Different kinds of hearing aids have differing numbers and kinds physical controls. As hearing aids become smaller and more automatic, and are able to be controlled with smart phone apps, some hearing aids don't have any buttons or switches at all. It is either up to the user to control the aid via an app, or simply rely on the programs an audiologist created during an office visit. Ideally the hearing aid has been properly fit and the user can wear the device without having to make adjustments.
Even the small hearing aids available today can usually accomodate a physical push button (click "View Button" under Figure 1 to see a typical program button). Such buttons can be configured to do various tasks, such as change between different sound processing programs (e.g., toggling between a program for quiet and noise), or adjust the volume (e.g., the right button increases and the left button decreases volume).
Hearing aids with extra power (for people with substantial hearing loss) are physically larger, in part to accommodate a bigger battery and speaker. Because there is more room on these aids, some manufacturers will include a switch (click "View Switch" under Figure 1 to see a typical switch). Modern hearing aid switches don't typically turn the aid on and off, but adjust the volume up and down. You might see an older hearing aid with a switch with three positions: O, T, M. Switching to the "O" position turns the hearing aid off. The "T" position is for use with a telephone program, which is outside the scope of this module. When the switch is in the "M" position the hearing aid is on ("M" stands for microphone).
Keep in mind that even though a hearing aid might have physical controls, that doesn't mean they are programmed to do anything! Especially for children's hearing aids, audiologists often disable any buttons or switches so the child does not accidentally make changes to the hearing aids.
So how do you turn a hearing aid on and off? Simply open or close the battery door. Most hearing aid battery doors have two positions: all the way open (for changing the battery), and open just enough to disconnect the battery from the hearing aid, but keep the battery from falling out.