Use PCT paper to roll Φ55mm red LED and photoresistor, put it in a color pen (black is good), glue both ends, it becomes a photocoupler, it is suitable for low frequency switching circuit, such as Figure (a) shows.
Figure (b) shows the power failure alarm circuit using the optocoupler , which can sound an alarm when the power is off. When there is mains supply, the red LED in the photocoupler is bright, and the resistance of the photoresistor is reduced by coupling, VT1 is cut off, and the oscillator composed of VT1 and VT2 does not work. When the mains is out of power, the resistance of the photoresistor increases, turning VT1 on, the oscillator starts to work, and an alarm sounds.
Silicone Rubber Keypads Membrane Switches
A Silicone Rubber Keypads Membrane Switch uses compression-molded silicone rubber with conductive carbon pills or with non-conductive rubber actuators. The compression molding process creates an angled web around a keypad center. When a keypad is pressed, the webbing collapses or deforms to produce a tactile response. When the pressure on the keypad is released, the webbing returns the keypad to its original position with positive feedback. The closure of the switch circuit occurs when the conductive pill or printed conductive ink makes contact to a PCB when the web has been deformed.
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