Bicycle anti-theft alarm

This article introduces two bicycle anti-theft alarms, which are composed of discrete components and integrated circuits respectively. With them, you can protect your bicycle. In addition to bicycle anti-theft, these two circuits can also be used for electric vehicle anti-theft.

Figure 1 is a bicycle anti-theft alarm made of discrete components. In the figure, VT1 and VT2 form an ultra-low frequency astable oscillator, and VT3 and VT4 form a complementary audio oscillator. When the two circuits start oscillating at the same time, the collectors of VT1 and VT2 alternately output high and low levels, that is, the base of the complementary audio oscillator VT3 alternates, and resistors R5 and R6 with different resistance values ​​are connected, so the oscillation frequency changes alternately, and the speaker B emits a "wooo! wooo" alarm sound similar to the sound of a police car whistle.

Thyristor VTH, voltage-controlled switch SB, etc. constitute a pressure control circuit, in which SB is installed under the seat of the bicycle. It is required that when a person sits on the bicycle, SB should be closed under the pressure of the person's weight. The small switch S should be hidden in a relatively hidden place under the seat. After parking the bicycle and locking it, the switch S should be closed before leaving safely. If a thief unlocks the bicycle and steals it, as long as he rides on the bicycle, the thyristor VTH will be turned on because SB is closed, and the speaker will emit a loud "Woo Wow! Wow Wow" double-tone alarm. After the thief inquires, he will panic and get off the bicycle, abandon the bicycle and flee. At this time, although SB jumps off, the thyristor is in the DC circuit. Once it is turned on, its gate loses its control function, so the alarm will continue until the power is exhausted or the power is cut off by disconnecting S. When the owner rides the bicycle, he can disconnect the hidden switch S in advance to cut off the power. Then, the circuit will not alarm when riding the bicycle again. If the car thief is bold enough to ignore the alarm and flee on his bike, the sound from the loudspeaker will surely attract the attention of passers-by and he will be quickly stopped and captured by the traffic police.

The circuit of the bicycle anti-theft alarm made of integrated circuit is shown in Figure 2. The pressure control circuit is exactly the same as Figure 1, except that the alarm sound source is changed to KD-9561 four-sound analog sound integrated circuit, and its first sound selection terminal SEL1 is connected to the positive power supply terminal VDD, and the second sound selection terminal SEL2 is left floating. Therefore, once the circuit is powered, the output terminal O/P outputs the most noticeable fire truck whistle sound, and is directly sent to the control terminal ⑤ of the TWH8778 power switch integrated circuit through R3, so that TWH8778 is in the gap of conduction and shutdown. In essence, it is a switch power amplification of the alarm signal output by KD-9561, and its audio power is much higher than that of Figure 1.

In the figure, VS, R1 and C1 form a simple voltage regulator, which mainly provides a suitable working voltage for KD-9561. VD is the freewheeling diode of speaker B, which can protect TWH8778.

In the above two figures, VTH should use a plastic-encapsulated unidirectional thyristor with a small trigger current, such as MCR100-6 and MCR100-8. SB is a normally open key switch, which can also be made by yourself. It is required that when a person sits on the seat cushion, the SB contact should be able to close reliably. G can be powered by 4-6 nickel-metal hydride or nickel-cadmium rechargeable batteries in series. This circuit can also be used for electric bicycle anti-theft alarm. The power supply can be directly taken from the battery of the electric vehicle. Since the battery voltage of the electric vehicle is relatively high, an LM7806 three-terminal voltage regulator integrated block should be added to the input end of the circuit power supply to reduce the voltage to 6V.