Bang-bang control is a type of control system that mechanically or electronically turns something on or off when a desired target (setpoint) has been reached. Bang–bang controllers, which are also known as two-step controllers, on-off controllers or hysteresis controllers, are used in many types of home and industrial control systems (ICS).
An old-fashioned house thermostat, for example, uses bang-bang control. When the temperature drops to a predetermined low set point, the thermostat switches the heating system on. When the temperature reaches a pre-determined high set point, the thermostat switches the heating system off. Depending upon the set points, this can result in a fairly wide range of acceptable temperatures and cause the heating system to have a fairly long response time. This is why bang-bang controllers are sometimes referred to as hysteresis controllers -- the word hysteresis describes a lag in response to change.
In product development, bang-bang control systems can be contrasted with proportional control systems. A smart thermostat in a hotel room, for example, might determine when there is an error between the setpoint and the current value of the process variable (the room's temperature) and respond to the slight deviation by opening or closing dampers. Proportional controls are more exact and have faster response times than bang-bang, all or nothing controls.
Light switches and dimmer switches also illustrate the difference between bang-bang and proportional controls. A light switch, which can be either on or off, uses bang-bang control. A dimmer switch, in contrast, reduces or increases power to the lighting load in order to achieve a lower or higher light output and uses proportional control.