Getting Started: Electronic Password Lock Based on 555 Monostable Circuit Design

The electronic password lock uses a digital logic circuit designed with voltage comparators, 555 monostable circuits, counters, JK triggers, UPS power supplies, etc. to achieve electronic control of the door, and there are various additional circuits to ensure that the circuit can work safely, with a very high safety factor. There are only 16 circuit passwords available for modification, but since others do not know the number of digits in the password and are required to unlock the lock in a certain order within a specified time, the probability of others unlocking the lock is very small. The following introduces the design of a new type of electronic password lock based on a 555 monostable circuit that makes people feel safer, more convenient, and more assured.

1 Introduction

With the improvement of people's living standards, the problem of how to achieve home anti-theft has become particularly prominent. Traditional mechanical locks are often pried open due to their simple structure. Electronic locks are favored by the majority of users due to their high confidentiality, good flexibility and high safety factor.

Two schemes were conceived when designing this topic: one is a single-chip microcomputer control scheme with AT89C2051 as the core; the other is a digital logic circuit control scheme composed of 74LS112 dual JK flip-flops. Considering that the principle of the single-chip microcomputer scheme is complicated and the debugging is relatively cumbersome, this paper adopts the latter scheme.

2 Overall design

2.1 Design Idea

There are a total of 9 user input keys, of which only 4 are valid password keys, and the others are interference keys. If an interference key is pressed, the keyboard input circuit will be automatically cleared, and the previously entered password will be invalid and need to be re-entered; if the user takes more than 40 seconds to enter the password (generally, the user will not take more than 40 seconds, and if the user finds it inconvenient, he can modify it), the circuit will alarm for 80 seconds. If the circuit alarms three times in a row, the circuit will lock the keyboard for 5 minutes to prevent illegal operations by others.

2.2 Overall block diagram

3 Design principle analysis

The circuit consists of two parts: the password lock circuit and the backup power supply (UPS). The UPS power supply is set to prevent the password lock circuit from failing due to power outages, saving users from trouble.

The password lock circuit includes: keyboard input, password modification, password detection, unlocking circuit, execution circuit, alarm circuit, and keyboard input times locking circuit.

3.1 Keyboard input, password modification, password detection, unlocking and execution circuit.

The circuit is shown in Figure 1 below:

Figure 1 Keyboard input, password modification, password detection, unlocking, execution circuit 3.2 Alarm circuit

The function of the alarm circuit is: when the time to enter the password exceeds 40 seconds (generally the user input will not exceed), the circuit will alarm for 80 seconds to prevent others from maliciously unlocking the door.

The circuit consists of two parts, 2-minute delay circuit and 40-second delay circuit. Its working principle is that when the user starts to enter the password, the circuit starts to count down for 2 minutes. If it exceeds 40 seconds, the circuit starts to alarm for 80 seconds. As shown in Figure 3

Figure 3 Alarm circuit

When someone approaches the door, he touches the TP terminal (the TP terminal is fixed on the keyboard, and its sensitivity is very high, ensuring reliable triggering of the circuit). Due to the electricity carried by the human body, the 2nd pin of IC10 appears low level, which causes the state of IC10 to flip, and its 3rd pin outputs high level, T5 is turned on (the base current of T1 can be controlled by R12), and the yellow light-emitting diode D3 connected to its collector lights up, indicating that the electronic lock is now in standby state, T6 is turned off, and C4 starts to charge through R14 (the charging time is 40 seconds, which is the time for the user to enter the password, that is, the time for the user to enter the password cannot exceed 40 seconds, otherwise the circuit will start to alarm. Since the user often enters the password and knows the password, the time to enter the password generally does not exceed 40 seconds), and IC2 starts to enter the 40-second delay state.

Start alarming: When the password entered by the user is incorrect or the time for entering the password exceeds 40 seconds, the potential of pin 2 of IC11 decreases as C4 charges. When the potential drops to 1/3Vcc (i.e., when the 40-second delay ends), pin 3 becomes high potential (low level during the delay), and T7 is turned on through R15 (the function of R15 is to limit the conduction current of T7 to prevent excessive current from burning the transistor). The red light-emitting diode D4 connected to its collector lights up, indicating that it is currently in an alarm state. T8 is also turned on, causing the buzzer to sound, making the thief afraid and realizing the alarm.

Stop alarm: When the alarm time reaches 80 seconds, the capacitor C5 connected to the 6th and 7th pins of IC10 finishes discharging, the 3rd pin of IC10 becomes low level, T5 is cut off, T6 is turned on, and the circuit is forced to be in a steady state. The 3rd pin of IC11 outputs a low level, which cuts off T7 and T8, and the buzzer stops alarming; or if the password entered by the user is correct, the collector of T10 in the unlocking circuit outputs a clear alarm signal, which is sent to T12 (PNP). T12 is turned on, and the base of T7 is forced to be low potential, thereby releasing the alarm signal.

3.3 Alarm times detection and locking circuit

If the user makes mistakes in operation for more than 3 times in a row, the circuit will be locked for 5 minutes. Its working principle is as follows: When the number of circuit alarms exceeds 3 times, the 3-bit counter composed of IC9 (74161) will generate a carry, and through IC7, the output clear signal is sent to the clear terminal of 74161 to achieve re-counting. Through IC8 (AND gate), it is sent to the 2nd pin of IC12 (555), so that the 3rd pin generates a 5-minute high-level locking pulse (the pulse can be calculated by the formula T=1.1RC), which is inverted by T9 and sent to the input terminal of IC6, so that IC6 outputs a low level, so that IC13 cannot be unlocked, achieving the purpose of locking. The circuit diagram is shown in Figure 4 below:

Figure 4 Alarm times detection and locking circuit 3.4 Backup power supply circuit

In order to prevent power outages, this circuit has a UPS power supply backup, which includes a mains power supply circuit, a power outage detection circuit, an electronic switch switching circuit, a battery charging circuit and a battery. The circuit diagram is shown in Figure 5 below:

Figure 5 Power supply circuit

220V mains power is stepped down to 12V AC power through transformer B, and then rectified by bridge rectifier, 7805 stabilizes the voltage to 5V and sends it to the electronic switching circuit. Since this circuit consumes less power, a 10W small transformer is selected. R8, R9, R6, R7 and IC14 form a voltage comparator. Under normal circumstances, V+V- IC14 outputs a high level, and the Darlington tube composed of T3 and T4 turns on relay J, which is normally open to connect the battery and the circuit, realizing the switching between mains power and battery power supply, and ensuring the normal operation of the electronic password lock (depending on the battery capacity and duration). Its circuit diagram is shown in Figure 6 below:

Figure 6 Power failure detection and electronic switch switching circuit

The battery automatic charging circuit composed of T1 and T2 will automatically stop charging after the battery is fully charged. D1 is on to indicate charging, and D2 is a working indicator. R4, R5, and T1 form a voltage detection circuit. If the battery voltage is low, T1 and T2 are turned on to charge it. After it is fully charged, T1 and T2 are turned off to stop charging, and D1 is turned off. The function of C4 in the circuit is to filter out interference signals. The circuit diagram is shown in Figure 7:

Figure 7 Battery automatic charging circuit

In summary, the password of this circuit cannot be forgotten. Once forgotten, it is difficult to open. Using a switch as the CLK pulse of 74LS112 is not very stable. Other high-speed switches or counting pulses can be replaced; there are only 16 circuit passwords available for modification, but since others do not know the number of digits of the password and are required to unlock the lock in a certain order within a specified time, the probability of others unlocking the lock is very small. Therefore, this anti-theft electronic password lock is still very reliable.