Each January my family, like many, undertakes the packing away of yuletide wreaths, bushes, ornaments, and different vacation paraphernalia. One of many least enjoyable of those chores is fixing strings of series-connected miniature incandescent lamps wherein a filament has opened up, the inner filament-bypass gadget has didn’t work, and an entire array of fifty or 100 lights has consequently winked out. This after all leaves the open-circuit-culprit visually indistinguishable from its harmless unpowered siblings. There aren’t many actions as well-calculated to dampen any lingering vacation cheer, and promote its Grinchy reverse, as having to do a number of of those irritating haystack/needle searches.
Wow the engineering world along with your distinctive design: Design Concepts Submission Information
The festive failed filament finder (FFFF) design concept seeks to ease the annual ache by speedily sniffing out faulty bulbs (and/or their sockets and wiring) by capacitively sensing the electrical fields they produce. See Determine 1.
Determine 1 Failed filament finder circuit that capacitively senses the electrical fields faulty filament bulbs produce.
To grasp how the FFFF works, contemplate a string of bulbs with an open-circuited filament as depicted. Intact filaments on both facet of the burned-out bulb full steady circuits from the AC plug all the best way round to reverse ends of the damaged filament. The AC voltage differential between wires resulting in any intact bulb is zero, as a result of present stream via the damaged string is zero. However the differential between the connections to the faulty bulb would be the full 120 Vrms. The presence of 120 VAC on a wire produces massive electrostatic fields which are simply detected via air and plastic insulation. The high-impedance CMOS inputs of “XOR” U1 can do exactly that.
Resistors R1 and R2, along with delivering working energy to the circuit, successfully droop U1 halfway between the AC provide rails. Thus, equal-amplitude (170 Vp-p) however opposite-phase voltage differentials will exist between U1 and each AC rails. R4 and C1 reference U1 pin 2 to AC “scorching”, whereas R3 and the “Check” electrode are used to capacitively probe the voltages current within the wires of the bulb string. U1 then performs as a phase-sensitive detector of these AC voltages.
Suppose the FFFF Check electrode is held near level A. As a result of A is steady with the “impartial” rail, the alerts at U1’s inputs may have opposing phases. One enter might be excessive (logic one) each time the opposite is low (logic zero). This causes XOR U1 to carry pin 3 excessive, so the three noninverting buffer components do likewise to their output pins, and D2 glows. If the probe is moved to factors B or C, the identical section relationship will persist and D2 will nonetheless glow. But when the probe is introduced close to D, the sensed section will reverse as a result of the circuit break lies between D and the impartial rail.
Consequently, “scorching” section voltage is induced within the probe. Pin 3 will subsequently go low, extinguishing D2, lighting D1, thus pointing the fickle finger of destiny on the offending filament. The faulty lamp can then be “merrily” changed.
In precise use, when the bad-bulb search is commenced, the preliminary section relationship might be arbitrary, so both D1 or D2 might glow. In both case, it’s the sudden change wherein LED is lit because the probe is moved alongside the string that signifies when the damaged filament has been discovered.
A phrase about voltage detector pens.
Another methodology for fixing faulty ornamental mild strings (a number of tutorial YouTube movies exist that advertise) includes non-contact voltage detector “pens” to seek out the place within the string AC voltage is current, the place it’s not, and use this data to deduce the situation of a nasty bulb. Within the movies this methodology appears to work properly. Nonetheless, in stated movies the string being examined is invariably conveniently stretched out on a countertop with the wires separated and simply recognized as to the place they fall within the connection sequence from plug to bulb to bulb. This enables an amplitude-sensitive voltage probe to be selectively introduced close to a selected wire with out undue interference from different wires.
Sadly, many actual life mild string restore eventualities contain strings completely put in into decorations like bushes and wreaths. This prevents free entry and the next of interconnections, making testing tougher and indications from an amplitude-sensitive probe ambiguous. This largely defeats the utility of such probes on this context. Cue the Grinch!
The phase-sensitive detection methodology utilized by the FFFF, in contrast, is way much less susceptible to confusion from, and works regardless of, these problems.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. In all, a complete of 64 submissions have been accepted since his first contribution was revealed in 1974.
Associated Content material
