Acoustic sensor for Christmas tree garland. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Acoustic sensor for Christmas tree garland. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Among the many electronic devices, a special place is occupied by simple acoustic signaling sensors, which, due to their versatility, can be used in everyday life almost unlimitedly: from security systems to automatic switches or components of more complex devices activated by noise (acoustic) exposure. For example, a signaling device for increased noise levels in a room (now such devices are becoming more and more relevant). As a special case, acoustic sensors can be used even in magic tricks, for example, on a New Year tree, when from the words "Christmas tree, burn!" lights will turn on automatically.

The basis for the above devices is the electronic assembly presented below. Its feature is a very high sensitivity, which is due to the combination of the VM1 piezoelectric element and transistors with high current amplification characteristics in the circuit.

(click to enlarge)

The node is an audio frequency amplifier (AF) based on transistors with a large static current transfer coefficient. The BM1 piezoelectric capsule serves as a sensor - it is he who converts the sound signal into electrical vibrations.

The node on transistors VT1 and VT2 is built on the principle of DC amplification. Sharp noise, shaking, popping near the capsule or touching the BM1 capsule will immediately be reflected by a voltage change in the base of the VT1 transistor by 1 ... 1,2 V. The sensitivity of the node is such that the device responds to a sharp noise at a distance of up to 5 m.

The second stage on the transistor VT2 amplifies the signal to the level of opening the transistor VT3. Fixed resistors R3 and R4 limit the collector current VT2 and the base current VT3, respectively, protecting these transistors from failure.

Capacitor C1 provides positive feedback between the input and output of the amplifier. Capacitor C2 smooths out the voltage ripple of the power supply.

When an audio signal is applied to the BM1 capsule, an amplified electrical signal is fed to the current amplifier (VT3 transistor) and opens it. A current flows through the winding of relay K1, as a result of which the group of contacts (3 and 5) K1.1 closes in the load circuit for 1 ... 2 s. In order to extend the turn-on time of the load, an oxide capacitor C3 is introduced into the device (shown in dotted lines in the figure). At the moment of acoustic noise near BM1, capacitor C3 is charged, and during a quiet acoustic background, it gives off energy.

Connection Features

As practice has shown, increasing the capacitance of the capacitor C3 over 10 μF is ineffective, since the stability of the operation of the entire node is lost - the accuracy of the relay turn-off delay fluctuates from time to time, the overall sensitivity to acoustic influences is noticeably lost (it takes time to charge C3).

In parallel with relay K1 (see diagram), an indicator circuit is connected, consisting of an HL1 LED and a limiting resistor R5. This circuit performs a dual role - it is convenient to monitor the function of the relay by the status of the indicator LED (since there are no other power indicators in the circuit), and in addition, this electrical circuit prevents reverse current surges through relay K1. If unnecessary, the circuit R5 - HL1 is excluded from the circuit.

Attention, this is important! This circuit can be turned into a less sensitive node if the upper (according to the circuit) output of the piezoelectric capsule BM1 is connected to the collector VT1, leaving the bias resistor R1. Such an option will be justified if, for example, VM1 is carried out on flexible wires (such as MGTF-0,6 or 0,8) at a distance of up to 2 m from the node itself. Now the device still turns on the relay with a sharp sound near the capsule, and pickups in the connecting wires are not terrible.

The device in the basic version (without alterations) can also be used as a sensitive sensor with a load disconnection delay. To do this, the VM1 capsule is excluded from the circuit, and the sensor contact is connected to the base of the VT1 transistor. For stable operation without false inclusions, the length of the connecting wire should not exceed 50 - 70 cm.

About details and installation

The device, mounted without errors and from serviceable parts, does not need to be adjusted and works reliably around the clock.

The printed circuit board was not developed. The elements of the device are compactly placed and mounted on a breadboard, their outputs are connected by jumpers from MGTF-0,6 wire segments. Connections to the power supply and to the switched circuits of peripheral devices are conveniently made using an electrical terminal block or any suitable connector.

The device is powered by a stabilized source at its voltage of 9-12 V. If the supply voltage is below 7,5 V, relay K1 (TRD-9VDC-FB-CL) will not work and will have to be replaced by another type of low-current electromagnetic relay (for example, TRU-5VDC -SB-SL) or use an electronic relay (suitable from the K449, KR449 series). The relay can also be replaced with RM85-2011 -35-1012, BV2091 SRUH-SH-112DM, TRU-9VDC-SB-SL and similar. All of these types of relays are designed to operate in a switching circuit of a load with a voltage of up to 250 V and a current of up to 3 A. Domestic relays can also be used, for example, RES10, RES15 and similar, however, they are designed to operate in switching circuits of not more than 150 V, and in addition In addition, in comparison with foreign analogues, they are (when buying at retail) much more expensive.

Sensitive acoustic sensor wiring diagram

During the operation of the device, it was noticed that the sensitivity of the node (ceteris paribus) increases with a decrease in the supply voltage. And with an increase in the supply voltage above 14 V, the device is self-excited, turning on the relay at regular intervals.

The current consumed in standby mode is 3 ... 5 mA, and when relay K1 is activated, it increases to 35 mA. All fixed resistors are of the MLT-0,25 type. Capacitor C1 - type KM-6 of the TKE H70 group or similar. Oxide capacitors - type K50-29.

The timing capacitor C3 (if there is a need to install it in the circuit) must be selected with a low leakage current (K53-4, K52-18).

Piezo capsule VM1 - ZP-22, but can be replaced with ZP-1, ZP-18, ZP-Z or another similar one. Silicon transistors VT1, VT2 can be any of the KT3107, KT502, C557 series. Transistor VT3 - KT815B, but you can use both KT815A and KT815G.

In the author's version, the node is used as an integral part of the security alarm complex. However, it is also effective as a separate sensitive sensor. The control voltage for other coupled devices is removed from point "A". In this case, the current amplifier on the transistor VT3 and the relay are excluded.

Author: A.Kashkarov

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