Among the many electronic devices a special place is occupied by a simple acoustic alarms, sensors which thanks to its versatility can be used in the home is almost unlimited: from security systems to automated switches or as parts of more complex devices activated by noise (acoustic) impact. For example, warning of high levels of noise in the room (now such devices is becoming more important). As a special case, acoustic sensors can be used even in the foci, for example, on a Christmas tree, when the words “Christmas Tree!” will automatically turn on the light garlands.
The basis for these devices is presented below electronic Assembly. His feature — very high sensitivity, which is caused by a combination circuit of the piezoelectric element ВМ1 and transistors with high performance current amplification.
The node is an amplifier of audio frequency (AF) transistors with a large static current transfer ratio. Sensor is piezooptical ВМ1 is it converts sound signal into electrical oscillations.
The node of the transistors VT1 and VT2 based on the principle of amplifying direct current. A sharp noise, the shaking, the cotton close to the cap or touch the capsule ВМ1 is immediately reflected by changing the voltage in the base of the transistor VT1 1 …1.2 V. the sensitivity of the node is such that the device responds to sudden noise at a distance up to 5 m.
The second cascade on the VT2 transistor amplifies the signal to the level of opening of the VT3 transistor. Fixed resistors R3 and R4 respectively limit the collector current VT2 and VT3 base current, preventing these transistors from damage.
Capacitor C1 provides positive feedback between the input and output of the amplifier. Capacitor C2 smoothes the ripple voltage of the power supply.
When exposed to a sound signal to the earpiece ВМ1 amplified electric signal is supplied to a current amplifier (transistor VT3) and opens it. Through the relay coil K1 current flows, resulting in a closed group of contacts (3 and 5), K1.1 in the load circuit 1…2 S. to extend the time of switching on the load in the device is administered oxide the capacitor C3 (shown in phantom). At the time acoustic noise near ВМ1 the capacitor C3 is charged, and during the calm acoustic background gives energy.
As shown, the increase in the capacitance of the capacitor C3 excess of 10 MK is inefficient, because you lose the stability of the entire node time ranges accuracy of off-delay relay, much lost the overall sensitivity to acoustic impacts (required time for the row C3).
Parallel to the relay K1 (see diagram) indicator circuit consisting of HL1 led and limiting resistor R5. This circuit performs a dual role — as indicating led convenient to monitor the function of the relay (as no other power indicators in the circuit), and in addition, this electrical circuit prevents a reverse surge of current through relay K1. When uselessness circuit R5 — HL1 from the schema rule.