GB2129592A - Motion sensor - Google Patents

Abstract

A small, compact motion sensing device suitable for use as a portable alarm system comprises a piezo-electric bar which is mounted with a small weight so as to provide an output voltage when disturbed in any way. The output is amplified using a field effect transistor with an unusually large source resistor thus requiring a very small operating current. When triggered a piezo-electric buzzer driven by a transistor, coil and diode arrangement ensures high electrical energy to sound energy conversion. The sensitivity is adjustable over a very wide range using a potentiometer which does not affect the low power consumption. Time delays and an automatic reset can be incorporated for greater convenience of operation. The system can also be used to control other external alarms and devices.

Description

SPECIFICATION Motion sensor This invention relates to a small, compact motion sensing device suitable for use as a portable alarm system. Current alarm systems often suffer from one or more of the following disadvantages; 1) Unnecessarily large physical size.

2) Relatively large drain current on standby and sounding modes limiting battery life.

3) Operation may be limited to certain orientations, e.g. mercury tilt switches.

4) The mechanism if very sensitive may be delicate.

The present invention aims to substantially improve on the above limitations in an inexpensive way.

According to the invention a piece of piezoelectic or piezoceramic material with a very small weight so attached as to maximise sensitivity to movement in any direction, produces a voltage which is fed into a high gain low current amplifying system. Because the weight is very small no movement limiting device is necessary to prevent damage even under severe shock conditions. Subsequent internal (or external) electronic circuitry operates warning signals which could be audible or visible.

In one embodiment the piezoelectric material is in the form of a small bimorphic bar operating in a cantilever mode fixed at one end and with the small weight at the other.

The output is fed into a high impedance direct current amplifier constructed with a field effect transister with an unusually large source resistor giving extreemly small standby current. (around 2 microamp) (This figure is about ten times smaller than that found in other related devices.) All the following stages have virtually zero standby current which is maintained until a two volt threshold is reached. This allows variable sensitivity to be controlled simply by a potentiometer forming the load to the drain of the previous stage without changing the overall standby current.

The sounding unit is a piezo buzzer driven by a transistor and coil chosen to optimize the ringing effect at resonance. A diode is connected in series with the coil and the sounder is connected in parallel so that the resonant voltage swings are not clamped in one direction by the driving transistor. Time delays can be incorporated for the following purposes; 1) To inactivate the unit for a period of time after switching on to allow the operator to walk away.

2) To switch the alarm off and reset the unit after a period of time in order to save the battery and/or in case of false alarm.

3) To allow a short interval between activation and sounding of the alarm thus permitting the rightful operator time to disarm the system silently.

In another embodiment the complete circuit is mounted on one circuit board together with the sensor and sounder. This unit is placed in a small container together with the battery and a security switch operated by a key. This whole system can easily be fitted to musical instrument cases, brief cases, etc.

In another embodiment the system is used to operate further internal devices registering the fact that certain thresholds of motion have been exceeded, or further external devices such as paging or auto-telephone call systems, either directly by wire, or by making the previous embodiment sound at an ultrasonic frequency or by modulation of a radio frequency carrier.

1. A Motion sensing device using a piezoelectric sensor.

2. A Motion sensing device according to claim 1 wherein the sensor is mounted in a cantilever mode.

3. A Motion sensing device according to claim 1 or 2 wherein the sensor is weighted at one end to increase sensitivity.

4. A Motion sensing device according to any preceding claim wherein the output from the sensor is fed into a high gain amplifier using a field-effect transistor.

5. A Motion sensing device according to any preceding claim wherein the sensitivity is adjustable by means of a potentiometer.

6. A Motion sensing device according to any preceding claim wherein an alarm whistle or other device is activated by the system.

7. A Motion sensing device according to any preceding claim wherein the alarm whistle is a piezo-electric buzzer driven by a transistor and coil.

8. A Motion sensing device according to claim 7 wherein a diode is connected in series with the coil.

9. A Motion sensing device according to any preceding claim wherein various time delays and reset mechanisms are incorporated.

10. A Motion sensing device constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Motion sensor This invention relates to a small, compact motion sensing device suitable for use as a portable alarm system. Current alarm systems often suffer from one or more of the following disadvantages; 1) Unnecessarily large physical size. 2) Relatively large drain current on standby and sounding modes limiting battery life. 3) Operation may be limited to certain orientations, e.g. mercury tilt switches. 4) The mechanism if very sensitive may be delicate. The present invention aims to substantially improve on the above limitations in an inexpensive way. According to the invention a piece of piezoelectic or piezoceramic material with a very small weight so attached as to maximise sensitivity to movement in any direction, produces a voltage which is fed into a high gain low current amplifying system. Because the weight is very small no movement limiting device is necessary to prevent damage even under severe shock conditions. Subsequent internal (or external) electronic circuitry operates warning signals which could be audible or visible. In one embodiment the piezoelectric material is in the form of a small bimorphic bar operating in a cantilever mode fixed at one end and with the small weight at the other. The output is fed into a high impedance direct current amplifier constructed with a field effect transister with an unusually large source resistor giving extreemly small standby current. (around 2 microamp) (This figure is about ten times smaller than that found in other related devices.) All the following stages have virtually zero standby current which is maintained until a two volt threshold is reached. This allows variable sensitivity to be controlled simply by a potentiometer forming the load to the drain of the previous stage without changing the overall standby current. The sounding unit is a piezo buzzer driven by a transistor and coil chosen to optimize the ringing effect at resonance. A diode is connected in series with the coil and the sounder is connected in parallel so that the resonant voltage swings are not clamped in one direction by the driving transistor. Time delays can be incorporated for the following purposes; 1) To inactivate the unit for a period of time after switching on to allow the operator to walk away. 2) To switch the alarm off and reset the unit after a period of time in order to save the battery and/or in case of false alarm. 3) To allow a short interval between activation and sounding of the alarm thus permitting the rightful operator time to disarm the system silently. In another embodiment the complete circuit is mounted on one circuit board together with the sensor and sounder. This unit is placed in a small container together with the battery and a security switch operated by a key. This whole system can easily be fitted to musical instrument cases, brief cases, etc. In another embodiment the system is used to operate further internal devices registering the fact that certain thresholds of motion have been exceeded, or further external devices such as paging or auto-telephone call systems, either directly by wire, or by making the previous embodiment sound at an ultrasonic frequency or by modulation of a radio frequency carrier. CLAIMS

1. A Motion sensing device using a piezoelectric sensor.

2. A Motion sensing device according to claim 1 wherein the sensor is mounted in a cantilever mode.

3. A Motion sensing device according to claim 1 or 2 wherein the sensor is weighted at one end to increase sensitivity.

4. A Motion sensing device according to any preceding claim wherein the output from the sensor is fed into a high gain amplifier using a field-effect transistor.

5. A Motion sensing device according to any preceding claim wherein the sensitivity is adjustable by means of a potentiometer.

6. A Motion sensing device according to any preceding claim wherein an alarm whistle or other device is activated by the system.

7. A Motion sensing device according to any preceding claim wherein the alarm whistle is a piezo-electric buzzer driven by a transistor and coil.

8. A Motion sensing device according to claim 7 wherein a diode is connected in series with the coil.

9. A Motion sensing device according to any preceding claim wherein various time delays and reset mechanisms are incorporated.

10. A Motion sensing device constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.