A manual actuator is any lever, wheel or button used to control something by human action rather than a power source. These may be as simple as an on-off switch or as complex as large control wheels for opening floodgates. The key to any manual actuator is that a person operates the device, rather than a computer or power source of any type.
Simple manual actuators are common and familiar to many people in their homes. Electrical switches are a manual actuator for turning on lights or other electrical appliances. Toilets can be flushed by pressing a flush arm or in some designs a button on top of the tank, which then allows water to flow. Electric, gas or microwave ovens may have electronic systems to operate them, but the user chooses the cooking and timer settings, making the controls manual actuators.
For hundreds of years and into the 21st century, many people have come to depend on a manual actuator to enter or leave their homes. Doorknobs or handles are simple actuators that move a latch mechanism as needed. In the later 20th century, some businesses switched to automatic door openers for customer convenience, but a doorknob remained as a common and effective way to open and close doors.
Vehicles have a wide range of manual actuators, including climate controls, exterior wipers in vehicles without automatic rain sensors, and even the accelerator pedal. The accelerator may be a cable linkage to the throttle assembly, or it may be an electronic switch depressed by the pedal, often referred to as "fly-by-wire" since there is no physical connection to the engine. There is a manual actuator for people who do not wish to use the accelerator, called the cruise control, activated by a switch.
Where water is used for irrigation, manual irrigation valves are often used to control water flow to fields. They are simple to use, rugged and require little maintenance. Manual gate or butterfly valves, which describe the shape of the valve seat, are most common and allow reasonable control over water flow rates.
Water can also be a nuisance, and flood control in smaller rivers and streams may use manual control valves. Valves can control floodgates and water directed to small generators or water wheels. Larger dams and water works may use powered control systems, but a manual valve can operate reliably for years. This can be particularly useful for flood control systems in remote areas without power, or for systems that are operated intermittently.
Industrial and chemical operations rely on manual valves as backup controls for powered systems. A manual control valve can be used in emergencies to control processes if power fails. Manual valves are often used as lockout devices, isolating parts of the process for maintenance and locked by the maintenance personnel to prevent injury.
In the 19th and early 20th centuries, adding machines and early calculation devices used levers and control arms to provide math functions. Adding machines commonly had rows of buttons that were depressed, and a control arm was pulled to activate each addition or subtraction. A manual actuator operated the machine, and manual calculators were widely used until electricity became more common, and eventually electronic calculators replaced calculator machines in the 20th century.