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I'd like to build a ghost detector. I'm only just beginning the design stage, and thought I'd reach out for some suggestions.

Receiver: I'm planning to end up with a handheld device with an analog backlit meter, which illuminates when the house lights turn off. The meter will display the present of a particular frequency. I have lots of old FPGA projects which I can modify to use as the radio. Or maybe an Arduino for the logic and ADC interface to the meter? I haven't used an Arduino before.

Transmitter: Now to generate the ghosts... The "ghosts" should be some RF signal or frequency which can readily be received in a common household. While I have a good bit of experience in building transceivers, they have always been over coax or fiber. I'm not familiar with all the FCC codes for OTA, so I'd rather stick to detecting "ghosts" (ambient signals) which already exist. Which leads me to my questions:

Q1 Which frequency should I look for? The lower the better, to keep costs down. The project is for my kids - I'd like for them to be able to walk around and find the presences of "ghosts" coming in and out of range. So the signal should be readily available, but intermittent (attenuation by the home's structure, perhaps). Would 60 Hz work?

Q2 I think it might be fun to have the house lights flicker when the meter hits a high mark. To do this, I would have to transmit after all, but the signal could be very low power - just enough to trigger a remotely controlled wall socket. Surely the socket is a COTS device, but if any of you have done this before, or know of a suitable unit, I'd appreciate the information.

Any other suggestions or ideas would be greatly appreciated.

Note I'm not looking for real ghosts here. It's just a toy for the kids.

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    \$\begingroup\$ Probably by people thinking you're asking for instructions on how to build a "real" ghost detector. (We get a surprising number of those.) You may want to make it more clear that this is meant as a toy. \$\endgroup\$ – duskwuff -inactive- Jul 14 '17 at 5:35
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    \$\begingroup\$ @duskwuff maybe. I did state that this is a toy, in bold, at the bottom of the question. Maybe I should move it to the top. Thanks. \$\endgroup\$ – Blair Fonville Jul 14 '17 at 5:38
  • \$\begingroup\$ Just another suggestion - Why don't you also try building ultrasonic generators powered by batteries? \$\endgroup\$ – R.Joshi Jul 14 '17 at 8:17
  • \$\begingroup\$ @R.Joshi Sounds too dangerous for kids to play with. This is a toy. \$\endgroup\$ – Blair Fonville Jul 14 '17 at 8:37
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    \$\begingroup\$ Dangerous? Maybe I need to change my phrasing! Do you know the really cheap proximity sensors (used for car parking and the such like). e.g. Ultrasonic Distance Sensor (HC-SR04). You can leave transmitters dotted around and search for them with your haldheld device. \$\endgroup\$ – R.Joshi Jul 14 '17 at 9:09
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Q1

60/120/180 Hz for sure

Those black-brick switchreg battery chargers: 50KHz to 1MHz.

LCD/CRT displays line-rate 50-200KHz

AM radio (540--1620Khz)

Use magnetic pickup: 10 turns of happy yellow solid-conductor wire, around a big oatmeal can (paper or plastic shell).

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  • \$\begingroup\$ AM! Didn't think of that. That might work nicely. \$\endgroup\$ – Blair Fonville Jul 14 '17 at 4:31
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There will be plenty of opportunity for detecting 60Hz around your house. You may however be disappointed with the distribution. There will be power and lighting wiring in almost any surface you look at.

At a frequency of 60Hz, you won't see any EM radiation, it will all be electric or magnetic near field.

The way conventional wiring is done, with pairs of wires running in close proximity, this tends to minimise the wire separation and loop area, which minimises (intentionally) the near field electric and magnetic coupling to other things. In various places where the conductors don't run close together, say the wiring behind a socket, or the distribution panel, then you'll have a local 'hotspot' of increased coupling. You will find a strong electric field in very close proximity to a paired cable, which is how 'cable finders' work, but you need to be within an inch or so, none of this 'getting warmer' tension. Electric field hotspots will always be present. Magnetic field hotspots will come and go depending on where current is being drawn in the house.

You do have the opportunity to plant some enhanced transmitters of 60Hz, by plugging in things to sockets around the house. This might not work for the scenario, as they may well notice visually odd things plugged into the wall. Unfortunately there's nothing I can think of on the market that you can just buy that intentionally generates nice strong external 60Hz fields, so you may have to make stuff. This would depend on your being safe working with mains components.

A better solution might be to buy some ISM 'garage door opener' boards. You could sprinkle a few battery-powered transmitters around, and use the matching receivers in your ghost hunters.

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