This is an early NCV (Non Contact Voltage) detector that uses a ceramic PCB with printed resistors.
I wonder if they used a ceramic substrate to give high stability at the very low currents involved in sensing electric fields.
The quality of the construction was also the downfall of the early units, since they used a rugged metal body. That feature was pounced on by the department of highly improbable electrical accidents, as posing a risk to unskilled labour who might shove it in amongst random live metalwork.
The principle of these things is that a capacitively coupled field is amplified and then used to drive an LED. In this particular unit there is a six-gate schottky inverter used to filter AC (already done by the detection method) and then drive a charge pump to get a decisive LED indication of a valid detection.
I get the feeling that the person who designed this went by the book and built it out of standard logical sections, but in doing so may have overengineered the design slightly.
I also looked at a much simpler version which uses one resistor, three cheap transistors and an LED to achieve the same effect. The cheaper version is the most common these days, but it's worth noting that some other clone units have clearly been "inspired" by the circuitry of the original design.
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