The circuit is built around two 2N4401 NPN transistors. The operating voltage of the circuit is 5V to 12V DC. If you are planned to operate the circuit with below 9V then change all 470 ohms LEDs current limiting resistors to 220 ohms. The circuit is using four 5mm LEDs. The number of LEDs can also be increased if desired. If you want to increase the LEDS then connect them same as we have connected in the circuit. The maximum number of LED should not be increased from 25, because each LED will require 20mA and the transistor can not handle more then 500mA. If you want to increase the number of LEDS upto 25 then use a higher output transistor like BD139 as Q2. A BD139 transistor can drive upto 50 LEDs easily.
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Here is a circuit of a simple sensitive sound activated LED flasher. The circuit flashes or blinks four LEDs when any sound is received by the electret microphone. The circuit is quite sensitive and can received sounds from many meters away. The sensitivity of the circuit can be adjusted with the 1M variable resistor.
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