NZ220217A - Pyro-electric sensor circuit - Google Patents

Description

" 1 ' '• f « ' 2202 17 O NEW ZEALAND PATENTS ACT 1953 No.: 220217 Date: 6 May 1987 COMPLETE SPECIFICATION "Improvements in or Relating to Alarms" I GRAYDON AUBREY SHEPHERD a New Zealand Citizen of -5 Severn-Street, Mt Eden, Auckland, New Zealand hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: OOfn <— 0 tL i Thio invention relates to an alarm particularly but not solely a pyro electric sensor movement alarm.

It is an object of the present invention to provide an alarm which will at least provide the public with a useful choice.

Accordingly the invention consists in an alarm including a dual element pyro electric sensor, amplifier means connected to said sensor, a first capacitor and a diode in series connected to the output of said amplifier means, an emitter follower across said diode, a second capacitor connected to ground charged through said first capaeitor and said diode, and trigger means to trigger the alarm when the voltage across said second capacitor reaches a selected trigger voltage, said trigger voltage being selected to be greater than the voltage expected across eaid second capacitor due to sensing of movement by one element of said pyro electric sensor and said second capacitor charging on increasing and decreasing outputs from said amplifier roeanB.

To thoBe skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limited.

One preferred form of the invention will now be described with reference to the accompanying drawings in which; Figure 1 1b a circuit diagram showing an alarm circuit suitable for uae in the alarm of the invention, and Figure 2 is a diagrammatic representation of a possible output from the amplifier means of the circuit of FiguuSe" l-j f»|i 2 5 JAN 19917 *1 yt o/ 220217 Referring to the drawings an alarm is provided as follows.

In a first aspect the alarm has a power input at 1 and alarm componentry which will be described further hereinafter.

Between the power input and the alarm componentry is a voltage regulation circuit. The voltage regulation circuit comprises two three terminal voltage regulation devices 3 and 4 which may be of known type. The voltage at the input 1 may be for example 13.8 volts DC and the three terminal voltage regulator 3 may reduce this to for example about 8 or 9 volts DC. This can then be reduced to for example about 5 volts by the three pin voltage regulator 4. At the stage of the first voltage drop there will be substantial attenuation of any ripple or transient supply voltage changes and at the second voltage drop at regulator 4 there will again be significant reduction in any remaining ripple and transient supply voltage changes. Thus there is a multiplier effect on the reduction of the ripple or transient supply voltage changes which results in severe attenuation of such changes and a significant reduction in the possibility of false alarms due to such occurrences.

Pyro electric detectors are also susceptible to false alarms caused by radio frequency interference for example from radio telephone transmissions which can occur for example from taxis police calls and the like.

In the circuit of Figure 1 a sensor such as a pyro electric detector 10 is included. The sensor has a gate 11 and drain 13, the gate 11 being taken to ground through a low value resistor such as IK ohm resistor 12. _ ?20217 Between the drain 13 and the gate 11 is connected a capacitor 14 of suitable value such as 0.1 microfarads.

Thus negative feedback is introduced between the drain and the gate. The effect of this is that "Miller effect" false alarms due to radio frequency interference are significantly reduced.

With sensors such as dual sensor pyro electric alarms the detectors can be positioned so that one sensor of a pair "sees" a different object to the other. This can happen for example at doorways, across re-entered corners in walls such as occur in an L shaped room and sometimes close to furniture or the like.

When this occurs one object such as a distant wall can be heated different to the other object such as for example a closer article of furniture or closer wall part. In particular this can happen where a window might be close to one or other of the articles or objects seen and thus the two sensors sense different heating patterns due to sunlight. It will be apparent that different heating patterns can arise in other manners.

With dual element pyro electric sensors the sensing of the different heating patterns can cause false alarms to occur. The dual elements 30 and 31 in the pyro electric sensor are usually connected in a back to back fashion so that essentially one sensor gives rise to a positive signal and the other sensor gives rise to a negative signal. In a non alarm situation the two signals cancel and the alarm does not trigger. In the circuit of Figure 1 the output from the dual element pyro electric sensor 10 is fed to an operational amplifier 20 and the o 2 6 OCT 198? 220217 quiescent output DC voltage at pin 7 of the operational amplifier 20 is set by a potential divider at pin 5 to approximately, for example, 1.75 volts DC. Positive and negative signal inputs to pin 6 will cause the output voltage to swing to ground or to approximately 3.5 volts DC positive. The output from pin 7 is taken to a potential divider formed by resistors R1 and R2 which in embodiment shown each have a value of IK ohm thereby reducing the voltage at point 21 to substantially half of the voltage at pin 7 of operational amplifier 20. Transistor TR1 acts as an emitter follower of any DC voltage appearing across capacitor C2 and thus the voltage at point 22 will tend to follow the voltage on the capacitor C2. If pin 7 swings high capacitor C2 will charge through the resistor R1 the capacitor CI and the diode Dl. Capacitor C2 therefore charges to a voltage somewhat less than 1.75 volts because of the presence of capacitor CI. This DC voltage also appears at the emitter of transistor TR1 since the emitter voltage tends to follow the base voltage. When the input signal at pin 6 causes pin 7 to swing to for example 0 volts that is to say when a negative pulse appears at pin 6 the left hand side of capacitor CI as shown in Figure 1 will move to 0 volts whilst the right hand will be at for example 1 volt DC due to the clamping action of transistor TR1 and previous charge voltage across capacitor C2.

When pin 7 again drives positive to for example 3.5 volts DC then the right hand side of CI will be at approximately 4.5 volts DC momentarily which will further charge caoacitor C2 to a o 260CTW* * 220217 higher voltage. This again raises the voltage at the base of transistor TRl and therefore the emitter of transistor TRl.

This causes the right hand side of capacitor CI to be clamped to a higher DC voltage. When pin 7 is again driven negative by an input signal at pin 6 then the left hand side of capacitor CI will be at say 0 volts while the right hand side is at say 2 volts. Figure 2 shows a possible pulse form at pin 7 of the operational amplifier 20 showing an initial positive signal followed by a negative signal as may be provided should movement occur across the areas seen by the two sensors in the dual sensor pyro electric sensing device.

When pin 7 swings positive 3.5 volts then the right hand side of capacitor CI will swing up momentarily to 5.5 volts DC. This further charges capacitor CI through the diode D1 so that capacitor CI attains a higher voltage level.

The circuit therefore functions as a type of staircase voltage generator across capacitor C2 which steps up on both positive and negative outputs from pin 7.

A threshold detector incorporated for example at pin 6 of timer 23 which may be for example an NE555 measures the level of the voltage across capacitor C2 and triggers to trip the relay 24 of the detector when the appropriate input signal have been received. The tripping being through pin 3 of the NE555 timer circuit.

In use as for example an intruder moves into a zone seen by the detector he will first cross a positive or negative element. Pin 7 swings positive for example but does not raise the voltage o 2 6 OCT 1989 220217 across C2 to a level sufficient to cause the alarm to trip.

This is during pulse 26 shown in Figure 2. As the intruder passes into a second zone a negative signal shown at 27 in Figure 2 appears at pin 7 (or pin 7 swings to ground). If this is the total intruder action then because of leakage within the pyro detector elements settling to DC occurs which lifts the right hand side of capacitor CI and charges capacitor C2 to a tripping voltage level. The tripping level is set at pin 6 to enable this occur thus for example the passage of one signal such as positive signal 26 is insufficient to trip the detector but the passing of two signals that is to say positive signal 26 and negative signal 27 is sufficient after settling has occurred to trip the alarm.

If however the intruder passes into a following positive zone the right hand side of capacitor CI is again lifted thereby charging capacitor C2 to the tripping voltage level.

Thus resistors R1 and R2 form a voltage divider to the right hand side of capacitor CI so that the voltage is raised only to say 1.75 volts (not 3.5 volts) and more positive and negative signals must be generated by the pyro electric detectors. This enables a pulse count like feature to be simply incorporated. By selecting the values of resistor R2 and resistor R1 the input voltage to capacitor CI can be made smaller than the 3.5 volts swing occurring at pin 7 of amplifier 20 so that capacitor C2 charges in smaller increments for each positive or negative output from pin 7 of operational amplifier 20.

When the trip level of capacitor C2 is reached capacitor C2 - 7 - O - b OCT 1989 220217 will trigger the following circuitry to release the relay and trigger the alarm.

Pin 6 of the NE555 timer senses the upper level or trigger voltage at capacitor C2 when this is reached and the output at 3 goes low and the relay drops out. When this occurs pin 7 of the timer 23 switches low and capacitor C2 is discharged through resistor R3 and pin 1. When capacitor C2 settles to its original level pin 3 of timer 23 goes high and relay resets after a selected time such as 3 seconds.

Thus it can be seen that at least in the preferred form of the invention an alarm is provided which has the advantage that the possibility of false alarms due to ripple and transient supply voltage changes is substantially reduced. Also the alarm is relatively insensitive to radio frequency interference and furthermore has circuitry which will tend to reduce the possibility of false alarms due to the two elements of a dual pyro electric sensor sensing surfaces having for example different heating patterns.

The circuitry described can be used for other purposes such as switching lights. The circuit is also advantageous over alarm detectors that count pulses. Such circuits trigger after a set number of pulses are received. This can lead to ease of masking of the detector. In practice the present circuit requires at least one positive and one negative pulse which reduces the possibility of false alarms due for example to uneven heating on spaced apart surfaces and also makes masking more difficult. Thus an installer can test the device simply by o ?. 6 OCT 198) » 220217 crossing a zone. If no alarm is triggered the the two elements within a sensor may be "seeing" different surfaces. This dual sequencing, i.e. a positive and negative signal in sequence substantially eliminates the requirements of pulse count so that only one zone need record an intruder to trigger an alarm rather than a number of zones as required by pulse count. Thus the limitations of pulse count (i.e. the ease of masking the sensor) when an intruder moves directly towards the sensor are substantially reduced. This is advantageous. i £ n r 6 OCT 198? o ro

Claims (5)

c ^ n '
1. WHAT I CLAIM IS l <" ° *" ' ';1-. An alarm Including a dual element pyro electric sensor, amplifier means connected to said sensor, a first capacitor and a diode in series connected to the output of said amplifier means, an emitter follower across said diode, a second capacitor connected to ground charged through said first capacitor and said diode, and trigger means to trigger the alarm when the voltage across said second capacitor reaches a selected trigger voltage, said trigger voltage being selected to be greater than (""*} the voltage expected across said second capacitor due to sensing of movement by one element of said pyro electric sensor and said second capacitor charging on increasing and decreasing outputs from said amplifier means.
2. 2. An alarm as claimed in claim 1 wherein said trigger voltage is set to a level between the voltages expected on said second capacitor due to two successive changes in output of said amplifier means.
3. 3. An alarm as claimed in either claim 1 or claim 2 further including a power input, and a voltage regulation circuit ^ between said power input and said dual element sensor, said voltage regulation circuit having a first voltage regulation component to cause a first voltage drop and at least a second . voltage regulation component to cause a second voltage drop so that in use the voltage from the input is dropped in two or more steps thereby causing substantial reduction in ripple and/or transient supply voltage fluctuations.
4. 4. An alarm as claimed in claim 3 wherein said regulation components are connected in series.
5. 5. An alarm substantially as herein described with reference to the accompanying drawing „ , dated this ^BAy OK A 0- A. J. PARK & SON oil pr?. •n ' 1T2 5 J AN 1991 ^j/ AOtNTS FOR TffHAPPCISNTS ■pet