Bender senses shocks
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Originally published in the April 26, 1990, issue of EDN
With the aid of a simple mounting system and some soldered-on weights, a piezoelectric “bender” can detect mechanical shocks. The bender comprises a piezoelectric-ceramic element bonded to a thin brass disc. Such assemblies form the heart of many telephone annunciators and wrist-watch or panel-mounted alarms.
Depending on the mounting scheme, the bender can sense shocks in one axis (Figure 1a) or three axes (Figure 1b). For one-axis sensing, solder one edge of the bender to a mounting bolt. Opposite the mounting bolt, solder a weight to increase the bender’s sensitivity. A small hook affixed to the mounting substrate limits motion so that the brittle piezoelectric element will not crack.
Figure 1 Solidly mounting one end of a piezoelectric “bender” and affixing a weight to the opposite edge transforms the bender into a shock sensor. Here, “a” is sensitive to vertical-axis shocks only; “b” senses shocks in all three axes.
For three-axis sensitivity, solder one edge to a mounting bolt as before. At the other edge, solder a flat-head bolt that points away from the mounting substrate. Use a pair of jam nuts to increase the assembly’s polar moment of inertia. The jam nuts’ position determines the bender’s sensitivity.
Figure 2 shows a simple alarm circuit. Giving the bender a good smack will develop several volts across R1, the 10-MΩ resistor. The dual-timer IC, IC1, will then pulse the output alarm for one minute at a 1-Hz rate. The alarm has its own driver circuit and sounds a piercing 90-dB tone when energized.
Figure 2 The circuit sounds a 1-sec alarm whenever you give the bender a good smack.
The bender and alarm are both available from Projects Unlimited, Dayton, OH.