Thursday, May 26, 2011

Sound-Activated Switch using Op amp 741

          Sound activated switch shows how to make an adjustable reference voltage of 0 to 100V. We use a 10 kΩ pot, 5 kΩ resistor, and +15V supply to generate a convenient large adjustable voltage of 0 to 10V. Next we connect a 100:1 voltage divider that divides the 0 to 10V adjustment down to the desired 0 to 100mV adjustment reference voltage. Again, signal source Ei is used as a microphone and an alarm circuit is connected to the output.

          With this sound-activated switch, control by sound may be very useful in different ways. For example, a sound-activated light responding to a knock on the door or a hand clap. The light will be automatically switched off after a few seconds. Actually, the practical application that uses a positive level detector is the sound-activated switch shown in Figure.

          Any noise signal will generate an ac voltage and microphone is used as an input. The first positive swing of Op amp of Ei above Vref drives Vo to +Vsat. The diode now conducts a current pulse of 1 mA into the gate, G, of the silicon-controlled rectifier (SCR). Normally, the SCR’s anode, A, and cathode, K, terminals act like an open switch.

Fig: A sound-activated switch is made by connecting the output of a non-inverting voltage-level detector to an alarm circuit.

However, the gate current pulse makes the SCR turn on, and now the anode and cathode terminals act like a closed switch. The audible or visual alarm is now activated. Furthermore, the alarm stays on because once SCR has been turned on, it stays on until its anode-cathode circuit is opened.

            The circuit of Figure can be modified to photograph high-speed events such as a bullet penetrating a glass bulb. Some cameras have mechanical switch contacts that close to activate a stroboscopic flash. To build this sound-activated flash circuit, remove the alarm and connect anode and cathode terminals to the strobe input in place of the camera switch. If we open the camera shutter and fire the rifle at the glass bulb, the rifle’s sound will activate the switch.

        The strobe does the work of apparently stopping the bullet in midair. If we close the shutter, the position of the bullet in relation to the bulb in the picture will adjusted experimentally by moving the microphone closer to or farther from the rifle.

           We use sound activated switch circuit in different ways. For light activated relay switches, machine gun sounds, sound activated FM transmitter, sound effects generator electronic circuit, auction of test equipment and many other works we use this circuit. This sound activated switch circuit makes our activities easy and comfortable.

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