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frequency resonance of a LM358 op-amp

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hans:
Some antennas can also be BPF filters.
It may have multiple resonsant bands though. Otherwise add SAW filter.

The sensitivity of a circuit like this is not very good. Certainly, nothing from the schematic makes the 3GHz reception look intentional. The opamp is setup as a transimpedance amplifier, which makes it suspectible to interference with Rf=6.8M, but at the same time AC input current is shunted to GND via the capacitor. However, because its such a shitty capacitor at RF (aluminium cap), you can probably look at it as a fairly high-value inductor for 3GHz..

It fixes the DC point, but how it behaves at 3GHz is a bit of an unknown.

As I eluded in my previous (emotional :-X) response.. I don't think these circuits are trustworthy at all. Sure they may light up if you hold them next to some device. But is that WiFi? GSM traffic? Proprietary 434/868MHz? Or even just 50Hz mains? From your neighbours? Who knows!
The poor sensitivity is perhaps advantegous for the 'neighbours' part. But other than that its anyone's guess what this circuit is receiving.

By the way, shielding does not work for self-resonance. This circuit purely operates in interference principles, and the antenna is already the most sensitive part (its directly on the net of the transimpedance amplifier). So I wouldn't expect it to be a gamechanger.

janoc:

--- Quote from: Andy Chee on March 10, 2024, 06:48:58 am ---
--- Quote from: jonovid on March 10, 2024, 03:13:37 am ---my intention was to search for hidden IoT in household appliances.

--- End quote ---

A suite of network sniffing tools on a Linux laptop with WiFi, should be all you need to search for hidden IoT devices.

--- End quote ---

I am not sure why everyone assumes that IoT == wifi. More often than not it doesn't because wifi is terrible esp. for anything battery powered. There is plenty of wireless hw around that uses Bluetooth, Zigbee, DECT, various 433MHz radios, ZWave ...

E.g. things like water and electricity meters that can be read remotely, heat meters on radiators often have low power radios for reading them remotely (in both cases it makes the life of the meter reader easier because they don't have to deal with people not being at home and having to come multiple times), heating thermostats (Zigbee and DECT are common there), various door and window sensors, lighting fixtures more often than not have wireless hw in today and many other things.

So the idea of using this sort of detector is fundamentally sound. However in practice the circuit posted is rather poor for this purpose. In addition, many of these devices use very low duty cycles to keep the power consumption low (e.g. many of those meters and thermostats) in order to extend the battery life. That will make the detection using this type of primitive equipment rather challenging.

pcprogrammer:
This reminds me of these stickers you could stick to your phone and when active a led would blink. All powered from the actual signal.

Like these shown here:

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