Door lock alarm with time recognition function

Door lock alarm with time recognition function The door lock alarm introduced here has time recognition function. When the owner opens the door, the door is opened normally. Generally, the door opening action can be completed within 30 seconds, so it will not alarm if a stranger tries to open the door or a thief pries the door. It does not open the door normally, which usually takes a long time. Once the time exceeds 30 seconds, it will sound an alarm, which has great practical value. working principle 　　　　 　　

The circuit is shown in Figure 1, which is mainly composed of a 555 time base integrated circuit and an alarm integrated circuit. The time base circuit IC1, R1, and C1 form a monostable trigger with a transient time T = about 4 minutes. Normally, IC1 is in a steady state, and its ③ pin outputs a low level, VT1 is cut off, VT2 is turned on, and the capacitor C2 is short-circuited by VT2. It cannot be charged. The threshold terminal pin ⑥ of the time base circuit IC2 is at high level, and pin ③ outputs low level. The alarm sound integrated circuit IC3 does not work without power supply, and speaker B is silent. M is the touch electrode piece, connected to the metal door lock. When someone unlocks the door, the negative half cycle of the clutter induced by the human body is added to the triggering end pin ② of IC1, causing IC1 to enter a transient state. Pin ③ outputs a high level. At this time, the transistor VT1 is turned on and VT2 is turned off. The power supply can be connected through R4. C2 is charging. As the charging time prolongs, the potential of IC2's ② pin continues to decrease. After about 30 seconds, IC2 flips over, and its ③ pin outputs a high level, causing IC3 to work and alarm. If the door opening time is within 30s, the ③ pin of IC2 is low level and the circuit will not alarm. The duration of the alarm sound is determined by the transient time T1 of IC1, T1=1.1R1×C1=4min. After 4min, pin ③ of IC1 outputs low level, VT1 is cut off, VT2 is turned on, IC3 loses power, and the alarm sound stops. If the door lock is touched again for more than 30 seconds, the circuit can re-alarm. The component selection and production component list is shown in the table below. No. Name Model Quantity No. Name Model Quantity R1 Resistor 2.2M 1 C3 Electrolytic capacitor 220u 1 R2 Resistor 10K 1 C4 Electrolytic capacitor 22u 1 R3 Resistor 5.1K 1 VD1 Switching diode IN4148 1 R4 Resistor 1M 1 LED Light-emitting diode 1 R5 Resistor 510Ω 1 VT1 -VT3 transistor 9013 3 R6 resistor 240K 1 IC1, 2 time base circuit NE555 2 R7 resistor 4.7K 1 IC3 alarm audio integrated circuit LC246 1 C1 electrolytic capacitor 100u 1 B speaker 8Ω 1 C2 electrolytic capacitor 47u 1 M homemade touch chip 1 power supply It can be powered by a battery pack (four dry batteries connected in series), but it must not be powered by a power circuit with capacitor voltage reduction to avoid electric shock!