GB2190254A - Condition responsive switching circuit - Google Patents

Description

GB2190254A 1 SPECIFICATION Two sensor terminals S 1, S2 are con nected,

respectively, to a positive supply rail A control unit 12 (connected to a battery power supply 7) and the base of transistor This invention relates to a control device. 70 A load 9 is connected between the collector According to the invention in a first aspect of P.N.P. transistor 5 and a negative supply there is provided a control device comprising rail 13 connected to the supply 7. A further sensing means for sensing the presence of a low-power load illustrated as a lamp 11 may, conductive liquid and/or an inductive field and optionally be provided, connected between the producing an amplified switching signal when 75 emitter of transistor 3 and the negative rail.

the liquid or field is sensed and switching In use, the presence of a conduction path means responsive to the amplified switching between terminals S1, S2 will cause a current signal for connecting a power source to a of flow through the base of transistor 1 which load. will cause the P.N.P. transistor 5 to be According to the invention in a second asswitched on by the cascade effect of the Dar- pect there is provided a control device com- lington pair 1, 3, thus connecting the battery prising first and second cascade connected power supply 7 to the load 9 (and also to the transistors responsive to an input signal ap- load 11, through the emitter base junction of plied to the base of the first transistor to proP.N.P. transistor 5 and transistor 3). Removal vide an amplified signal, the input signal being 85 of the conduction path causes the device to indicative of a predetermined sensed condi- switch off.

tion; and a switching transistor responsive to In a preferred application, the sensor termi the amplified signal to provide a switched out- nals S1, S2 are arranged externally of the de put. vice and disposed so that if a conductive Preferably the second transistor provides a 90 liquid is disposed between terminals S1, S2, a further switched output. current will flow acuating the circuit. The pres Preferably the sensed condition is the pres- ence of the Darlington pair 1, 3 provides the ence of a conductive liquid between a first significant advantage that only very small cur sensor terminal connected to the base of the rents are required to actuate the circuit. Thus, first transistor and a second sensor terminal 95 the circuit may be actuated not only by rela arranged to be connected to a voltage supply. tively good water-based conductors such as Preferably the first and second sensor termi- sea water but also by water having very small nals are arranged to act as aerials so that the levels of impurities, for example de-ionised tap device is further responsive to the presence of water or reservoir water.

an inductive field, the presence of said field 100 In a particular application, the device may be providing a further sensed condition. arranged to actuate a light or radio beacon 9, Alternatively the device may include damp- and may be attached to a life jacket as a ing means for example a capacitor, to prevent distress device which will switch on automati actuation of the device in response to a said cally when water connects the sensor termina- inductive field. 105 tor S1, S2.

In a preferred application, the device is ar- Optionally at least one diode 21 (e.g.

ranged to connect a power supply to a load IN4001) may be provided for reducing the vol which may comprise a light source or a radio tage level applied to the loads 9,11.

beacon which is/are switched on when the In a further application, the circuit may be sensor terminals come into contact with 110 arranged so that the terminals S 1, S2 act as water. aerials, responsive to an inductive field. Due

An embodiment of the invention will now to the sensitivity of the circuit, such inductive be described by way of example with refer- field need only be of sufficient strength to ence to the accompanying drawings in which: develop a very small current for the Darlington Figure 1 is a circuit diagram of an embodi- 115 pair 1, 3 to switch the transistor 5. Thus the ment of the invention; circuit can be arranged to be responsive to Figure 2 is a circuit diagram showing a mo- the inductive field developed by a human or dification to the embodiment of Figure 1; an animal or the field developed by an active

Figures 3A and 313 show a general view of power circuit. In this respect, the device has an embodiment of the invention, with Figure 120 particular application for e.g. alarm systems as 313 being a sectional view and Figure 3A be- a trigger.

ing taken in the direction of arrow A of Fig. Figure 2 illustrates a modification to the cir 3B. cuit of Figure 1 designed to prevent actuation With reference to Figure 1, a circuit diagram of the circuit due to inductive field effects, of an embodiment of the invention is shown. 125 when these are undesirable for a particular ap The device includes a Darlington pair of plication. Similar components are represented N.P.N. transistors 1, 3 (e.g. BC109). The com- by corresponding reference numerals.

monly connected collectors of transistors 1, 3 In Figure 2, a capacitor 19 is provided be are connected to the base of a P.N.P. power tween the base of transistor 3 and the nega- transistor 5 (e.g. BC327). 130tive power supply rail 13. The capacitor is 2 GB2190254A 2 preferably a silicon disc capacitor and prevents circuit coupled to one of said electrodes and the sensor terminals S1, S2 acting as aerials. switching means coupled and responsive to The capacitor 19 charges up from the d.c. the ampification circuit, for connecting the supply and then forms a -dead-fall- against power source to a load upon sensing of a possible actuation of the device. 70 said liquid and/or field at said electrodes.

The value of the capacitor 19 need only be 2. A device as claimed in claim 1 wherein very small (e.g..001 micro-farad, 1 kV type) the amplification circuit includes a pair of cas so that the power dissipation of the capacitor cade connected transistors.

19 is virtually negligible. 3. A device as claimed in claim 1 or claim 2 Also, in Figure 2, an alternative load ar- 75 wherein said switching means includes a bipo rangement is shown in which a lamp is pro- lar transistor.

vided as load 9, with the second switched 4. A device as claimed in any one of claims output, via transistor 3, being connected to 1 to 3 wherein the amplification circuit is fur ground and not used. ther arranged to connect the power source to In Figure 3 a practical embodiment of the 80 a further load having a lower power require device of Figures 1 or 2, for use as a water ment than the load connectable to the power sensor 22 is shown, in which the main com- source via the switching means.

ponents are mounted on a circuit board 23 5. A device as claimed in any one of the encapsulated in resin and retained, together preceding claims further comprising damping with battery cells 7 within a housing 25. The 85 means coupled to the amplification circuit for space around the cells and circuit is filled with preventing actuation of the device in response 916 RTV Silastic. The housing 25 is sealed to a said inductive field.

with a thermoset plug 27 from which three 6. A device as claimed in claim 5 wherein insulation covered pins 13, 15, 17 project. Pin the damping means comprises a capacitor 13 is connected to the negative voltage sup90 connected to said amplification circuit.

ply rail with pins 15, 17 being connected, 7. A control device as claimed in any one respectively, to the collector of transistor 5 of the preceding claims wherein the power and the emitter of transistor 3, these pins source comprises at least one battery cell and providing the switched outputs. The projecting the device further comprises a housing con- pins 13, 15, 17 are arranged for connection 95 taining the cell, amplification circuit and to a further unit which includes the loads 9, switching means, the first and second elec 11. Terminals S1, S2 are also provided trodes projecting from the housing.

through the plug 27 and are arranged flush 8. In combination, a control device as with surface 29 thereof. In use, the unit in- claimed in any one of the preceding claims cluding the loads 9, 11 is attached to the 100 and a load connected to the switching means.

sensor 22 so as to spaced from surface 29, 9. A combination as claimed in claim 8 which is open to the surroundings. Thus, if wherein the load comprises a light source for the sensor 22 is immersed in water, a con- a radio becon.

nection will be formed between terminals S1, 10. A combination as claimed in claim 7 or S2 thereby actuating the circuit and providing 105 claim 8 further comprising a further load con power to the loads 9, 11. nected to said amplification circuit.

Whilst the device has been shown con- 11. A combination as claimed in any one of nected to a battery source 7 this is not to be claims 8 to 10, wherein the load or loads are construed as limitative and the device may be retained within a unit separate from the de- used connected to a mains supply. In such a 110 vice, the device and unit being electrically con use, the device may preferably include a LED nected via connectors projecting from the de indicator. Also, it will be appreciated that with vice.

an alternating current source, the capicitor 19 12. A control device substantially as herein would not provide effective damping against before described with reference figure 1 and inductive field interference. However, the pro- 115 figure 3 or figure 2 and figure 3.

vision of an alternative damping means would 13. A combination of a control device and a be within the scope of one skilled in the art. load or loads substantially as hereinbefore de Also, whilst specific applications of the de- scribed with reference to figure 1 and figure 3 vice have been mentioned, these are not to or figure 2 and figure 3.

be construed as limitative and the device is Printed in the United Kingdom for applicable for other applications for which the Her Majesty's Stationery Office, Dd 8991685, 1986, 4235 sensing and switching function as disclosed is Published at The Patent Office, 25 Southampton Buildings, applicable. London, WC2A l AY, from which copies may be obtained-

Claims (1)

1. A control device comprising sensing means for sensing the presence of a conductive liquid andlor inductive field, the sensing means including first and second electrodes coupled to a power source, an amplification