CCTV Microphone Review – ElectroSchematics.com

I was playing around with some cheap CCTV microphone modules and managed to create a monophonic spy ear bug for common DVRs. They can be directly powered from the existing CCTV camera power supply (12VDC).

Following is a quick review/teardown of an omnipresent CCTV (closed circuit television) microphone module that I’ve used for my initial experiments. I’ve only considered a few things that I found relevant for my personal use.

A Quick Inspection

The CCTV microphone module comes from China in the form of a heat-shrinked rectangular PCB with three flying leads (RED – BLACK – WHITE). The flying leads (+12V – GND – AUDIO OUT) allow to directly solder the microphone module to any “3+1” CCTV cable (https://5.imimg.com/data5/CB/OE/DY/SELLER-1552586 /d-link-dcc-whi-180-4-premium-high-performance-cctv-cable.pdf) to connect it to the output of the CCTV camera power supply unit and to the audio input of the DVR (digital video recorder ).

I simply removed its sheath to see the concealed electronics. It’s a simple one-transistor ECM (electret condenser microphone) preamplifier built around a few cheap chip parts. The bottom side of the PCB (not exposed here) holds nothing except a well-dispersed ground plane.

This is the reverse-engineered schematic of the CCTV microphone module (prepared by me in my own way).

This is a quick closeup of the PCB.

As you can see, the design is centered around the general-purpose NPN small signal transistor MMBT3904L (SMD code = 1AM , SOT-23 package). Datasheet https://www.onsemi.com/pdf/datasheet/mmbt3904lt1-d.pdf

On closer inspection, you will find that this design has been around us for many decades! Anyway, let me examine the circuitry.

When taking the schematic above, resistor R4 biases the ECM through the decoupled DC12V power supply available across capacitor C1.

Note that the ECM has a preamplifier inside usually made of a field effect transistor (FET) which is wired in common source configuration. So, it needs to be powered through a suitable resistor – denoted as RL in the below figure. Now see an ECM sample datasheet http://www.farnell.com/datasheets/2869767.pdf

The coupling capacitor C3 in the schematic is to remove the DC offset to render a “mV-level” AC signal output which however needs to be amplified even more. Luckily, a self-biased single-transistor electret condenser microphone amplifier works well here.

This section – the so called self-biasing transistor small signal audio amplifier circuit – works on the principle that the base resistor (wired between collector and base) is loosely chosen to supply just the demanded current to turn the transistor on such that it rests in the just-about-linear region. I won’t go into an in-depth theoretical explanation on this because you can find a great post here https://www.electronics-tutorials.ws/amplifier/transistor-biasing.html

An Agile Test Report

As usual, I made a quick trial to see how/if it works. In order to conduct that test, I used a variable output lab power supply which’s initially dialed to 12VDC.

Then, I linked the module’s audio output directly to one channel of a stereo headset (32Ω x 2). I also tested the module with a battery-powered pocket stereo audio amplifier. Both seemed okay at the moment!

The module that works happily with 9 to 12 volt DC inputs, has a moderate overload margin, and can cope with anything from a whisper to a loud shout. However, the more sensible capture range is not up to the mark ie, it’s around 5cm to 15cm only that doesn’t put out much juice (perhaps a budget fit for certain indoor security camera installations).

Despite the negative(s), I like the classic circuit of this module because it’s robust and straightforward. Also, its output impedance is well suited for driving cables over a long distance and a screened cable is not very necessary. If we do not look for delightful signal quality and plenteous sound output, simple ideas like this one will do fine.

I forgot to mention the role of the output coupling capacitor (C2 in my schematic/C3 in the module) which stands before the audio output wire. Yes, it’s again a dc offset filter but this time resides at the output of the small signal amplifier. Note at this point that a dc offset filter capacitor often works as a high pass filter along with the input resistance of the next audio circuitry as well. So, choose it wisely in your own designs!

Inserted as an aside is the photograph of a “high-impedance line-level” security camera microphone that can be connected to a digital video recorder or audio amplifier. Understanding Impedance: https://www.soundonsound.com/techniques/understanding-impedance

Next CCTV Microphone

I’ve been searching for another cheap CCTV microphone which can provide better features. Having exhaustively read through various blogposts it became pretty clear that there’re currently two major cheap candidates on the market that fulfill the current requirements. Since I already bought the first one, I took the next step to get a bunch of “golf ball shaped” security camera microphones (see below) from an online store. My intention is to test and compare both products so that I can (hopefully) provide you with a more objective opinion from an electronics maker’s point of view. Well, they’re cheap enough to buy heaps of them!

So, what’s inside the “golf ball” ? I decided to open up it to see what makes these devices work. The audio electronics inside the golf ball shaped enclosure looks as follows:

The key component is the RC4558 IC (https://www.ti.com/lit/ds/symlink/rc4558.pdf). Yes, the non-discrete transistor designs that use op-amps are more popular because they’re more stable, make less noise, and are pretty compact.

The next thing to be analysed is the actual schematic of this different device. But I’m not going into that right now because I started making a simple parabolic microphone and I will extend this part too through the documentation of that construction project. Please be patient.

In conclusion

A CCTV MIC (surveillance mic) should be designed for high sensitivity, omnidirectionality, and it must have an integrated preamplifier circuitry. If so, the audio output can be directly connected to the LINE IN of a DVR or an active speaker. On the hardware part, its inside electronics adds a voltage stabilizer circuit and a front audio/small signal amplification stage which is why the CCTV microphone module requires DC input supply.

If you are on the hunt for an entry-level solution to your CCTV microphone woes, look no further. Both devices covered here, despite their low price, will do that job fine. But, as it seems, the second type has been designed to pick up voices more clearly, thus offers much better (adjustable gain) audio quality. Besides it has a “live indicator” inside the golf ball which will alert intruders to its current existence.

Anyway, each application will have different priorities and budgets so what works for me, may not work for you. So, always do your own research and determine which surveillance microphone will work best for your proposed project. Thank for reading. Stay tuned!

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