pieces with attention to resistors placed in series with the human parts to control the current for safety. Although early e-stim units used only a simple, pulsed, sinusoidal wave, newer units use more complex wave forms and also allow for the use of ambient sound or prerecorded wave forms like music or specially designed computer files for specific types of stimulation. There are now sites dedicated to the creation of MP3 files specifically for erotic journeys or symphonies, which can include such routines as rewards, punishments, very strong, and pleasantly soft portions. Types of power sources Medical power sources There are repackaged TENS and EMS units marketed as erotic electrostimulation power sources. EMS units are designed to cause muscle contraction. Body toning and massage units An increasing number of "body toner" or "electromassage"-type power sources are being marketed directly to consumers. Though lacking in options compared to the more expensive specialized units, these have proven to be an inexpensive method for entry level practice. Homemade power sources Some people craft "homemade" electrostimulation power sources, or adapt or modify commercial products (such as a Hifi or DVD system) that were never intended for electrostimulation of the human body. These can be dangerous practices; such improvised devices not specifically designed for use on the human body can easily cause injury. The risk is mostly twofold. First the device can supply too much power. Second the insulation of most devices is not suited for medical equipment. For example, a transient over-voltage on the mains input can damage the isolation of the transformer, resulting in the output terminals becoming live. Erotic electrostimulation power sources Erotic electrostimulation power sources are specifically designed and manufactured for erotic use on the human body. The first analog devices became popular during the mid-1980s, and during the late 1990s digital devices also became available. Both types usually allow for adjustments of frequency and power output levels, some with complex preset "programs" and computer controls. The setups usually consist of a "box" and electrodes connected by wiring. Many of the boxes are portable and can be powered by batteries or come with built-in rechargeable batteries. Some units can be connected to remote operators via an Internet-connected computer or controlled via radio frequency key fobs. Units which can be powered by a 9 volt battery are preferable to those plugged into mains as they reduce the risk of accidental injury. Other