Trigger alarm experimental circuit using 4011

1. Circuit Introduction

The function of this circuit is that after the trigger switch is turned on, the alarm will sound to alert people; the sound will not stop until someone presses the release switch AN. The circuit diagram is shown below.

2. Working principle of trigger alarm

The sound-generating part of this circuit has the same principle as that of a pager beeper.

Here we mainly introduce the control circuit composed of gate 1 and gate 2. In the circuit, the two input ends of gate 1 and gate 2 are connected in parallel to form two ordinary NOT gate circuits. Gates 1 and 2 form a "level self-locking" circuit; the output end of gate 1 is directly connected to the input end of gate 2, and the output end of gate 2 is connected to the input end of gate 1 through resistor R2. In this way, when the input end of gate 1 receives a high-level external control signal, it outputs a low level to the input end of gate 2; the output end of gate 2 outputs a high level. This high level is output to the input end of gate 1 through resistor R2. If the external control signal at the input end of gate 1 disappears, the input end of gate 1 still remains at a high level; the output states of gates 1 and gate 2 remain unchanged, and the output end of gate 2 remains at a high level. Conversely, when the input end of gate 1 receives a low-level control signal, the output end of gate 2 of the circuit will become a low level and remain unchanged.

The other end of the trigger switch connected to the input end of door 1 is connected to the positive pole of the power supply; one end of the release switch is connected to the input end of door 1, and the other end is connected to the ground. In this way, when the trigger switch is activated, the input end of door 1 receives a high-level signal, and door 2 outputs a high level and maintains a high level; the alarm sounder behind is controlled to work and emits sound and light alarm signals. After pressing the release switch, door 2 outputs a low level, door 3 outputs a high level, and door 4 outputs a low level. The alarm stops alarming and enters the next round of alarm preparation state. If the alarm stops alarming after pressing the release switch, and the alarm resumes alarming after releasing the switch, it can be concluded that the trigger circuit is in a closed state and should be eliminated.

Different from the above BB machine buzzer circuit which uses PNP transistor 9012, this circuit uses NPN transistor 9013 to drive the buzzer, so its base should be connected to the output terminal B of gate 4 to ensure the normal operation of the circuit.

3. Alarm trigger delay circuit

The trigger switch can be made of mechanical switch, reed switch, etc. In order to avoid false triggering, the circuit is designed with an alarm trigger delay circuit. It can avoid the false operation of the alarm caused by external interference signals to a certain extent. The delay time of this circuit is between 0.1 and 0.12s; that is, the alarm will not operate for a trigger signal with a maintenance time of less than 0.2s, and only a trigger signal exceeding 0.2s can trigger the alarm to send out an alarm sound and light signal. The alarm trigger delay circuit is composed of resistors R1, R2 and capacitor C1. Its working principle is as follows. When the trigger switch circuit gets a high level, the voltage charges capacitor C1 through resistor R1.

The time constant of R1 and C1 is 0.25s, so if this level lasts for more than 0.2s, the voltage on capacitor C1 will increase, causing gates 1 and 2 to flip. If the high level obtained by the trigger switch circuit lasts for less than 0.2 seconds, the voltage of gate 1 is not enough to cause gate 1 to flip, and the alarm will not sound.

The resistance of resistor R2 is 1M, which is much larger than 51k of R1, so the voltage transmitted by resistor R2 has no substantial effect on either the trigger signal or the release signal. However, when the trigger signal or the release signal disappears, resistor R2 takes effect and transmits the self-locking voltage to the input of gate 1.

The trigger delay time can be changed by changing the values ​​of resistor R1 and capacitor C1. The smaller their values ​​are, the higher the trigger sensitivity is; the larger their values ​​are, the slower the trigger response is. However, the proportional relationship between R1 and R2 should always be maintained, so that R1 is much smaller than R2.

The release switch AN is connected to the input terminal of gate 1 through resistor R3. What is the function of R3? It mainly reduces the current passing through AN. Especially when using a small push button switch, if AN is directly connected to capacitor C1, at the moment the switch is pressed, the electricity stored in capacitor C1 will form a large current on the switch, which may cause damage to the switch.