You may use any magnet to construct the circuit and make the machine functional for treadmills that use a magnet as part of the key. Treadmill safety keys are really inexpensive. Amazon sells a variety of keys for less than $10. If you have children or pets, the key is critical for their protection. You should buy a new key every time your treadmill's belt moves 15 feet or more because the old one will no longer function. The best way to install a new key on a treadmill from scratch is to remove the original key first. Then follow the instructions below to construct a new magnetic circuit and attach it to the back of the unit with double-sided tape. This process takes about 30 minutes total. Once complete, be sure to replace the original key with the new one before using the treadmill again. The good news is that once you've done this process once, replacing the key is as easy as taking off the old one and putting in the new one. Each key lasts for several years of continuous use if you don't push it too hard! For help installing a new belt, see our article on how to install a new belt.