GB2045493A - A trigger circuit and method for detecting changes in light levels in the field of scan of a camera - Google Patents

Abstract

A trigger circuit and method for detecting changes in light levels in the field of scan of a camera is provided with a trigger circuit adapted for actuation on a sudden change in light levels in the field of scan of a camera, which circuit comprises means for monitoring a signal from said camera to establish a base value and trigger level detection means adjustable to said base value and adapted for actuation on a disturbance of said base value to generate an alarm signal. The camera may be merely a lens in a light field enclosure having a light sensitive device such as a photo-cell situated at or adjacent the focal point of the lens. The trigger circuit device is particularly useful in security systems since it can be utilized to switch in a video camera or other alarm device on any variation in average light values occasioned, for example, by an intruder crossing a space. <IMAGE>

Description

SPECIFICATION A trigger circuit and method for detecting changes in light levels in the field of scan of a camera The present invention relates to a trigger circuit and a method for detecting changes in light levels in the field of scan of a camera, and particularly to a method of detecting a disturbance in a field of scan of a TV camera and camera device, said device being a light dependant cell associated with a camera lens.

Security systems utilizing a TV camera and a TV monitor therefor are well known and are used in commercial and domestic premises.

When switched on the TV monitor shows a picture representing a field of scan of a TV camera and persons or objects entering this field are readily seen by a person watching said monitor. Unfortunately the system is useless if the monitor is switched off, or no one is actively watching it.

The present invention seeks to overcome this problem by utilizing a change in the picture signal from the camera to actuate an alarm mechanism.

According to the present invention, therefore there is provided a method of detecting a change in light levels in a field of scan of a camera as defined, which comprises monitoring a signal from said camera and adjusting a trigger level detector accordingly to give a base signal value, generated by a change in light levels entering the camera actuates the level detector to give an alarm signal.

The term "camera" as used herein is to be understood to include a light-sealed enclosure having a lens and means for generating a light dependant signal therein.

In a preferred embodiment the sensitivity of the circuit is adjusted to actuate on a sudden change in light levels.

In a further feature of the invention there is provided a trigger circuit adapted for activation on a sudden change in light levels in a field of scan of a camera as herein defined, which circuit comprises means for monitoring a signal from said camera to establish a base value, and a trigger level detector means adjustable to said base value and adapted for actuation on a disturbance of said base value to generate an alarm signal.

The invention also includes a TV camera incorporating and/or utilizing such a trigger circuit and to security systems incorporating such a TV camera. It will be appreciated that said change may be caused by an increase or a decrease in light entering the TV camera.

Alternatively said camera may comprise a lens and a light dependent cell, said cell acting to provide said signal for monitoring.

Accordingly, the trigger circuit may comprise a voltage level detector, an amplifier and an adjustable trigger level detector.

Such a circuit detects the voltage (via switch SW1) from a TV camera, monitor or light dependent resistor LDR1 associated with a camera lens, which voltage is representative of a picture signal which is proportional to the light level from light and dark areas in the field of scan entering the said camera.

In an alternative embodiment a base signal value is derived from one or more discrete spaced positions (usually four) on a monitor screen, said value being stored in a register.

After each picture formation a comparison value is fed to a comparator means provided in association with said register thereby each successive value may be compared with said base signal value. Where a disturbance is of sufficient magnitude said comparator initiates an alarm signal. This system has an advantage that the base signal value can automatically be updated at any predetermined interval to take account of average light values (where for example the system is used outside). So long as there are no sudden changes in light levels therefore, the circuit can adjust to average light values automatically without an alarm being operated. A sudden change in the level of light entering the camera can trigger the circuit independently of a TV monitor if desired; ie: the circuit may be actuated even when the monitor is switched off.

The trigger circuit may actuate an alarm directly which may be audible or visual and may be in addition to the TV monitor. Alternatively said circuit may actuate a video tape recorder or may be utilized to automatically switch on a TV monitor or to switch to another channel (the camera channel) a TV receiving set already receiving a network transmission whereby the TV picture is only displayed when the TV camera picks up a change in light levels.

As will be appreciated, the value of the signal generated by the base picture signal will depend upon the light value in the area used by the camera. Thus, the circuit described above is suitable mainly for applications where the field of scan is lit by artificial light of a substantially constant value.

Where the camera is used in daylight, (eg: in a porch), it may be necessary to include compensator means in or adjacent the camera to adjust the trigger level detector means to accord with the exterior light values so that only sudden changes in light values entering the camera actuate said trigger circuit. In the absence of such a compensator means either the level detector must be constantly manually adjusted at dusk and dawn or the trigger circuit will be actuated merely by the overall change in light values.

Such a compensator means may include a light dependent resistor (LDR2) and a comparator for use therewith. Whereby a signal from said compensator means acts to adjust the base picture value in accordance with an exterior light value.

One embodiment of the invention will now be described, by way of example only, with reference to the accompanying circuit diagram; it will be appreciated that since an embodiment of this device works on a low frequency video signal as do closed circuit TV cameras and TV monitoring systems, if the device is used with UHF devices such as domestic TV adjustments must be made accordingly. Where a closed circuit TV camera a separate RF unit is used with a domestic TV no modifications are necessary.

In the device of the present invention a filter and a level detector, are utilized to gauge the light values of light and dark areas reaching the TV camera. The filter is used to by-pass the unwanted time pulsing signals and allow the picture signal to be amplified by transistors T1 and IC1. The resultant output is then fed into a level detector IC2 which is adjusted by controls P1 and P1A to cause IC3 to trigger when an increase of "white" (light) information is received beyond a preset basal level set by control P3.

The opposite and so far unused polarity of the picture signal, which is "black" (dark) information from IC2 is fed into IC4. This will trigger when an increase of dark information is received beyond a second preset basal level set by control P4. The triggered output from 1C3 may feed into T3 which in this embodiment illuminates a bulb B1 or operates a relay to actuate any electrical or warning device.

Alternatively the triggered output from IC4 because it is of the opposite polarity to that of IC3, is fed into an Inverter T2 and then into T3 to operate the bulb or relay.

Controls P1 (fine) and P1A (coarse) adjusts the basic sensitivity of the device to give a level on the meter Ml. This indicates that a workable output level from IC2 will be obtained when a change in signal information is received from the TV camera.

The control P2 and the light dependant resistor LDR2" are required in the circuit when no substantially constant light is available to illuminate the camera surveillance area. To set up the LDR2* circuit the control P2 is adjusted in minimum light conditions so that M1 registers a null point. Once set up the LDR2" circuit automatically adjusts the basic sensitivity to the slowly changing light levels received from dawn until dusk (for example). A faster change in light levels will trigger level detector IC3 and IC4 and an alarm will operate.

In an alternative a flip-flop circuit comprising additional components, Diode D2, capacitor C1 and switch SW2" may be added to lock on the triggered signal feeding B1 for a length of time predetermined by the value of capacitor C1. Alternatively the alarm may be locked on indefinitely by closing switch So2*, whereby the alarm will continue to operate until reset, usually manually.

In operation the circuit functions as follows:- an increase in "white" camera signal information triggers 1C3 and its now changed polarity output signal feeds through integers D1, D2 and C1 back to its input, causing the IC to remain triggered on. Alternatively the flip-flop circuit comes into operation again when IC4 is triggered by an increase in "black" signal information. The signal which feeds into T2 is routed through D2 and C2, reinitiating the beforementioned sequences of events.

This device enables a security guard for example, to be freed from the necessity of constant watch on his TV monitors since the trigger circuit of the present invention will actuate an alarm if there is anything to be seen. Further, the alarm signal output can be used to operate a relay which may switch on any other electrically operated security device.

Such security devices may include a video recorder, a TV monitor or monitors, or operate a land-line to summon help.

Further, the trigger circuit of the invention may be used to automatically switch the standard (domestic) TV set onto a monitoring channel even when the set is tuned to a broadcasting station. This means that an untoward movement in a field of scan of a TV monitor can automatically be drawn to the attention of the person watching the programme from said broadcasting station.

Furthermore, in broadcast network programmes it is usual to arrange that the signal values of commercial advertisements are of a higher value than those of the network programmes. These signal values may be utilized by the circuit of this invention to switch off or indeed on a video recorder recording from a TV receiver, when said advertisements are broadcast; thus allowing continuity of recording. In this instance the circuit is incorporated in the circuitry of the video recorder, preset to the desired signal value and arranged to automatically cancel the alarm signal when the average signal value has fallen from the high level.

Finally, the trigger circuit of this invention has a particular application in the monitoring and control of laboratory animals for use in in vivo experiments in medical research. In such experiments it is usual to use animals who are trying to operate a trigger bar on a given stimulus (usually a light associated with food).

The training of these animals, usually rodents is time consuming and thus expensive, Thus, when, for example, a group of rodents is used to test the efficacy of a narcotic, it is important that precise sleeping times are recorded.

At present rodents awaking are given an intermitent light stimulus, and if sufficiently awake will operate a trigger bar to receive food thereby establishing their return to consciousness. By the circuit of this invention with a camera, an alarm can be actuated, or clock stopped, as soon as the first significant movement of the rodent occurs. This system is not only more accurate, but also much cheaper as the rodents do not have to be trained before the experiment can be performed.

It will be appreciated that the invention encompasses each and every one of the foregoing particular embodiments.

Claims (11)

1. A method of detecting a change in light levels in a field of scan of a camera as defined, which comprises monitoring a signal from said camera and adjusting a trigger level detector accordingly to give a base signal value, the arrangement being such that a disturbance of said basal value generated by a change in light levels entering the camera actuates the level detector to give an alarm signal.

2. A method according to claim 1 wherein the sensitivity of the trigger level detector is adjusted to actuate upon a sudden change in light levels.

3. A method according to either of claims 1 or 2 wherein a compensator means is provided to adjust the trigger level detector to the average light values outside the camera.

4. A method according to any one of the preceding claims wherein the signal for monitoring is desired from a camera comprising a photo-cell disposed at, or adjacant, the focal point of a camera lens.

5. A method according to claim 4 wherein said monitored signal is used via the trigger detector to actuate a separate TV camera.

6. A method of detecting a change in light levels in a field of scan of a TV camera and monitor assembly which comprises deriving a base signal from one or more discrete spaced portions on a monitor screen, storing said value or values in a register, feeding a comparison value from said TV camera and said base signal to a comparator means provided in association with said register thereby to compare each successive comparison value with said base signal value and when said values differ by more than a predetermined amount, generating an alarm signal.

7. A method according to any one of the preceding claims wherein the alarm signal is utilized to switch a video camera and/or recorder.

8. A trigger circuit adapted for activation on a sudden change in light levels in a field of scan of a camera as herein defined, which circuit comprises means for monitoring a signal from said camera to establish a base value, and a trigger level detector means adjustable to said base value and adapted for actuation on a disturbance of said base value to generate an alarm signal.

9. A trigger circuit according to claim 8 wherein compensator means are provided to adjust the trigger level detector to the average light values outside the camera.

1 0. A trigger circuit according to either one of claims 8 or 9 wherein the circuit comprises a voltage level detector, an amplifier, and an adjustable trigger level detector.

11. A method of detecting a change in light levels in a field of scan of a camera as defined substantially as hereinbefore set forth.

1 2. A method of surveillance which comprises utilizing a camera as defined to detect a change in light levels substantially as hereinbefore set forth.

1 3. A trigger circuit substantially as herein before set forth.

1 4. A trigger circuit substantially as hereinbefore set forth with reference to, and as illustrated in, the accompanying drawing.