frequency resonance of a LM358 op-amp - Page 1
Forum section: electronics / general

author: jonovid
author_rank: Super Contributor
timestamp: March 09, 2024, 12:34:39 pm
content: this LM358 circuit is supposed to receive up to 3GHz from a cell phone.please explain-

author: Kleinstein
author_rank: Super Contributor
timestamp: March 09, 2024, 01:18:30 pm
content: This is a rather bad circuit: 2 x 6.8 milliohm in series across the 9 V supply draws a lot of current.If the resistors are meant to be 6.8 Mohm, the value is a bit high for the BJT base OP-amp.With 100 nF from the antenna to ground there should ideally be not much AC voltage left. If at all the parasitic inductance and similar could make a difference, but they are not shown in the plan.The plan is using the pin-out of the chip instead of the standard OP-amp symbols. This makes it hard to read - though it may be justfied if one would really follow the layout to include the parasitic efffects.The LM358 is a plain, slow OP-amp. It may react to RF frequencies by rectification it the input nodes. How much is however hard to tell and it depends on the layout / manufacturer.

author: hans
author_rank: Super Contributor
timestamp: March 09, 2024, 02:04:53 pm
content: Quote from: jonovid on March 09, 2024, 12:34:39 pmNo it doesn'tAnd the bulb is drawn like a Star of David, so stop the troll.Sorry, apologies, stress has gotten the better of me.

author: Andy Chee
author_rank: Super Contributor
timestamp: March 09, 2024, 02:29:52 pm
content: Found another LM358 phone detectorhttps://www.electronicsforu.com/electronics-projects/mobile-phone-detector-using-lm358Even includes a PCB etching pattern!

author: Zero999
author_rank: Super Contributor
timestamp: March 09, 2024, 02:35:18 pm
content: Quote from: Kleinstein on March 09, 2024, 01:18:30 pmYes, the resistors are probably 6M8, which is on the high side, although the LM358 has fairly low bias currents so it's not that bad.Quote from: hans on March 09, 2024, 02:04:53 pmIt's just a coincidence. The star just shows the bulb flashing. Don't let the Current Thing affect your judgement.It's probably relying on the LM358 reifying the RF signal, which is unreliable. The circuit might work under some conditions, but most likely won't most of the time.

author: jonovid
author_rank: Super Contributor
timestamp: March 09, 2024, 08:26:24 pm
content: the 2 images are from video captures of 2 YouTube videos.unorthodox use of circuit symbols is not my doing.thank you for your response.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 10, 2024, 01:06:23 am
content: The opamp is just meant to act as a demodulator, it could be dead slow too. I haven't checked the values on this pseudo-schematic, but the basic idea is workable.Remember how we kept getting those annoying audio signals in cheap amplifiers when a cell phone was nearby, before we started to properly protect inputs against RF. Internal diodes just act as demodulators.But it's going to be a gadget that doesn't have any tuning and will work on Murphy's principle, reacting to any RF signal strong enough and probably not doing anything much when you'd find it actually useful.

author: jonovid
author_rank: Super Contributor
timestamp: March 10, 2024, 03:13:37 am
content: Quote from: SiliconWizard on March 10, 2024, 01:06:23 ammy intention was to search for hidden IoT in household appliances.with a simple circuit suitable for further experimentation & improvement.like adding a LM3914 analog level display to the backend of the circuit.or more challenging is to add a 2SC3358 GHz pre-amp at the frontend.with a bandpass filter in the antenna. rf shielding to prevent self oscillation. mounted on a double sided copper clad pcb.

author: EPAIII
author_rank: Super Contributor
timestamp: March 10, 2024, 04:15:57 am
content: There are cell phone APPs (I hate that word) that will show wifi signal strength and the devices that are connected with it. Probably a lot easier to use and more dependable.You should know the ID of the legit devices on your wifi. If you don't know which appliance is showing up as an unknown, you can just unplug the suspicious ones, one at a time, until the mystery device disappears from the line-up. But I wouldn't leave the refrigerator unplugged for very long.

author: Andy Chee
author_rank: Super Contributor
timestamp: March 10, 2024, 06:48:58 am
content: Quote from: jonovid on March 10, 2024, 03:13:37 amA suite of network sniffing tools on a Linux laptop with WiFi, should be all you need to search for hidden IoT devices.

author: hans
author_rank: Super Contributor
timestamp: March 10, 2024, 08:53:41 am
content: 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) 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.

author: janoc
author_rank: Super Contributor
timestamp: March 10, 2024, 09:51:57 am
content: Quote from: Andy Chee on March 10, 2024, 06:48:58 amI 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.

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 10, 2024, 10:18:09 am
content: 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: