Early every January, my household, like many, undertakes the “Ordeal of Post-Christmas Undecoration.” One of the chores associated with dismantling and packing away Christmas ornaments is fixing those strings of series-connected miniature incandescent lamps in which a filament has opened up, the internal filament-bypass device has failed to work, and a whole array of 50 or 100 lights has consequently gone dark. I’ve discovered there aren’t many activities as well-calculated to dampen any lingering holiday cheer as having to do several of these haystack/needle searches. The circuited illustrated here comes to the rescue by sniffing out defective bulbs by capacitively sensing the electric fields they produce (see the figure).
To understand how it works, consider a string of bulbs with an opencircuited filament, labeled in the schematic as “String Under Test.” The intact filaments on either side of the burned-out bulb complete continuous circuits from the ac supply all the way around to the opposite ends of the broken filament. The ac voltage differential between wires leading to any intact bulb is zero, but the differential between the connections to the defective bulb will be the full 110 V. The presence of 110 V ac on a wire produces large electrostatic fields that are easily detected through plastic insulation. The high-impedance CMOS inputs of “XOR” U1 can do just that.
Resistors R1 and R2, in addition to delivering operating power to the circuit, effectively suspend U1 midway between the ac supply rails. Thus, equal-amplitude (160 V p-p) but opposite-phase voltage differentials will exist between U1 and both ac rails. R4 and C1 reference U1 pin 2 to ac “neutral,” while R2 and the “Test” electrode are used to capacitively probe the voltages present in the wires of the String Under Test. U1 then performs as a phase-sensitive detector of those ac voltages.
Suppose the Test electrode is held close to point A. Because A is continuous with the “hot” rail, the signals at U1’s inputs will have opposing phases. One input will be high (logic one) whenever the other is low (logic zero). This causes XOR U1 to hold pin 3 high, so D2 glows. If the probe is moved to points B or C, the same phase relationship will persist and D2 will still glow. But if the probe is brought near D, the sensed phase will reverse because the circuit break lies between D and the hot rail. Consequently, neutral phase voltage is induced in the probe. Pin 3 will therefore go low, extinguishing D2, lighting D1, and thus designating the offending filament. The defective lamp then is replaced.
In actual use, when the bad-bulb search is commenced near the powerplug end of the String Under Test,the initial phase relationship will be arbitrary, so either D1 or D2 may glow. In either case, it’s the sudden change in which a LED is lit as the probe is moved along the String Under Test that indicates when the broken filament has been found.