Security microphone, how good would it have to be ?

[MathWizard]

I live in a city, and there's too many punks that don't respect property rights, and use an alley right by my basement door, and I don't trust a single 1 of them. Until I get a real camera system, I want to try making an open door and loud noise alarms. (I was supposed to be making an FM TX/RX so I can listen to my PC in other rooms...)

 I can easily hear someone walking on the gravel out there, so I'm just wondering how cheap of a microphone I would need, placed next to my door. What about a basic electret mic's ? Are jelly bean ones as good as this article claims? I have some of them. I'm not expecting audio quality, just something the modulates the carrier when there's a loud enough noise.

https://www.homemade-circuits.com/spy-bug-circuits/

A tried 2-3 in ltspice and they work w/ a 1mV input signal, so I was going to try something like #8 below. Then figure out what to do w/ the TX'ed info.

[Kleinstein]

The ubiquous cheap electret microphone are usually not that bad when it comes to noise and frequency response. So they should be good enough. The problem may be more to distinguish normal city sounds and close by noise.

[NiHaoMike]

You can get an ESP32 camera module for $5 or so.

[mc172]

I don't think this is a good idea. If your neighbour slams their door a bit too hard, do you really want that to make an alarm go off? What if a gust of wind is enough to exceed the threshold and sound the alarm? What if you closing your own doors is enough to trip the alarm via conducted vibrations?

[MathWizard]

I'll get a proper 2k camera or something down the road, I want 1 on the mailbox too. For now I just want a light or noise to go off in my room, if someone opens the door, so I'll make that too.

I built the circuit I posted, on the BB, and it only RF oscillates if I remove the 39pF cap, and then only 100mVpp or so. But it works in LTSpice, so I'll make it on a copper clad board and see if I can't get it working without all the parasitics. These electret mics I have are the cheapest of the cheap too, so I should experiment w/ 1 of them too.

I know I'm not doing DSP on the signal, to see what noises are out there, but maybe it will work ok as something I could listen to, and/or alert me if it goes over some threshold.

[Ian.M]

No microphones are necessary or desirable! See https://www.canadiansecuritymag.com/the-tricky-business-of-audio-surveillance/

A PIR activated high intensity floodlight over the door, mounted 'two drunks high' (i.e. out of reach of someone with a baseball bat sitting on someone else's shoulders), angled not to bother the neighbors, with its sensor shielded or angled  so that walking down the middle of the ally (assuming its a legitimate public thoroughfare) does not trigger it but approaching the door does, set for a short on time to maximally affect the night vision of the ne'er-do-wells in question should go a long way to improve security.

Add a vibration sensor on the door, set coarsely enough not to trigger unless the door is hit and a reed switch to trigger an alarm if its opened more subtly. 

[MathWizard]

Those mic's really do work ok, I made a 1BJT amp, and yeah they do pick up a fair bit. I made the FM TX circuit on copper-clad, but no dice, so far the colpitt's osc. is not oscillating, my homemade inductor is too small I think. Either way I guess my loop gain is too low or out of phase, for my tank circuit to get going.

At such high freq, it's a good excuse to try using Cu and Cpi capacitance's of BJT's in calculations again. I've looked at papers on Colpitt's osc. analysis before, plenty to learn before I that for practical purposes.