CHRISTMAS TREE!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.
Connection features
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.
Attention, this is important! This scheme can be converted to the less sensitive node, if the top (the scheme) the output of platapus ВМ1 to connect to the collector VT1, leaving the bias resistor R1. This option enable will be justified if, for example, ВМ1 shall be made with flexible wires (such MGTF at 0.6 or 0.8) to a distance of 2 m from the site. Now the device still includes relay with a sharp sound near the primer, and pickup in the connecting wires is not terrible.
The device in the basic version (without alterations) can be applied as a sensitive sensor with a delay of load shedding. This schema eliminates the primer ВМ1, and touch down to the base of transistor VT1. For stable operation without false inclusions the length of the connecting wire should not exceed 50 — 70 cm.
For details and installation
The device is mounted without errors and of defective parts, adjustment is not needed and works reliably around the clock.
PCB was not designed. The device elements are compactly arranged and mounted on the prototyping Board, their findings are connected with a jumper of pigtails MGTF-0,6. Connect to a power source and to the switched circuits of the peripheral devices which are easy to do by using electrical terminals or any suitable connector.
The device is powered by a stabilized source when the voltage of 9-12 V. If the supply voltage is below 7.5 volts, relay K1 (TRD-9VDC-FB-CL) will not fire and have to be replaced with another type of low-voltage electromagnetic relay (e.g., TRU-5VDC-SB-SL) or to use electronic relays (a series К449, КР449). Relays also can be replaced by RM85-2011 -35-1012, BV2091 SRUH-SH-112DM, TRU-9VDC-SB-SL and similar. All of these types of relays designed for operation in the circuit switching load voltage up to 250 V and current up to 3 A. you Can apply and domestic relay, for example РЭС10, РЭС15 and similar, but they are designed to work in circuits of switching no more than 150, and in addition, in comparison with foreign analogues, cost (if buying retail) is much more expensive.
Electrical circuit sensitive acoustic sensor
The electrical circuit sensitive acoustic sensor
In operation of the device observed that the sensitivity of the node (ceteris paribus) increases with decreasing supply voltage. If you increase the voltage more than 14 V, the device snowspeeders, including relays with equal intervals.
Current consumption in standby mode — 3…5 mA, while the relay K1 is increased to 35 mA. All fixed resistors type MLT-0,25. The capacitor C1 is of type KM-6 groups of TKE Н70 or similar. Electrolytic capacitors of the type K50-29.
Timing capacitor C3 (if necessary install it in the scheme) it is necessary to choose low leakage current (K53-4, K52-18).
Piezooptical ВМ1 — SN-22, but can be replaced by SN-1, SN-18, SN-W, or equivalent. Silicon transistors VT1, VT2 can be anything from КТ3107, КТ502, С557. The transistor VT3 — КТ815Б, but you can use КТ815А, and КТ815Г.
In the original node is used as an integral part of the security signaling complex. However, it is effective and as a separate sensitive sensor. Control voltage for other paired devices removed from point “A”. In this case, the current amplifier transistor VT3 and the relay rule.
A. KASHKAROV, St. Petersburg

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