Have you used an exercise bike, elliptical, or rowing machine recently? If so, you've probably noticed the machine's electronics turn on only after you start moving. That's because the exercise machine is converting your physical movements into electricity, and using this electricity to power the various screens and readouts on the device.

A modern exercise bike uses a dynamo to turn pedaling into electric current.

Now, as a survivalist, you may wonder if this electricity can be re-purposed to charge batteries or power your electronics off the grid. The answer is yes—and it's surprisingly simple and inexpensive to do so with an exercise bike. It can even be accomplished using parts from a junkyard or yard sale, plus a few items from an auto parts store.

It may not look pretty, but it'll help you stay physically fit and keep the lights on at night.

Here's a basic rundown of the items you'll need for a DIY bicycle generator:

• Bicycle in decent working condition
• Bicycle stand (or some wood, nuts, bolts, and screws to make one)
• DC motor, such as one from an old treadmill or electric scooter
• Chain, belt, or other method of coupling the bike's pedal drive to the motor
• Car battery charger, connected to the motor to regulate its charging current
• Car battery, connected to the battery charger
• Optional: AC inverter to connect electronics to your car battery
• Optional: Zener diodes to prevent reverse current flow or accidentally overloading the battery charger

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That may sound complicated, but it's really not so bad. Here's a video from The DIY World that shows how it works:

Here's an alternate step-by-step guide from Instructables on how to build a bicycle generator.

There's one caveat to all this, and that's the fact that this setup isn't very efficient. In other words, you're not going to be producing an immense amount of electricity. Depending on your motor and how fast you're pedaling, it may take up to 40 minutes to fully charge a smartphone, or up to 9 hours to recharge a car battery. If you're able to pedal faster and output more amps of electrical current, these times can decrease.

Regardless, building a DIY bicycle generator helps you stay in shape (something that's essential to survival) and also generates some useful electricity to recharge your electronics off the grid. That's what we call killing two birds with one stone.

Written by Patrick McCarthy