This example describes the light-control safety switch can be installed in the machine more prone to accidents and other special parts . The safety switch used as an optical signal photosensitive resistor sensor, when the staff or other parts of the site near the danger , the safety switch to brake quickly Starter bed , then machine stops running , which played a security role. When staff leave the danger zone , the machine can resume normal operation . Circuit works The light-control safety switch circuit from the power circuit , control circuit and control the implementation of the optical circuit (switch circuit ), is shown in Figure 8-128 . Power circuit from the power transformer T, rectifier bridge and filter capacitor C2 UR heap formed . Light control circuit by the photosensitive resistor RL, resistors Rl-R3, potentiometer RP form transistors Vl and capacitor Cl . Control the implementation of the circuit by the diode VDl and VD2, transistor V2 and V3 and the relay K composition . AC 220V voltage by T Buck , UR rectified and filtered by C2 , as the light control circuit and control circuit to provide the implementation of 6V DC voltage . In the light control switch is in static state security , RL low resistance state , Vl voltage lower than 0.7V for the base and is off , its collector is high potential , VDl and V2, V3 are in a closed state , K can not pull , Machine braking device does not work. When the hand near the danger zone , RL hand because light is blocked and was high-impedance state , Vl high potential for base becomes saturated conduction , its collector goes low potential , VDl and V2, V3 are conduction , so that K pull its normally open contact connected to the brake system of a machine tool movements to stop running . When the left hand after the danger zone , RL restore low resistance state , Vl-V3 are closed , K release , machine tools continue to operate . RP 's resistance adjustment can change the light sensitivity control circuit action . Components Selection Rl-R3 selected 1/4W carbon film potentiometer or metal film resistors . RP selected membrane or solid variable potentiometer variable resistor . Use light resistance RL is less than or equal to lOkO, dark resistance is greater than 2MO the photosensitive resistor . Cl used monolithic capacitor or polyester capacitors ; C2 selection pressure over the value lOV aluminum electrolytic capacitors . VD1 use 1N4148 silicon switch diode ; VD2 use lN4007 type silicon rectifier diode . UR selected lA, 5OV the rectifier bridge pile . Vl use S9013 or 3DG6, 3DG2O1 silicon NPN transistor ; V2 -type germanium PNP transistors used 3AX3l ; V3 use 3DGl2 or C8050 silicon NPN transistor . T use 3-5W, 6V secondary voltage power transformers . K use more than one set of 6V DC relay contacts , each group used to use normally open contact in parallel .