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This circuit is intended to signal, through a flashing LED, the exceeding of a fixed threshold in room noise, chosen from three fixed levels, namely 50, 70 & 85 dB. Two Op-amps provide the necessary circuit gain for sounds picked-up by a miniature electret microphone to drive a LED. With SW1 in the first position the circuit is off. Second, third and fourth positions power the circuit and set the input sensitivity threshold to 85, 70 & 50 dB respectively. Current drawing is 1mA with LED off and 12-15mA when the LED is steady on. Circuit diagram : Room Noise Detector Circuit diagram Parts List : R1____________10K 1/4W Resistor R2,R3_________22K 1/4W Resistors R4___________100K 1/4W Resistor R5,R9,R10_____56K 1/4W Resistors R6_____________5K6 1/4W Resistor R7___________560R 1/4W Resistor R8_____________2K2 1/4W Resistor R11____________1K 1/4W Resistor R12___________33K 1/4W Resistor R13__________330R 1/4W Resistor C1___________100nF 63V Polyester Capacitor C2____________10µF 25V ...

My wife was working on a doctoral dissertation and needed to do some field work involving personal interviews in various settings. What would be the best way, technically speaking, to record the interviews? To pass a tape recorder or microphone back and forth seemed too awkward and clipping wired microphones to interviewees didn’t make for a particularly informal atmosphere. Radio microphones seemed overly expensive, too. After some thought, I can up with the "Room Recorder", an add-on microphone preamplifier circuit for use with a tape recorder. While I don’t make any great claim to originality for the circuit, it has produced first class results over one year of interviews and might prove useful to anyone doing similar work. Circuit diagram: Room Recorder Circuit Diagram The preamplifier was plugged into a Sony Cassette-Corder (any similar device will work) by means of a long, screened microphone cable and placed in a central location in a room or on a bench. The circuit wi...