Electronic Design
Low-Cost Impact Sensor  Uses Piezoelectric Device

Low-Cost Impact Sensor Uses Piezoelectric Device

Although piezoelectric elements have been used for shock and vibration detection for some time, this circuit provides a low-cost alternative for solid-state general-purpose impact sensing. The circuit uses a standard piezoelectric device (PZT) to detect a mechanical shock or vibration in its vicinity (see the figure).

The output of IC1, and LM555 monostable, initially is low. When an impact flexes the PZT, it generates a voltage. The base of transistor T1 is biased using the PZT, so the transistor amplifies the sensor signal, which triggers IC1’s input (pin 2) and changes the IC’s state.

IC1’s output (pin 3) goes high for a finite time determined by the values of timing components R4 and C3. IC1’s output is routed to the inputs (pin 2 and pin 3) of IC2, a dual-LED and phototransistor opto-coupler in an eight-pin plastic dual-inline package (TLP621-2).

When IC1’s output is low, IC2’s first LED (across pins 1 and 2) lights and activates the first phototransistor (across pins 7 and 8). When the monostable’s output goes high, the first LED goes out and the second LED (across pins 3 and 4) lights, switching on the second phototransistor.

The result is a solid-state dual-mode output (N/C and N/O) from IC2 that can be interfaced with an external circuit. You may need to alter the values of R5 and R6 in some applications to optimize the switching in IC2.

After constructing the circuit on a Veroboard, enclose it in a suitable ABS cabinet. Then connect the PZT to the circuit using a short length of shielded cable. Glue a rounded rubber washer on the face plate of the piezo element and attach it to the required surface with the washer facing the surface so the PZT can flex to detect impacts.

Anoop’s Analysis

The author makes two incorrect statements.

The first involves the triggering of IC1 when pin 2 changes its state. Actually, the timer triggers only when pin 2 goes below VCC/3, not when the pin changes state.

Second, the author tells readers to “Glue a rounded rubber washer on the faceplate of the piezo element.” A rubber washer will dampen the shock and make the sensor less sensitive to impact and vibrations.

Next, there are four design issues.

First, because of the piezo element’s high output amplitude, there is a risk of base-to-emitter reverse breakdown in T1 during a negative voltage swing. A diode should be added with its cathode connected to the base of T1 and the anode to the emitter of T1 to protect the transistor.

Second, a resistor in series with T1’s base will protect it from any overcurrent condition in the base-to-emitter junction.

Third, because of timing capacitor C3’s high value (10 µF), when it discharges through pin 7 of the LM555, the discharge current can go as high as several amps and can destroy the open-collector transistor inside the IC. You can prevent this by adding a resistor in series with pin 7.

Finally, D1’s role is not mentioned. In fact, D1 is not necessary for the operation of the circuit and can be removed.

The Author’s Reply

I’ll start by addressing the incorrect statements.

First, the LM555 can be triggered by momentarily grounding its triggering terminal. However, in some applications, the 555 is triggered by a pulse. Generally, the current required for triggering is about 0.5 µA for a period of 0.1 µs. Triggering voltage ranges from 1.67 V (5 V VCC) to 5 V (15 V VCC).

Second, I agree. A rubber washer will dampen the oscillation and reduce the overall sensitivity. However, it may also prevent malfunction by eliminating unnecessary triggering. As an alternative, the piezo element can be moved to the surface with a suitable spacer at the bottom side of the sensor element.

Next, I’ll address the design issues.

First, the addition of a free-wheeling diode at the base of T1 is a good idea.

Second, in my opinion, a resistor in series with the base of T1 is superfluous. The current level is not very dangerous.

Third, a resistor in series with pin 7 of the LM555 is not necessary. Refer to its datasheets.

Finally, D1 was added to introduce a one-diode drop (0.7 V). This reduces the chances of a timer latch-up in some applications. One example is when a designer wants to use electromagnetic relays instead of IC2 at the 555’s output.


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