ESD monitors

Hi everyone.
I want to know how do ESD monitors work, those who detect if someone is wearing the ESD wrist band or not. The problem is that there is only one wire feedback from the wrist band so i don't know how can you measure anything from just one wire.
Thank you.

there are several ways, the two most common are body capacitance, which can be sensed with an oscillator fed into a frequency-to-voltage converter. you become part of the tuned circuit of the oscillator, and so being connected to the wrist strap changes the oscillator frequency. the other method is an electrometer, which is an amplifier with a VERY high input resistance. the input resistance is high enough to be able to detect nearby electrostatic fields, so if you sit down at the workbench, and are carrying a charge, the electrometer detects it. when you connect to the wrist strap, you are now grounded, and the charge bleeds off.

unclejed613 thanks a lot for your reply i really like the idea of the oscillator into the frequency to voltage converter, but still i can't figure out how to integrate the human body into the circuit with only one wire, I'd appreciate further explication on how to do it.

using a Hartley oscillator, there is a single tuning capacitor, one side is grounded, and one side goes to the grid/base/gate (depending on whether a vaccum tube, bipolar transistor or FET is used). if the "hot" side of the capacitor is connected to an isolated contact of the wrist strap, your wrist is connected to the oscillator, and lowers the resonant frequency, and either a bandpass filter or a F/V converter can detect the change in frequency. https://en.wikipedia.org/wiki/Hartley_oscillator

unclejed613 said:

using a Hartley oscillator, there is a single tuning capacitor, one side is grounded, and one side goes to the grid/base/gate (depending on whether a vaccum tube, bipolar transistor or FET is used). if the "hot" side of the capacitor is connected to an isolated contact of the wrist strap, your wrist is connected to the oscillator, and lowers the resonant frequency, and either a bandpass filter or a F/V converter can detect the change in frequency. https://en.wikipedia.org/wiki/Hartley_oscillator

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Well thanks a lot that was very helpful and very well explicit, thank you very much.