Acoustic relay instead of a switch. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Lighting
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Acoustic switches are indispensable in walk-through rooms, where ordinary key switches quickly get dirty and break, as well as for those who like to surround themselves with super comfort. The advantages of the proposed scheme are high repeatability, availability of the element base, versatility, and the ability to install walls in a niche instead of a conventional switch.
Acoustic relay (AR) (Fig. 1) consists of the following main units: microphone, amplifier, detector, comparator, trigger, key.
(click to enlarge)
The current from the resistor R1 passes through the BM1 carbon microphone. Sound vibrations change the resistance of the microphone, modulating the voltage drop across it. This voltage is amplified on the DA1.1 amplifier. The resistor R5 regulates the gain, and hence the sensitivity of the device. Amplified to 0,25 ... 1 V, the signal through the capacitor C4 is fed to the amplitude detector on the diodes VD1, VD2. The integrator R7, C6 protects against short-term sounds, thereby increasing noise immunity.
A voltage comparator is assembled on the DA1.2 chip. The signal at its output occurs when the signal voltage exceeds the threshold voltage, which is set by the divider R8, R9.
A switching element is assembled on the DD1 chip, which responds to the leading edge of the comparator pulse. When the trigger DD1 is started, a high potential appears at its non-inverting output, which opens the transistor VT1. In this case, the voltage from the divider R12, R13 is supplied to the thyristor VS1. The thyristor turns on and the HL1 lamp lights up.
The circuit is powered through the diode bridge VD3-VD6. The quenching resistor R10 limits the current through the microcircuits and, together with the capacitor C5, forms an effective RC filter to smooth out power ripples.
When choosing the element base, the main criterion was the dimensions of the niche in the wall, so together with the outdated carbon microphone, surface mount parts are used.
The printed circuit board is shown in Figure 2.
This device does not require any adjustments. Resistor R5 sets the sensitivity to the clap of the palms. When mounting into a wall, a cover is cut out of a hard material, on which a microphone is glued, and the device itself is placed in a niche.
During operation, the acoustic relay showed high reliability and noise immunity.
Author: A. Rishtun
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