The circuit discussed in this article is of a non-contact mains AC field detector which displays the presence of a mains AC field from a distance of more an 6 inches.
Locating Fault in AC Lines without Physical Contact
The circuit can be used for locating faults in house wiring without the need of making physical contact with the inner conductor of the wire and becomes useful in locating the breaks in a wire by pin pointing the area where the AC mains may be blocked due to a breakage.
The circuit is basically high gain non inverting amplifier which is configured using a few opamps and a few other inexpensive passive electronic components.
Just a couple of opamps have been incorporated here from the IC 324 for the required operations.
Looking at the figure we notice the following things:
The non-inverting input of the IC is grounded making the sensitivity of the configuration to the maximum.
Similarly a feed back loop created by connecting the output of the opamps to the inverting input helps to increase the gain of the set up many folds.
The input is applied to the inverting input 2 of the IC through a blocking capacitor.
The signals entering via the antenna is quickly picked up by the opamp inverting input and sent to the
preceding circuit for the required processing and amplification.
It may be interesting to note that the sensitivity of the design can be simply varied by changing the value of the feedback resistor R1, for maximum sensiticity this resistor can be omitted.
However this can make the circuit a bit unstable and might provide false results.
Second Series opamp Amplifier Function
The next stage includes another identical amplifier which is just the repetition of the previous input stage.
This stage has been included in order to make the response of the circuit instant and so that the circuit is able to pick even the slightest of RF or the AC field within a certain range.
In case the circuit is intended to be used for detecting mains phase only at touching proximity, the sensitivity may be reduced to the required levels or the second stage may be excluded from the design.
The LED connected at the output is used for displaying the presence of the AC field; an illuminated LED identifies the presence of the field while no light from it provides the opposite conclusion.
By connecting a 1V FSD moving coil meter at the output, the device can be used to detect and measure the average strength of the AC mains present in that particular vicinity.
R1 = 2M2,R2 = 100K,R3 = 1K,C1 = 0.01uFA1, A2 = IC 324