide enough current for the peak demand. Motors can draw a lot more current when first starting.

They sell exactly the thing you look for. They are marketed as USB battery eliminators. They have a USB plug on one end and 3 dummy batteries on the other end. You stick the dummies in and plug in the USB and you’re good to go. They even step the voltage down for you.

It’s perfectly fine. Fresh batteries are higher voltage than 1.5V each anyway.

Your plan is perfectly sound, done it many times myself. Just clip the USB cable and wire it to the battery contacts (or cable, internally, whatever is convenient for you) or use a purpose made adapter. Simple as can be!

Only thing I’ll say is to use a vaguely okay quality 1A or 2A charger if you have one kicking around, and a decent quality (thicker power wire gauge) USB cable too. Not necessary for most 4.5v -> USB conversions, but the motor’s draw could cause dropouts with a dinky little guy.

For about $5 you Can order via AliExpress a five volt power supply. You’ll have to wire a 3-wire grounded plug to it, and provide leads from it into your device. It’s good to go, it’s got over voltage protection, And it’s Rock solid. If you really need 4.5 volts, there’s a little screw next to the output terminals that can lower it down to around four volts.

5V should be fine since it’s just 10% more. As Diligent_Nature says, what could fail is the charger not supplying enough current to the motor, but the easiest way to go about it is hook it up and try: cut off the micro-USB connector off an old cable, connect the + and – wires, cut short the other two wires if present.

If the motors are sluggish, look at the amperage rating, e.g. “Output 5V ⎓ 1A” and see if you can find one that supplies 1.5A or 2A.

You can buy ‘USB boost buck convertors’ from e.g. AliExpress for only a few dollars. 5V from USB goes in, anything from 1.2 to 24V comes out. (You can change the output voltage with an onboard potmeter.)

I’ve done this countless times with holiday decor in a themed attraction and it works great. You can solder leads directly to a USB adapter, which is the best way. I’ve also had great results with using cheap USB to DC alligator clip adapters bought for pennies on Amazon. You clip the negative to the spring terminal in the battery compartment and you’ll have to gently pry the flat positive terminal up and out, then clip the positive to that. It takes less than a minute to do it.

AAA battery are closer to 1.6V than 1.5V. So will work just fine.

If you are worried, just add a diode in series, which will drop the voltage by 0.3-0.8V depending on the model you use.

However, be mindfull that battery can give quite alot of instantanious current and can be abused. If the automatic door uses a high instantanious current, like when the motor start, battery will still work, usb adapter might not like it and goes into protection as they don’t like abuse. This is however unlikely, but a small possibility.

Done it with stuff like flashlights. Most stuff that runs on 3x AA/AAAs will work fine on 5v from USB or 3.7v from a lithium cell

The voltage is fine. A little off is not going to matter with small devices.

What matters more is the current drawn. What you need to make sure is that the device (your automatic door) is drawing LESS current (during peak draw) than the power supply can provide, otherwise the power supply side will overheat or worse. USB has current cap, the cap depends on which USB you intend to use. Just compare the two, that’ll tell you whether it’ll be enough.

many electronics have a good tolerance for input voltages, especially if it’s not sensitive and if they have internal power filtering (like step downs)

5V is unlikely to fry something requiring 4.5V, given the variance in voltages AAA batteries normally experience anyways

Edit: Realized its half a volt, just throw in a diode for the diode drop or just feed it 5v itll probably be fine its like 10%.

I’ve done this with a handful of different things.  Haven’t had an issue yet, but ymmv

You may need to add a capacitor. Even washing machines plugged into 120v need a capacitor to start the motor. It just stores a bit of charge that is available instantly so the initial current draw for the motor is satisfied.

Go on eBay and get a 110v transformer that outputs 4.5v dc.

Just verify rechaargable batteries dont use 1.2v nominal or you’ll come up short

I’ve done it for toys for my daughter and electronic locks without a problem. There are kits that can do the conversion for you.