Replace Batteries With Gravity Power

This is an off-grid, gravity-powered light that I installed in my closet.

I live in a house that doesn’t have any light fixtures in the closets. So, I bought a couple of rechargeable motion sensor lights -designed specifically to be used in closets. And they worked, but when the batteries died, I just kept forgetting to take the LEDs out to recharge them and put them back. So, I finally decided to try and hack the LEDs to run on electricity generated by a gravity battery when I need them. And I decided to try and do it using stuff I already had, like a synchronous motor that I pulled out of a broken oscillating fan a few years ago.

It works by pulling down on a string guided by a few pulleys to turn a wheel that's connected to a synchronous motor. As the wheel turns, the synchronous motor generates electricity that powers an LED light. Pulling the string also lifts up a counterweight in the form of a glass bottle filled with water. Then, when the string is let go, the bottle acts as a gravity battery and turns the wheel in the opposite direction to generate electricity as it falls, which gives me about 10 seconds of hands-free lighting when I can grab what I need out of the closet.

The mechanism, the generator, the bridge rectifier, and the capacitors, work together to replace the power provided by a rechargeable lithium-ion battery, which is about what you’d get out of a couple of AA or AAA batteries - around 3 volts. And, it turns out that it can run other battery-powered devices as well.

Geared Permanent Magnet Motor: Synchronous Alternating Current (AC) Motor or Brushed Direct Current (DC) Motor will work

(4) 1N4001 Rectifier Diodes

Capacitor(s)

Low-Wattage Bulb or LED Light Bar

Breadboard

Jump Wires

Lamp Wire

Quick Splice Wire Connectors

String or Cord

3 or 4 Pulleys

Bottle or Container

A medium to use as a weight, like water or sand

Plywood

Nuts and Bolts

Jigsaw

Choosing the right motor depends how much electricity you need to generate. A geared permanent magnet motor that will work turning clockwise and counterclockwise is your best bet. That means you will likely be choosing between an AC synchronous motor or a brushed DC motor

I chose to use an AC synchronous motor rated for: 110 vac, 60 hertz, 5 rpms, 4 watts, CW/CCW. Synchronous motors have an internal gear system to give them more torque, so you don’t have to turn the motor shaft very fast to make them work as an alternator to produce a useful amount of electricity. And they’re designed to work when they're turned clockwise or counterclockwise.

I also tested geared brushed DC motors, and they worked just as well as the synchronous AC motor, but they weren't rated for as much power, so I didn't choose to use them for this project.

Direct current is required to replace the batteries in an electronic device and a bridge rectifier converts alternating current to direct current. A bridge rectifier also allows a DC motor shaft to be turned in either direction -clockwise and counterclockwise- but still send electricity in the same direction. So, even if you are using a brushed DC motor, you will still need to use a bridge rectifier because the counterweighted pullstring mechanism works by turning the motor shaft both directions.

If you want to build your own rectifier circuit, you will need 4 1N4001 rectifier diodes and at least one capacitor. Diodes only allow electricity to flow in one direction, so a bridge rectifier circuit configures 4 diodes so that whichever way an alternator is sending the positive and negative current, the diodes will reroute the current so that a consistent flow of electricity will come out of the circuit as direct current.

Adding one or more smoothing capacitors across the positive and negative DC outputs of the rectifier circuit will make the current more consistent, like the current from a battery. You may have to calculate the appropriate amount of capacitance for your project, but a 50v 470uf capacitor is likely to work for generator that is small enought to replace a few batteries.

NOTE: If you are using a synchronous AC motor with an AC bulb, you may not need a rectifier circuit at all. Some synchronous motors are rated for household AC frequency (60 hertz) and will light up certain low-watt lightbulbs. And if you don't mind some flickering, you may be able to run the light bulb directly off the motor.

And if you don’t want to have to build your own bridge rectifier circuit, I did find a product for operating model trains that’s basically the exact circuit that you’d need for this project. They cost about 3 bucks each. Try searching for "Full Bridge Rectifier Filter Power Supply Module for Model Train Railroad."

Fasten a lever to the motor shaft. This can be a piece of wire or wood, whatever works.

Try pushing on the lever at different distances from the motor shaft to feel how much effort it will take to turn it.

Choose the distance that you think will work best for your project.

If you want to use a wheel to turn the shaft continuously for multiple turns, you will need to need to cut it out using the distance you chose from the shaft to the point on the lever as the radius for your wheel.

Use a jigsaw to cut at least two pieces of plywood into the appropriate sized circles.

Cut out a slightly smaller plywood circle to sandwich between the other plywood circles to create a spindle that the string can be wrapped around.

Using a bolt, fasten the wheel or lever, to a piece of plywood.

Connect a string on either side of the wheel or lever.

Tie a handle to one of the strings, you will tie the counterweight to the other.

You may want to add some pulleys to either side of the mechanism to redirect where the pullstring and counterweights will be located. Pulleys can be fabricated just like the wheel spindle and they can also be used to improve the amount of leverage you get. Just be aware that if you leverage the counterweight, you may have to adjust how much weight you'll need.

The motor shaft has to be connected to the wheel somehow.

You may be able to use an off-the-shelf coupler or you may have to fabricate your own.

I used some plywood hole saw offcuts and fit them onto the motor shaft and held them in place by pushing a piece of hanger wire through the plywood into the hole in the shaft. Then I wound the wire around a couple of screws projecting from the wheel.

There are many ways to connect a motor shaft to a wheel or lever, it just depends on what works best for the particular mechanism.

Start out with a container of some kind, like a bucket or a basket and tie that to the string that will hold the counterweight.

Then, fill the container with whatever heavy materials you have on hand until you reach the best weight for your mechanism. I originally used a basket and filled it with tools.

Once your container is at the right weight, put it on a scale to find out the exact weight.

Then you can find something that will work as you permanent counterweight.

Bottles work well as counterweights because you can add water or sand to the bottle until the desired weight is reached.

You will know you've reached the optimal weight when it falls at just the right speed to turn the wheel.

Installing the light is an important consideration when it comes to finishing your project.

The pullstring needs to be easily accessible when you want to operate the mechanism and the counterweight needs to have a place to land after it falls.

I used a cleat to hang the mechanism so that the weight of the mechanism would be properly supported. And then, I added another pulley to the door frame to let the string hang in the most convenient place for me to pull it.

I can imagine a lot more applications for this, and I'm planning to make more of these. I know there could be much better ways of doing this, but so far this has been working for me and I really just enjoy using it!