Piston area 1 square inch Vent System pressure Deboost pressure Piston area 5  square inch To brake cylinders From power brake control valve 1,500 psi 300 psi ANTISKID INBD OUTBD AUTO BRAKE ANTI SKID INOP ANTI SKID INOP INOP ON ON OFF OFF MED MIN MAX OFF 13-62 Figure 13-105.  Antiskid switches in the cockpit. Figure 13-106.  A wheel sensor (left), a control unit (center), and  a control valve (right) are components of an antiskid system. A  sensor is located on each wheel equipped with a brake assembly. An antiskid control valve for each brake assembly is controlled from  a single central control unit. Figure 13-104.  Brake deboosters. Wheel sensor Control valve Control unit Control Units Wheel Speed Sensors its own anti-skid control valve. Typically, a single control  box contains the anti-skid comparative circuitry for all of the  brakes on the aircraft.  [Figure 13-106] Wheel speed sensors are transducers. They may be alternating  current (AC) or direct current (DC). The typical AC wheel  speed sensor has a stator mounted in the wheel axle. A coil  around it is connected to a controlled DC source so that  when energized, the stator becomes an electromagnet. A  rotor that turns inside the stator is connected to the rotating  wheel hub assembly through a drive coupling so that it  rotates at the speed of the wheel. Lobes on the rotor and  stator cause the distance between the two components to  constantly change during rotation. This alters the magnetic  coupling or reluctance between the rotor and stator. As  the electromagnetic field changes, a variable frequency  AC is induced in the stator coil. The frequency is directly proportional to the speed of rotation of the wheel. The  AC signal is fed to the control unit for processing. A DC  wheel speed sensor is similar, except that a DC is produced  the magnitude of which is directly proportional to wheel  speed.  [Figure 13-107] The control unit can be regarded as the brain of the anti- skid system. It receives signals from each of the wheel  sensors. Comparative circuits are used to determine if any  of the signals indicate a skid is imminent or occurring on a  particular wheel. If so, a signal is sent to the control valve of  the wheel to relieve hydraulic pressure to that brake which  prevents or relieves the skid. The control unit may or may  not have external test switches and status indicating lights.  It is common for it to be located in the avionics bay of the  aircraft.  [Figure 13-108] 