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Design IdeasJanuary 16, 1997 |
Machine vibration activates hour meter
Battery-powered digital hour meters commonly monitor how long a machine is in operation by detecting when the machine is supplied with power. However, it is often difficult or dangerous to connect a battery-powered hour meter to a line-powered machine. When the machine being monitored produces de-tectable vibrations, you can use the circuit in Figure 1, which doesn't require any power-line connections. The circuit uses just one IC and allows you to turn a battery-powered hour meter on and off according to a machine's vibration.
An off-the-shelf, 1-in.-diameter piezoelectric device, such as an alarm beeper or sound generator, detects machine vibrations. Almost any device from 1 to 1.7 in. in diameter works, but the larger diameter parts provide more sensitivity. Also, slightly bending the flat edges of the piezo device to form a thin dome enhances vibration sensitivity. You can then solder the bent device directly to a pc board, which you can then glue to the machine. The fiberglass epoxy pc material provides good electrical isolation.
As Figure 1 shows, the signal from the piezoelectric device drives a simple amplifier consisting of one section of a CD4069UB CMOS hex inverter. You can reduce the value of the 22-megohms feedback resistor to decrease vibration sensitivity. A diode, resistor, and capacitor rectify and filter the output of this stage. The resultant signal connects to a second amplifier stage, which acts as a logic buffer. The output from the second stage drives the digital hour meter with a 3V-to-ground signal swing. You can obtain the 3V to power the circuit from two #76 silver-oxide button cells. Because the circuit draws only about 2 µA, the two cells should power the circuit for about 5 years.(DI #1978)
Figure 1 |
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| 1An IC and a piezoelectric device sense machine vibration to turn a battery-powered hour meter on and off. |
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