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NOTES ON THE “WALL BUG”. THE SCHEMATIC IS AVAILABLE FROM COLIN MITCHELL’S WEBSITE SITE: TALKING ELECTRONICS. DOWNLOAD HIS BOOK. IT’S FREE. I HAD TO MODIFY THE PCB FOR COMMON US COMPONENTS… BECAUSE BC547’S ARE NOT REALLY POPULAR HERE WITHOUT SPECIAL ORDERING. BUT I ALSO DIDN’T HAVE 2N3904 ON HAND. I SUBSTITUTED A 2N5210 (LOW NOISE) FOR THE PIEZO INPUT AMPLIFIER, AND A PN918 FOR THE OSCILLATOR. BOTH OF THEM HAVE E‐B‐C PIN SEQUENCE SO THEY WORK FINE. ALMOST ANY COMMON NPN TRANSISTORS WILL PROBABLY WORK FOR THE PREAMP AND OSCILLATOR. THE 2N3563 IS A TO‐106 PACKAGE WITH A ROUND TOP AND ODD PINNING. ANOTHER REASON FOR THE PCB CHANGES. USE #22 AWG MAGNET WIRE FOR THE OSCILLATOR COIL. THEN USE #24 FOR THE 8T CHOKE AND THE 6T BUFFER. WRAP THEM CLOCKWISE—DO NOT USE A MACHINE SCREW FOR THE FORM, THEY ROLL THE COIL IN THE WRONG DIRECTION. FOR THIS PROJECT I USED A 3.6V RECHARGEABLE NIMH BATTERY PACK WITH A “JST” PLUG; OLD CELL PHONE PACKS FROM THE SURPLUS STORE. I SOLDERED SHORT WIRES TO A JST JACK AND ATTACHED THEM TO THE PCB. I DISLIKE SOLDERING WIRES DIRECTLY TO BATTERIES—WHICH IS WHAT THE INSTRUCTIONS IN LOTS OF COLIN’S PROJECTS DO. THE TRANSMITTER CAME ON STRONG, FIRST TRY. IT’S A VERY GOOD DESIGN. 88.1MHZ ON THE FM DIAL. THE PEIZO WILL PICK UP VOICES THROUGH WALLS, BUT IT ALSO PICKS UP AIR MOVEMENT THROUGH HEATER VENTS, WATER PIPES, FLUSHING, WALKING AND SCUFFING NOISES, AND ANY OTHER VIBRATIONS—REQUIRING A LOT OF CONCENTRATION TO DISTINGUISH AND UNDERSTAND VOICE, IF IT’S NOT SWAMPED ALL TOGETHER. A GOOD PLACEMENT WILL BE KEY TO GETTING INTELLIGIBLE SOUND. [email protected]
