Simple Vibration Sensor with No Moving Parts.

by Forrest M. Mims III

Simple vibration and motion sensors can be made using a pendulum switch, mercury switch or one of many other methods. The circuit shown here uses no moving parts to detect vibration. Instead, a piezo speaker element is used as a sensitive vibration sensor.

How It Works

The piezo speaker is connected to the input of an op amp operated as a comparator. This operation is achieved by eliminating the usual feedback resistor between the output (pin 6) and the inverting input (pin 2). In operation, subtle vibrations cause the piezo element to generate a small voltage. The LED glows when the voltage exceeds that applied to pin 3 of the op amp by sensitivity control R2.


Figure 1. Circuit for the vibration sensor.

R2 controls the sensitivity of the circuit by controlling the voltage applied to pin 3 of the op amp.

Parts You Will Need

The following parts were used to assemble a breadboard version of the circuit:
IC1- LTC1050 op amp (1745688)
R1 - 3.9MΩ resistor (691711)
R2 - 1MΩ trimmer pot (42982 or similar)
R3 - 1KΩ resistor (690865)
LED - any red (333973) or green LED (34606)
Piezo Speaker - (1956776 or 1956784)

Miscellaneous: Perforated prototype board (616622), 9 volt battery, battery connector clip (216427), double-sided tape or 9 volt battery holder (105794), wire jumpers (JE10 Wire Jumper Kit 19290).

Note: While the components listed above were used for the prototype, substitutions can be easily made. For example, many different piezo speaker elements (without a built-in driver) can be used.

Prepare the Board and Install the Components

The circuit was assembled on a solderless breadboard and tested. When the circuit was operating properly, the components were transferred to a 1 x 3.5 inch (2.5 x 9 cm) board cut from a perforated prototype board (616622) and soldered in place.

If you want to make a permanent version of the circuit, you can follow your own parts layout, and you might consider installing the circuit in a small enclosure. Or you can simply copy the layout in Fig. 2 to make a trial version of the circuit.




Figure 2. Parts layout for an assembled version of the vibration sensor.

You can follow the steps below to duplicate the prototype circuit shown in Fig. 2 or you can proceed on your own.

1. The prototype board shown in Fig. 2 was cut along rows 35 and 47 and the cut edges were filed smooth. This provides a 1 x 3.5 inch (2.5 x 9 cm) board with an appropriate foil pattern on the back side for the parts layout shown in Fig. 2. Note that each hole in the board can be identified by its respective row number and column letter. Cutting the board along rows 35 and 47 will keep the row numbers but eliminate the column letters printed along the outer edge of the board. To use the alphanumeric hole indicators, you can print them along one edge of the cut board. Or use a large binder clip to temporarily attach the cut board to a section of board bearing the alphabet letters for each column.

2. Orient the board so that row 35 is along the bottom side. Insert the LTC1050 op amp IC into the top side of the board (without the foil pattern) so that pin 1 is in hole R42 and pin 5 is in hole O45. Bend pins 1 and 5 slightly outward to secure the IC in place.



Figure 3. Close up view of the components in the vibration sensor.

3. Insert trimmer R2 in the board as shown in Fig. 3 with the center pin at hole P36. Bend the pins slightly outward to hold R2 in place.

4. Insert R1 into holes Q41 and O41 as shown in Fig. 3. Note that its leads bridge pins 2 and 4 of the LTC1050.

5. Insert R3 into holes S45 and Q46 as shown in Fig. 3. Note that one lead goes to pin 7 of the LTC1050.

6. Check to make sure all components are properly positioned. Then use a low-wattage soldering iron with a well-tinned, small tip to solder in place all the pins and leads of the above components. Be careful to avoid applying too much solder.

Caution: Be sure to work in a well ventilated room when using lead solder.

7. Trim the leads of the resistors so that 1/4 inch (6 mm) or so remains for additional connections.

Caution: Wear eye protection when using a wire cutter to trim leads and pins.

8. Insert the LED into holes P46 and S46 as shown in Fig. 3. The lead closest to the flat spot at the base of the LED must be inserted into hole P46 (facing the right side of the board in Fig. 3). Solder the leads in place and clip off the excess.

9. Use a hobby knife or small drill to form a 1/8-inch (3 mm) hole in the board at hole S39. This hole is for the leads from the battery connector and piezo speaker (see Fig. 3). Check to make sure all four wires will easily fit through the hole.

10. Insert the battery connector leads through the top side of the board and tie them into a knot on the foil side of the board. Leave at least 3 inches (75 mm) of wire length on the top side of the board.

11. Flip the board over and trim the red and black leads to about 1 inch (2.5 cm). Carefully remove about 3/16 of an inch (5mm) from the ends of the red and black battery clip leads.

12. Solder the red lead to the leftmost pin of trimmer R2 (as viewed from the top side of the board) at hole Q37.

13. Solder the black lead to the end of R2's lead connected to pin 4 of the LTC1050 at hole O41.

14. Use wrapping or other small gauge insulated wire to connect the red battery lead at hole Q37 to the junction of R3 and pin 7 of the LTC1050 at hole Q46.

15. Use wrapping or other small gauge insulated wire to connect the center pin of R2 at hole P36 to pin 3 of the LTC1050 at hole P39.

16. Solder a short piece of bare wire between holes O37 and O39.

17. Attach the piezo speaker to the top of the board using double-sided tape, cement or 2-56 screws and nuts. Run the wires from the speaker through the battery wire hole.

18. Solder the red speaker wire to the red battery lead at hole Q37 or hole Q46.

19. Solder the black speaker wire to R1 at hole Q41.

20. Put on safety glasses and clip off all the excess wire lengths emerging from the back side of the circuit board.

21. Carefully check the circuit for any wiring errors. Remove any solder bridges between foil traces using braided solder remover.

22. After the circuit has been checked and any problems corrected, attach the battery to the board using double-sided tape or a 9 volt battery holder (see Parts List).


Testing the Circuit

Use a small screwdriver to rotate R2's shaft until the LED just turns off. If the LED stays either off or on, immediately disconnect the battery and check for wiring errors. When the circuit is working properly, tapping the piezo speaker or the entire circuit board will cause the LED to flash.

Going Further

This circuit can be made far more sensitive and used to detect seismic vibrations by replacing the enclosed piezo speaker element shown in Fig. 2 above with a bare piezo element (such as 1956784). The bare element should be attached to the circuit board with cement. The circuit board should be firmly mounted to a heavy base (a brick, concrete block, etc.) or to a fixed structure. A rectangular length of thin aluminum stock should be cemented to the upper surface of the piezo element so that the end of the bar is suspended in free space to form a horizontal pendulum. I tried this by mounting the end of an aluminum ruler to the upper surface of a piezo speaker attached to a circuit board that was mounted on a brick. The brick with the extended ruler was placed on a compacted caliche driveway. The LED flashed when a second brick was dropped onto the driveway from a distance of about 40 feet.


About Forrest M Mims III

Forrest M. Mims III has written more than sixty books about science, lasers, computers and electronics. Many of his books describe electronic circuits andprojects that he personally builds and tests. When he isn't writing books, he does various kinds of scientific research, writes magazine and newspaper articles, and teaches experimental science at the University of the Nations in Hawaii. For more information visit www.forrestmims.com.