Piezo Film Vibra Tab Mass Sensor Problem

Michele

I'm trying to use a piezo Vibra Tab sensor. I've set up the circuit shown in the documentation and it works fine - with the sensor plugged directly into the circuit board. However, if I try to remote the sensor by more than about 12" of wire, I get a continual "triggered" indication. This sensor is totally useless if it can only be used directly on the board. Is it really so sensitive that the resistance of a foot of wire throws it off? My only idea is to remove the 220 ohm resistor from the circuit. Is there any reason I shouldn't do this?

Unsoundcode

The 220 ohm resistor is there to protect the Stamp from excess current and really should not be removed. You could try a pull down resistor of say 10K ohm, one end connected to the junction of P0 ( or whichever pin you are using as the input ) and the 220 ohm resistor and the other end to VSS.

Jeff T.

Chris Savage

Hello,

I don’t believe the sensor is the problem…It is the length of wire which is acting as an antennae. You may need to use some sort of front end on the sensor to be able to extend it out. Perhaps an op-amp of comparator and CAT-5 or other shielded cable will help you.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Chris Savage
Parallax Tech Support

Michele

Thanks to both Jeff and Chris for replying. The real problem was my lack of knowledge about piezo technology. This sensor seems to sit at an odd interface between the digital and analog worlds. Adding a pulldown resistor solved the problem, but only for large sensor deflections. To use this as a vibration sensor (which generates just a tiny output), I'm afraid there's probably no getting around the need to use a charge amplifier, as Chris suggests. That would no doubt also fix the wire length problem. Perhaps some mention of this could be made in the Parallax documentation for this sensor.

Learn something new every day, I suppose.