GB2117126A - Circuit testing device - Google Patents

Abstract

A circuit tester comprises a body 12 from which a first electrically conductive probe 14 extends and which has a second electrical probe 16 flexibly joined to the body. An oscillator generates a sound output in response to electronic switches, such as NOR gates; oscillator excitation only occurring when the resistance between the probes is less than a relatively low figure. An additional circuit is present having one of three states depending upon the AC voltage detected across the probes, the first circuit state occurring when the AC voltage is about zero, the second state when a certain voltage is exceeded and the third state when a higher voltage is exceeded, the circuit also controls an indicator for visually indicating the existence of at least the second and third states. <IMAGE>

Description

SPECIFICATION Improvements in circuit testing devices This invention relates to circuit testing devices.

It is very useful for the home-handy man or for someone testing or repairing electrical appliances to be able to check whether particular points in a circuit are live and if so at what an approximate voltage, and whether any particular lines are continuous.

Existing devices for making such checks are either extremely expensive and therefore only likely to be worth while to a professional or exceedingly simple and therefore not of great practical use. At the professional end of the scale, there is the well known "Avo meter" for checking voltage and continuity and that is very expensive.

At the simple end of the scale, screwdrivers having a built-in neon lamp one end of which the user can earth with his finger, will give an indication of an AC live point but that is all.

It is therefore an object of the invention to provide a relatively simple and inexpensive circuit testing device but one which has relatively wide ranging usefulness.

According to the invention there is provided a circuit testing device comprising a body from which a first conductive probe extends, a second electrical probe flexibly joined to the body, an oscillator capable of generating a sound output, electronic switch means controlling the excitation of the oscillator so that excitation only occurs when the resistance between two probes is less than a relatively low figure, circuit means having one of three states depending upon the presence and level of an AC power detected across the probe, the circuit means having a first stage when the AC voltage is below zero, a second state when a certain voltage exceeded and a third state when a second higher voltage exceeds, the circuit means controlling an indicator for visually indicating the existence of at least the second and third state.

In order to increase the versatility of the circuit tester, a neon lamp can be connected between a touch point on the device and one of the probes to that when the user touches the touch point with the probe in contact with a mains AC source, the neon lamp will be illuminated.

The overall circuit testing device can be relatively simple and of a compact size. In addition any circuit is desirably in the form of an integrated circuit chip.

The device preferably has its own power supply, e.g. a small battery, and this is used to power the oscillator. This can be a RC oscillator arranged with an electronic switch to open the oscillating circuit and so prevent oscillations or to close and allow the oscillator to function. The electronic switch can be one or more NOR gates whose opening and closing to allow the oscillator to function are controlled by the voltage drop across a resistor resulting when the battery power is supplied across the two probes. Thus when a relatively high current passes in the situation that the probes are connected to a continuous wire, then the voltage drop across the resistor will cause switching of the NOR gates and equally when there is a high resistance between the probes and so only a very low voltage drop, the NOR gates do not switch.

The circuit means can also comprise a number of electronic switches such as NOR gates which have a non-conductive state to control the illumination of light emitters comprising the indicator. Thus the light emitters can be arranged to be illuminated depending upon the level of the AC detected across the probes. The switching off of these gates can be controlled by rectifying the AC source detected across the probes and comparing that with the battery voltage to provide appropriate logic signals for the NOR gates.

A circuit testing device according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram illustrating the operation of the device; Figure 2 is an external view of the device; and Figure 3 is a diagram showing the truth table of NOR gates used in the circuit of Figure 1.

The circuit testing device 10 shown in Figure 2 of the drawings comprises an insulating body 12 from one end of which projects an electrically conductive probe 14. A second electrically conductive probe 1 6 having a sheath 1 7 of insulating material is connected via a flexible electrical lead 18 to an input jack 20 which is plugged into one of two inputs 22 and 24 on the body.

Within the body 20 is an electrical circuit 30 as shown in Figure 1. This includes three main parts, a first part 32 for detecting an AC source, a second part 34 for continuity testing and a third part 36 for testing the levels of AC voltage.

Turning first to the first part 32 for detecting an AC source, the circuit includes a high resistance resistor 38 in series with a neon lamp 40. The point E is connected to the probe 14 whilst the other side of the serially connected resistor 38 and lamp 40 is joined to a touch stud 42 set in the body 12 and insulated from the probe 14. This part 32 operates like a conventional current tester.

Thus, if the probe 1 4 is in contact with a live wire carrying AC and the user, whilst holding the insulating body 10, touches the stud 42 with his finger, a small current will pass through the resistor 38 and lamp 40 and the latter will light up. The high resistance of the resistor 38 prevents the user from being exposed to a high voltage. If of course the probe 14 is not in contact with a live wire, the lamp 40 will not illuminate.

In order to test for continuity or a short the part 34 of the circuit is used. This comprises a RC oscillator arranged so that when one of its controlling poles is open-circuited, oscillation will not occur and vice versa if it is short-circuited.

Two NOR gates (IC1 and IC2) are used to compose the RC oscillation circuit.

The frequency of oscillation of the oscillating circuit is determined by the values of resistance of the resistor 44 and capacitance of the capacitor 46.

One of the inputs of gate IC1, namely pin 1, is used as a controlling point. When pin 1 of gate IC1 is in contact with a logic "1" potential, for example the positive pole of a battery, the NOR gate circuit will not oscillate since the output at pin 3 will be logic "0" as seen from the truth table shown in Figure 3 where A corresponds to pin 1 of gate IC1, B corresponds to pin 2 and OUT corresponds to pin 3. On the other hand if pin 1 is in contact with a logic "0" potential, for example the negative pole of a battery, the NOR gate circuit can oscillate because the output on pin 3 will then be logic "1". Based on this the controlling point and the negative pole of a battery can be used for testing a short in any wire.If the wire has an open circuit, there will not be any oscillation and if the wire has a short circuit there will be oscillation.

Turning to Figure 1, the output pins 3 and 4 of gates IC1 and IC2 are connected across a ceramic buzzer 48 whilst the positive pole of a battery 50 is connected to pin 1 through a resistor 52. Also connected to pin 1 via a resistor 54 is the input 22 and, when measuring continuity, the jack 20 is plugged into the input 22 and not the input 24.

Thus, in effect, the probe 1 6 at the end of the flexible wire 1 8 is connected to the positive pole of the battery whilst the probe 14 is of course connected to the negative pole of the battery 50.

When two probes 14 and 1 6 are connected at the ends of a wire which is to be tested for continuity, it there is no continuity, i.e. the resistance is above say 100 ohms, there will be little or no current passing between the probes 14 and 16 and so the voltage at the input 22 will not significantly effect the voltage on pin 1 generated from the positive pole of the battery via the resistor 52. Thus, in effect, the pin 1 will remain at a logic "1" potential corresponding to the positive pole of the battery 50 and the NOR gate IC1 will have an output on pin 3 which is logic "0" so preventing any oscillation.By contrast, if the resistance between the probes 14 and 1 6 is very low, indicating a short circuit or continuous wire, the resulting current generated will cause a large voltage drop across the resistor 54 which will in turn reduce the voltage on pin 1 of gate IC1 to logic "0" and as a result the output pin 3 will switch to logic "1" and so allow the circuit 34 to oscillate. Oscillation will in turn give a sound output e.g. 800 Hz generated by the buzzer 48 which can be heard through the grille 56 in the body 12.

As can be seen, an on/off switch 58 is provided in series with the battery 50 together with a fuse 60.

To protect the overall circuit, the user needs to be warned not to touch a voltage AC source whilst conducting this continuity test. Accordingly, linked with the input 22 and actuated by the insertion or removal of the jack 20 is a switch 64 controlling the illumination of a red LED 66 in series with a current limiting resistor 68. When the jack 20 is inserted in the input 22 the switch 64 is closed and the LED 66 lights up to give this warning.

The circuit making up the third part 36 for detecting various levels of AC voltage, i.e. O, 1 1 OV or 220V, comprises two further NOR gates IC3 and IC4. When such a test is to be made, the jack 20 is removed from the input 22 and inserted into the input 24 so connecting the probe 1 6 to the circuit 36.

It will be seen that the input pins 12 and 13 of gate IC3 are connected together and so always maintained at the same logic state. Before any testing is started, these input pins 12 and 13 are at a logic "1" state because of their connection to the positive pole of the battery 50 through the resistor 70. Referring to the truth table shown in Figure 3, the output pin 11 will therefore be at a logic "0" state and so a green LED 72 in series with the output pin 11 will not be energised. At the same time the input 9 to the gate IC4 will also be at a logic "0" state since it is directly connected to the output pin 11. The input pin 8, on the other hand, will be at the logic "1" state, again by virtue of its connection to the positive pole of the battery 50 through the resistor 70 and further resistors 74 and 76.Therefore again referring to the truth table shown in Figure 3, it can be seen that the output pin 10 of that gate will be at logic "0" state and so a yellow light emitting diode 78 in series with the output will also not be illuminated.

When the probes 14 and 1 6 are now connected across an AC source, the resulting AC potential will be applied across the input 24 and the point E. The potential is rectified by a halfwave rectifier 80 and if the voltage is about 11 OV, the resulting voltage generated at the point X relative the point E will be --10V. By an appropriate choice of the resistance values of the resistors 70 and 74 it can be arranged that the pins 12 and 1 3 will have at least half the voltage of the battery (4.5V) so that they remain in the logic "1" state and so that the output pin 11 also remains in logic "0" state As a result the LED 72 will still not be illuminated.However the input pin 8 of the gate IC4 will now have changed to a logic "0" state and, since both pins 8 and 9 are now at a logic "0" state, the output pin 10 will convert to a logic "1" state and so allow illumination of the diode 78. The illumination of this diode 78'is visible against the marking 11 OV on the body 12 and so the user knows that he now has his probes 14 and 16 E connected across a source of AC voltage of about 11 OV.

When, on the other hand, the probes 14 and 16 are connected to a source of voltage of 220V the potential of the point X will reduce to a sufficiently low negative value that the input pins 12 and 1 3 reach a logic "0" state. Referring to the truth table shown in Figure 3, it can be seen that, as a result, the output pin 11 will become logic "1" state so allowing the diode 72 to become illuminated. In turn the input pin 9 of the gate IC4 now becomes the logic "1" state and inevitably therefore the output of pin 10 converts to a logic "0" state so causing the diode 78 to cease to be illuminated.

Thus the user can see that by the illumination of the diode 72 against the marking 220V, he now has a voltage of that level across the probes 14 and 16.

For convenience the four gates IC1 and IC4 are: provided on an integrated circuit such as for example that sold under the number MC 14001 B by Motorola or that sold under the number CD4001. A Germanium diode 62 is provided to protect the integrated circuit in case the battery is inserted with an incorrect polarity.

An advantage of the circuit 30 is that it can be very small and compact particularly if the NOR gates are provided on a single integrated circuit and so the circuit tester 10 can be quite small and convenient to hold and use. It does however give indications of voltages, continuity and whether particular points in a circuit are live in a very simple and effective way and so it is very convenient for use, for example, by the homehandy man. In addition the device 1 2 is relatively simple and so can be relatively inexpensive.

Claims (9)

1. A circuit testing device comprising a body from which a first electrically conductive probe extends, a second electrical probe flexibly joined to the body, an oscillator capable of generating a sound ouput, electronic switch means controlling the excitation of the oscillator so that excitation only occurs when the resistance between two probes is less than a relatively low figure, circuit means having one of three states depending upon the presence and level of an AC power detected across the probe, the circuit means having a first stage when the AC voltage is about zero, a second state when a first intermediate voltage is exceeded and a third state when a second higher voltage is exceeded, the circuit means controlling an indicator for visually indicating the existence of at least the second and third states.

2. A circuit testing device as claimed in Claim 1 in which a neon lamp is connected between a touch point on the device and one of the probes so that when the user touches the touch point with the probe in contact with the mains AC source, the neon lamp will be illuminated.

3. A circuit testing device as claimed in either preceding claim which has its own electrical power supply.

4. A circuit testing device as claimed in any preceding claim in which the oscillator is a RC oscillator arranged with an electronic switch to open the oscillating circuit and so prevent oscillations or to close and allow the oscillator to function;

5. A circuit testing device as claimed in Claim 4 in which the electronic switch comprises one or more NOR gates whose opening and closing to allow the oscillator to function are controlled by the voltage drop across a resistor.

6. A circuit testing device as claimed in any preceding claim in which the circuit means comprise a number of electronic switches which have a non-conductive state to control the illumination of light emitters comprising the indicator.

7. A circuit testing device as claimed in Claim 6 in which the electronic switches are NOR gates.

8. A circuit testing device as claimed in Claim 6 or Claim 7 in which the switching off of the electronic switches is controlled by rectifying any AC source detected across the probes and comparing that with the voltage of a power supply for the device to provide logic signals for the NOR gates.

9. A circuit testing device substantially as herein desicribed with reference to the accompanying drawings.