GB2204703A

GB2204703A - Microwave radiation detector

Info

Publication number: GB2204703A
Application number: GB08708486A
Authority: GB
Inventors: Damien Gerard Mcdonnell; Paul Bonnett
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1987-04-09
Filing date: 1987-04-09
Publication date: 1988-11-16
Also published as: GB8708486D0

Abstract

A microwave radiation detector includes two thermistors (14, 16), an electronic detector circuit (18) and a LED bar-graph display (20). One thermistor (14) is coated with a conducting layer (24) formed from a liquid suspension of colloidal graphite. A radiation-induced rise in temperature of this layer causes a difference in resistance between the two thermistors (14, 16) which is detected by circuit (18). Display (20) indicates the radiation intensity. An alternative to the display (20) would be an LED alarm circuit (50) (Fig 4 not shown). <IMAGE>

Description

MICROWAVE RADIATION DETECTOR This invention relates to a microwave radiation detector. In particular, it relates to detectors which detect microwave radiation intensity.

Microwave radiation detectors are known. Commercially available detectors include a crystal diode and bulky dielectric lenses.

However, these are unnecessarily complex and expensive for applica tions where only a qualitative indication of microwave radiation intensity is required. An example of such an application is the detection of hazardous radiation leakage from a microwave oven.

The use of prior art devices for monitoring of microwave ovens outside a factory or laboratory environment is unrealistic due to their size and expense.

It is the object of this invention to provide a microwave radiation detector of relatively cheap and simplified construction.

The present invention provides a microwave radiation detector including: (1) first and second electrically responsive temperature sensors; (2) a microwave radiation absorber in thermal contact with the first sensor; (3) an electronic circuit arranged to receive electrical signals from the sensors and to provide an output in response to sensor signals corresponding to differences in temperature between the first and second sensors.

The microwave radiation detector of the invention provides the advan tage that it includes relatively cheap and commercially available components, and is of simplified construction.

In a preferred embodiment the microwave radiation absorber is a film formed from a liquid suspension of colloidal graphite. The sensors themselves may conveniently be thermistors. The temperature difference between the first and second thermistors may be detected by an electronic circuit arranged to respond to a difference between the resistances of the two thermistors. The circuit may include an operational amplifier, the thermistors being connected to respective inputs of the operational amplifier.

The microwave radiation detector may have a LED bar chart display arranged to indicate an output voltage of the electronic circuit.

The microwave radiation detector may have an alarm circuit activated by the output from the electronic circuit to produce an alarm indication. The alarm circuit may include a latch arranged to maintain an alarm indication after activation thereof has ceased.

In order that the invention might be more fully understood, one embodiment thereof will be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a plan view of a microwave radiation detector of the invention with a section removed to facilitate viewing of internal detail; Figure 2 shows a schematic external view of the Figure 1 microwave radiation detector in use; Figure 3 is a schematic drawing of an electronic circuit included in the Figure 1 microwave radiation detector; Figure 4 is a schematic drawing of an alarm circuit and for use with the Figure 1 microwave radiation detector.

Referring to Figure 1, there is shown a plan view of a device 10 of the invention. A section has been removed as indicated at 12 to facilitate viewing of internal detail. Two thermistors 14 and 16, an electronic detector circuit 18, and a LED bar-graph display 20 are contained in a plastic container 22. Thermistor 14 is coated with a conducting layer 24 formed from a liquid suspension of colloidal graphite. Thermistor 16 is uncoated. Thermistors 14 and 16 are connected to the circuit 18 which itself is connected to the display 20. A display surface 26 of the display 20 is exposed for external viewing.

Thermistor 16 may be shielded from microwave radiation by local metallisation (not shown) of container 22.

Referring now to Figure 2, there is shown a schematic external view of the Figure 1 device 10 in use. The device 10 is held in the immediate vicinity of a microwave oven 28. Microwave radiation leakage from oven 28 is indicated by the display 20.

Referring now also to Figure 3, there is shown the electronic circuit 18 , contained within device 13, for monitoring output signals from the thermistors 14 and 16. The circuit t8 is a closed-loop operational amplifier configuration. A 1 Mn feedback resistor 31 is connected across an operational amplifier 32 between its non-inverting output 33 and its inverting input 34. Operational amplifier 32 is a commercially available type designated LM 355 and supplied by Radio Spares. It is powered by two 9 V batteries 35 and 36, which supply a total of 18 V. An offset potentiometer 37, connected between the offset terminals of the amplifier 32 has its slider 38 connected to a positive power rail 39.Thermistor 14 and a 50 n resistor 40 are connected in series between a node 41 of the positive rail 39 and the inverting input 34. The other thermistor 20 and a 100 n variable resistor 42 are connected in series between a node 43 of the positive rail 39 and a non-inverting input 44 of the amplifier 32.

A 1 Mn resistor 45 is connected between a common rail 46 and the non-inverting input 44 of amplifier 32. Any output signal appears across terminals 47 and 48, connected to the common rail 46 and the output 33 of amplifier 32 respectively.

Before describing the operation of the device 10 the operation of the circuit 18 will be described with reference to Figure 3.

Let the total resistance of the thermistor 14 and the resistor 40 in series be represented by R1, the total resistance of the thermistor 16 and variable resistor 42 in series be represented by R2, and the resistances of resistors 31 and 45 be represented by R and R' respectively. Circuit potentials to be mentioned later will be referred to the common rail 46. The potentials of the nodes 41 and 43 will be represented by E i and the potential of the output ter- minal 48 will be represented by Eo The potential at the inverting input terminal 34 will be represented by À and the potential at the non-inverting input terminal 44 will be eB.Also let the currents flowing through resistors 14, 16, 31 and 45 be IR1 IR2r IR and 1R' respectively. Therefore, using simple circuit theory and operational amplifier theory, assuming ideal circuit components, the following relationships hold: = = EB ; because of amplifier feedback (1) IR1 = IR ; assuming infinite impedance of the (2) inverting input of amplifier 32 1R2 = IR( assuming infinite impedance of the (3) non-inverting input of amplifier 32 From equation (2) the following relationships hold using simple circuit theory

therefore

From equation (3)

therefore

From equations (4), (5) and (1)

which gives

let a be defined by #o = aì; therefore

If the thermistors 14 and 16 are matched and have resistances RT1 and RT2 at respective absolute temperatures T1 and T2, the following relationship holds

where B is a constant depending on the properties of the thermistors 14 and 16.

In operation of the circuit 18 the variable resistance 42 is adjusted so that R1 equals R2 when thermistors 14 and 16 are at the same temperature, ie Eg is set to zero for this condition. If the temperatures T1 and T2 remain the same, Eo will remain zero, ie circuit 30 corrects for changes in local ambience. If thermistors 14 and 16 are a perfectly matched pair, resistors 40 and 42 may be omitted.

Let the resistance of resistor 40 be RA and the resistance of variable resistor 42 be RB. Then R1 = RT1 + RA R2 = RT2 + RB as RA and RB are arranged to be small compared to RT1 and RT2, and therefore: R1 t RT1 R2 " RT2 Moreover, since R2 is small compared to R':

therefore using (7)

Using (8)

let

therefore

If the difference in temperature of thermistors 14 and 16 is 10C and they are both near room temperature (250C), then using (10)

and B = 3390 was measured experimentally for specific thermistors 14 and 16.

Therefore ss = 0.038 as ss 1, 1, exp(-ss) s 1 - accordingly

assuming R = R' then a = 0.038, and as Ei = 18 V, eO = 342 mV.

The amplifier 32 produces an estimated output voltage of 342 mV for a difference of 10C in the temperatures of thermistors 14 and 16.

If thermistors 14 and 16 are mismatched and resistors 31 and 45 not accurately matched then this value will vary. For instance, if resistors 31 and 45 have 10% tolerances then the output voltage Eo may differ by as much as 20% from that calculated theoretically.

The mode of operation of device 10 will now be described. When microwave radiation is incident on conducting layer 24, the electric vector of the incident radiation induces ac currents in layer 24.

Consequently the layer 24 heats up and causes thermistor 14 to be at a higher temperature than thermistor 16. Electronic circuit 18 responds to this difference in temperature via the differences in resistances between thermistor 14 and 16. The output voltage from electronic circuit 18 is proportional to the incident microwave radiation intensity. The output is fed to the LED bar-graph display 20 which indicates the output voltage from circuit 18 and thus the microwave radiation intensity.

An alternative to the LED bar-graph display is an alarm circuit which would activate an alarm when a preset intensity level is reached. This would provide a warning of dangerous microwave radiation intensity levels.

A suitable alarm circuit 50 is shown in Figure 4. Alarm circuit 50 also includes a latching mechanism to retain an alarm state. It uses a comparator 52 which switches on an LED 54 in response to an input signal with a voltage above a fixed reference voltage (VREF).

A resistor 56 and a variable resistor 58 holds the inverting input 60 at the voltage VREF, which is preset by adjusting resistor 58.

An input 62 is connected to a non-inverting input 64. To provide latching of the output, an output terminal 66 is connected to the non-inverting input 64 via a switch 68. Circuit 50 is powered by the same source as circuit 18 of Figure 3.

Circuit 50 operates as follows. A signal applied to input 62 produces an output at terminal 66 if that signal has a voltage above VREF. The output from comparator 52 is fed to the non-inverting input 64 to retain an output at terminal 66. The LED 54 is switched on by a high output from comparator 52. To reset the device when the signal is no longer present at input 62, switch 68 is depressed.

This reduces the voltage at the non-inverting input 64 below VREF, and the output of comparator 52 falls.

An additional operational amplifier may be employed to amplify the signal from the circuit 18 if increased output is required.

Claims

1. A microwave radiation detector including: (1) first and second electrically responsive temperature sensors; (2) a microwave radiation absorber in thermal contact with the first sensor; (3) an electronic circuit arranged to receive electrical signals from the sensors and to provide an output in response to sensor signals corresponding to differences in temperature between the first and second sensors.

2. A microwave radiation detector according to Claim 1 wherein the microwave radiation absorber is a film, formed from a liquid suspension of colloidal graphite.

3. A microwave radiation detector according to Claim 1 wherein each of the first and second temperature sensors is a thermistor.

4. A microwave radiation detector according to Claim 3 wherein the electronic circuit is arranged to respond to a difference between the resistances of the first and second thermistors.

5. A microwave radiation detector according to Claim 4 wherein the electronic circuit includes an operational amplifier, with the first and second thermistors connected to respective inputs of the operational amplifer.

6. A microwave radiation detector according to Claim 1 including a LED bar chart display arranged to indicate an output voltage of the electronic circuit.

7. A microwave radiation detector according to Claim 1 and including an alarm circuit device arranged to provide an alarm indication in response to activation by the electronic circuit.

8. A microwave radiation detector according to Claim 7 and including latching means arranged to maintain the alarm indication after activation thereof has ceased.

9. A microwave radiation detector substantially as herein described with reference to and as illustrated in the accompanying drawings.