ECT in Schools : Datasheets : Movement Sensor

What does it do?

Signals

The movement sensor is a switch. It is ‘on’ (closed) if the sensor is not moved. With the circuit arrangement suggested this gives a low output signal. If the sensor is moved (accelerated) the switch opens and the signal goes high.

How does it operate?

These notes apply to the Rapid movement sensors (Order codes 78-2088 – horizontal sensor, and 78-2104 – vertical).

Both sensors act as a normally closed switch. If they are moved the switch opens. They are quite sensitive to movement – shaking by hand opens the switch.

To work reliably, the sensor needs to be mounted in either the horizontal or vertical position, depending on the type.

Movement sensor circuit

Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. Note that there is no Livewire Symbol for a movement sensor - a push to break switch has been used.

In the suggested circuit diagram the movement sensor switch is in the lower part of a potential divider. In the normal switch subsystem the switch is in the upper part of the potential divider.
The result is that, when the sensor is not moved (and the switch is closed) the output signal is low. When the sensor is moved the output goes high.

The signal is only high while the sensor is accelerating, and this is usually only for a short time. Therefore the subsystem to which the sensor is connected must be able to respond quickly to this brief high signal.

Possible applications

An alarm system (detecting vibrations from an intruder)
A collision detector on a vehicle
A hand-held game that responds to movement

Making

How part of the PCB might look

If there is no symbol for the movement sensor in the PCB design package you are using then place pads in suitable positions (0.5” spacing for the horizontal movement sensor).

In the example on the left a 0.5” long resistor was used to mimic the dimensions of the movement sensor.

Testing

Make sure that the signal going out (on the green PCB track) briefly changes from low to high when the sensor is moved.

Fault finding

If there is a fault, check that:

The voltage at the ‘top’ of R1 is high (the supply voltage)
The voltage at the ‘bottom’ of the sensor is 0V.
The output signal is normally low

If there is a fault, check the tracks and solder joints.

Alternatives

The tilt switch, microswitch and optoswitch can also be used to detect movement.

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