This is a Forrest Mim's III Original!

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

Parts List:

IC, LTC1050CN8, DIP-8

1/4 watt resistor, 3.9M Ω

1/2 watt potentiometer, 1M Ω

1/4 watt resistor, 1k Ω

5mm red LED

Piezzo transducer, 90dB, 4500 Hz

9 volt battery clip

9 volt battery 1 8-pin IC socket

Metal 9V battery clip 1 PCB

Step 1: 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.

Step 2: LTC1050 Op Amp & Socket

U1- Take note of the orientation of the IC and IC socket by looking at the notch and matching the notch of the IC to the notch of the PCB. See Figure 1.

Step 3: Non-Polarized Resistor Color Code

R1 is a 3.9MΩ resistor that can be installed in any direction. (orange - white - green - gold) R2 is a potentiometer, solder all three leads on the board. R3 is a 1K resistor that can be installed in any direction, 1K (brown - black - red - gold)

Step 4: Piezo

Connect the red positive lead to the square pad inside the circle labeled PIEZO. Then connect the ground (black wire) to the circular pad next to it. Step 4 – LEDs: D1 - Connect the cathode (short side) leg of the LED to the square pad labeled D1. Be sure to have the cathode end (shorter lead) facing the side with the flat edge. See Figure 2.

Step 5: LED

D1 - Connect the cathode (short side) leg of the LED to the square pad labeled D1.

Be sure to have the cathode end (shorter lead) facing the side with the flat edge. See Figure 2.

Step 6: Battery and Attaching It All Together

U2- Connect the battery positive (red wire) to the "+" pad on the PCB board, and the ground (black wire) to the "–" pad on the PCB.

You can use double sided tape or hot glue to attach the piezo to the PCB.

Step 7: Testing the Circuit

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.

Step 8: Going Further

This circuit can be made far more sensitive and used to detect seismic vibrations by replacing the enclosed piezo speaker element with a bare piezo element. 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.