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This tester can detect leakage from microwave oven above a certain value (among its other uses). It has only resistor, transistor (darlington), LED and two batteries. As far as I understand, it can detect hot wire without making a contact with the wire thanks to the capacitive coupling effect.

But, how does this continuity/mains tester detect microwave oven RF?

  • 3
    \$\begingroup\$ Short answer: It's a radio. You could hear the 50/60Hz hum of the mains modulated on the oven RF if you replace the LED by a speaker. It doesn't have a tuned circuit but that doesn't matter at all - you aren't interested in filtering out anything but 2450MHz. The piece is so insensitive, it won't light up by any waves coming in <1W. So if the LED lights up the culprit could only be the leaking microwave oven you hold it at. \$\endgroup\$
    – Janka
    Commented Dec 27, 2016 at 19:51
  • \$\begingroup\$ I understand.But how does it detect that RF waves?Does the metal's leg of the transistor base(or other component metal's legs),acts as an antenna and turn the 2.4Ghz electric wave that they sense into current that flows through the circuit(amplifier...),my body and to the ground,also,by capacitive coupling effect? \$\endgroup\$
    – xchcui
    Commented Dec 28, 2016 at 12:29
  • \$\begingroup\$ For AC currents, capacitors are complex value resistors. Capacitive coupling is a term used with DC and very low frequency AC only. In the GHz range, anything is a complex value resistor. Anything. That's how it heats your meals. See yourself as a complex value resistor. \$\endgroup\$
    – Janka
    Commented Dec 28, 2016 at 21:19
  • \$\begingroup\$ Ok.But what about the way that this tester received the RF electromagnetic waves.It doesn't have any antenna,what part of the tester is received those RF wave,in order to turned them into current? \$\endgroup\$
    – xchcui
    Commented Dec 29, 2016 at 11:36
  • 4
    \$\begingroup\$ You don't need a special antenna to receive RF waves. Any conductor longer than lambda/4 makes a great antenna, but shorter conductors also work. The reason why you get such a clear indication is the enormous amplification of the darlington transistor together with >1W leakage RF. The circuit can only receive nW from this >1W, but this is sufficient to drive the darlington. \$\endgroup\$
    – Janka
    Commented Dec 29, 2016 at 14:21

2 Answers 2


The E-field for microwave is AC modulated and gets rectified to look like E field from proximity to line signals near outlet.

The plastic has ~7kV/mm insulation and a dielectric constant ~ 4 with a coaxial conductor in the probe to feed the Darlington which rectifies the E field current in ~<=1uA to ~ few mA pk drawn from the Lithium 3V*2 cells which have inherent current limiting to < 20mA from ESR.

The proximity effect is inverse squared per mm gap from the slot which also acts as an RF antenna (perhaps minimized by geometry).

The oven is also well shielded by the Faraday cage with holes in window shield < 5% of lambda in the door unless extremely dirty on the seal creating a gap with E field leakage.

Microwave ovens were also known to cause EMI on analog cordless phones due the 120 HZ modulation of the magnetron's E Field. Shielded pacemaker technology is no longer threatened by this leakage.

This is a useful tool but the LED energy comes from the battery.


Contrary to myth that microwave ovens heat from the inside out, the polar properties of moisture conduct well with ions (salt) but still has skin effect but since much lower frequency ( by several decades) than conventional thermal ovens which do have a much shallower skin effect.

Microwave has a deeper skin depth in food. Research has shown BBQ is the worst for harmful effects on food due to transformations from burning. Contrary to the myth that microwave cooking is harmful, it is excessive food temperature that is harmful. Just consider the frequency of a 1um thermal wave.

At one time the crystalline structure of some dentist amalgam fillings were once known to act as piezo AM detectors and hearing works with vibration on any bone in the head. So in rare cases, some could hear strong AM radio stations.


The Darlington base-emitter junction is a rectifier, or a diode. With only a few milliVolts on the base, nothing happens.

With 20 or 30 milliVolts, you get significant 2nd-harmonic current flow which, added to the fundamental, becomes IMBALANCED.....i.e. there is a DC shift and this DC_shift turns on the transistor.

Model this in SPICE; apply 2GHz at 20milliVolts PP thru 1pF cap, in a transient simulation, not AC simulation.


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