201305.pdf 43279KB May 08 2013 11:07:04 PM

designideasCLASSICSOriginally published in the April 26, 1990, issue of EDNBender senses shocksDon Sherman, Maxim Integrated Products, Sunnyvale, CA↘With the aid of a simple mountingsystem and some soldered-onweights, a piezoelectric “bender” candetect mechanical shocks. The bendercomprises a piezoelectric-ceramic elementbonded to a thin brass disc. Suchassemblies form the heart of many telephoneannunciators and wrist-watchor panel-mounted alarms.Depending on the mountingscheme, the bender can sense shocksin one axis (Figure 1a) or three axes(Figure 1b). For one-axis sensing, solderone edge of the bender to a mountingbolt. Opposite the mounting bolt,solder a weight to increase the bender’ssensitivity. A small hook affixed to themounting substrate limits motion sothat the brittle piezoelectric elementwill not crack.For three-axis sensitivity, solder oneedge to a mounting bolt as before. Atthe other edge, solder a flat-head boltthat points away from the mountingsubstrate. Use a pair of jam nuts toincrease the assembly’s polar momentof inertia. The jam nuts’ position determinesthe bender’s sensitivity.In both cases, apply your solderingiron as briefly as possible to the benderto avoid damaging the piezoelectricelement’s bond to the brass disc.Figure 2 shows a simple alarm circuit.Giving the bender a good smackwill develop several volts across R 1,the 10-MΩ resistor. The dual-timerIC, IC 1, will then pulse the outputalarm for one minute at a 1-Hz rate.The alarm has its own driver circuitand sounds a piercing 90-dB tone whenenergized.The bender and alarm are bothavailable from Projects Unlimited,Dayton, OH.EDNFigure 1 Solidly mounting one end of a piezoelectric “bender” and affixing a weight to the opposite edge transforms the benderinto a shock sensor. Here, “a” is sensitive to vertical-axis shocks only; “b” senses shocks in all three axes.Figure 2 The circuit sounds a 1-sec alarm whenever you give the bender a good smack.[ www.edn.com]May 2013 | EDN 55