Presented here with a door alarm recognition time, the owner opened the door is normally open, normally completed within 30s to open the door movement, so it does not alarm, a thief breaking a stranger tried to open the door or door is not normal, usually a long time, Once more than 30s, it will sound the alarm, with great practical value.
The circuit in Figure 1, mainly by the 555 components of integrated circuits and alarm circuits.
Time base circuit IC1 and the R1, C1 transient time T=4min composed of about one-shot, usually IC1 in a steady state, the pin output low, VT1 end, VT2 conduction, capacitor C2 is short-VT2 access can not be charged, when the base end of the threshold circuit IC2 pin is high, pin output low, integrated circuit IC3 alarm does not work without power supply, speaker B silent.
Components list the following table:
Number Name Type Number Number Name Type the number of resistors R1 2.2M 1 C3 Electrolytic capacitor 220u 1 R2 resistor 10K 1 C4 Electrolytic capacitor 22u 1 R3 resistors 5.1K 1 VD1 switching diode IN4148 1 R4 Resistor 1M 1 LED light-emitting diode 1 R5 Resistor 510O 1 VT1-VT3 resistance transistor 9013 3 R6 240K 1 IC1, 2 time base circuit NE555 2 R7 Resistor 4.7K 1 IC3 alarm sound IC LC246 1 C1 electrolytic capacitor 100u 1 B speaker 8O 1 C2 electrolysis Capacitive touch chip self-47u 1 M 1
available battery power (four batteries in series) power supply, but must not use the capacitor step-down power circuitry in order to avoid an electric shock!