Do you remember the cctv microphone review published a while ago? (https://www.electroschemics.com/cctv-microphone-review/).
This is a continuation of that post. This time my target is another popular cctv microphone, commonly referred to by most vendors and technicians as the golf ball type microphone!
We can add this golf ball microphone in-line, so the microphone can draw its power from the same power supply cable as our cctv camera. This means we only need to use a single dc power supply adapter, and we don’t have to run a pair of cables for the camera and microphone separately.
Let’s see the technical specifications (copied from a seller’s page):
- Voltage: 12VDC
- Current Consumption: 20mA
- Enclosure Design: Concave/Omnidirectional Sound Capture
- Monitoring Range: 5 to 100 square meter
- Frequency Range: 100-20kHz
- Output Impedance: 600Ω
While most do it yourself security camera projects have audio electronics built-in, many commercial products don’t. This little audio device can be used to get audio with a commercial cctv camera. Note that this is a “pre-amplified” or “line-level” cctv indoor microphone. It works well with most digital video recorders (DVRs) and surveillance cameras.
Hooking up a microphone to your DVR gives you an extra layer of security. You can see and hear what’s going on. Surveillance camera microphones are usually very easy to install, especially if there’s a dedicated audio port on your DVR. Most DVRs will probably use an RCA connector (see below), but this may be different on your DVR (quite natural).
Spoiler: I’m currently working on an inspired birdbox camera project (not yet completed).
Although ready-made birdbox/nest box cameras are available, my idea is to build my model with a cheap car rear-view camera (https://www.codrey.com/electronic-circuits/poor-mans-door-video- security-camera/).
However, since that minuscule 12VDC waterproof night vision car rear-view camera did not have an inbuilt microphone, I planned to use a golf ball microphone for my little project as the microphone is an affordable and useful audio device. What drew me to it? To get the right answer, let’s first look at the electronics inside it!
We can see a neat PCB with an electret condenser microphone fitted in the dome (even its flying wires are neatly arranged). In addition to the power supply indicator LED, we can also see a small trimpot to adjust the gain of the microphone preamplifier; Definitely a good addition!
This is how its schematic looks:
The JRC4558 IC, located at the heart of this simple audio circuitry, (https://www.rcscomponents.kiev.ua/datasheets/jrc45584i743ncft874nfdt34ufguygf43.pdf) is a high performance monolithic dual operational amplifier which’s pin to pin compatible with the common MC1458 /LM358.
Frankly, this design seems like a mere adaptation of the “textbook” circuit of a single-supply electret condenser microphone (ECM) preamplifier (see below). So, it does not require any in-depth explanation, but I decided to include a few applicable points that may be useful for some newbies.
This single-supply microphone preamplifier amplifies the output signal of the electret condenser microphone to audio line levels. Note, the electret is a thin, Teflon-like material with a fixed charge bonded to its surface which is housed between two electrodes, and the structure forms a capacitor which contains a fixed charge. Soundwaves (air pressure variations) move one of the electrodes of the capacitor back and forth, changing the distance between the two electrodes, and modulating the capacitance of the structure. Remember, most electret microphones have an internal JFET which buffers the microphone capacitor. An example construction of the electret microphone is shown below (Thanks to Texas Instruments).
Note that this simple electret microphone preamplifier is tailored for single-supply operation by biasing the non-inverting input of the op amp to the mid-supply (½ Vcc) point. The internal JFET of the electret microphone is biased by resistor R5, and C7 is the input coupling capacitor. Because of the potential divider (R4+R6), maximum voltage available for the electret microphone is around 6V when powered by a 12VDC power source. If it’s a 2V electret microphone, the microphone’s operating current then will be around 1mA (max).
Here’s the sample datasheet of an electret condenser microphone https://www.endrich.com/fm/2/SOB-413S42-EM.pdf
The capacitors C1 and C2 filters and decouples the input supply voltage and prevents parasitic oscillations, while diode D1 is a good safeguard against accidental reverse voltage input.
And, the trimpot TM1 provides bias to the negative input of the op amp through resistor R3, while together with R2 it finally sets negative feedback and therefore the voltage gain as well. The capacitor C3 blocks dc bias from being affected by R2, as it’s low resistance would lower the negative input bias from half of the supply voltage, to fractions of a volt. The C4 capacitor (sees to be an optional component in many similar circuits) sets the high frequency roll off. At last, the dc component of the output audio signal is removed by the output coupling capacitor C5. Look, audio circuits are often ac-coupled so that only the audio signal is passed between them, and any dc voltages are moved out.
While concluding my preliminary evaluation, I’m happy to relay that everything worked out and the outcome was better than I expected.
DC Input Range: In the light of my quick trial runs, it’s obvious that the minimum voltage required for an effective operation is 9VDC, hence it might work even when the 12V camera cable is too long.
Gain Adjustment: The gain set trimpot is useful to some extent, but you need to adjust it properly. This means, as long as your golf ball microphone is working properly, there’s nothing about it that will hinder you from making better recordings. But still, you might have concerns about your microphone, and one of them is its gain.
Gain structure is very important as some microphones sound better at certain gains than they do at other gains. Logically, you should be testing your cctv microphones under circumstances as close to yours as possible, but you must also look at the performance of them under other contexts.
The method of gain adjustment is somewhat tedious but a continuously variable gain control allows you to do it a bit smooth. The common practice to set the gain is to raise it to whatever value that is necessary to achieve a clean and strong audio capture without clipping or distortion or troublesome acoustic background noise.
Output Impedance: On any given cctv microphone specification sheet, you’ll find a value for its output impedance. A “low-impedance” roughly denotes an output impedance in the range of 50 to 600Ω. On paper, low-impedance microphones allow for long cable runs without any noticeable degradation of the audio signal.
At this point, note that a microphone’s output impedance is actually frequency specific, and manufacturers typically provide microphone output impedance specifications as nominal impedance, which’s the approximate output impedance of the microphone averaged across the de facto audible frequency spectrum scale.
As a side note, I’m not an audio guru, so as far as I know, the easiest way to measure the output impedance of a device is to:
- Measure the output voltage of the device without a load while a signal generator is connected to the input (the output impedance of the signal generator should be low compared to the input impedance of the device under test)
- Connect a resistive load to the output and observe the output voltage
- Lower the load resistor until the output level is half of the output(-6 dB) compared to the situation without a load
- Finally that this does not cause distortion (in this case the load resistor is the same as the observe output impedance of the device)
For more “impedance” tales, go to this page http://www.sengpielaudio.com/calculator-InputOutputImpedance.htm
Ultimately, you need to listen how your surveillance camera microphone holds up at the final destination, whether that destination is your everyday DVR, another sophisticated audio/voice recorder, or the audio segment of your new do it yourself project. Admittedly, everyone’s situation is a little different, so you must decide what is best for you!
What’s more, recently I’ve been doing some “Arduino Audio” projects, I will cover one of them in one of my future posts. But now I’d like to share a funny idea to start your play with your golf ball cctv microphone and an Arduino Uno (or Nano) microcontroller!
The idea is pretty simple. Since the audio signal output from the golf ball microphone is a varying voltage, we can measure the sound level by taking multiple measurements to find the peak-to-peak amplitude of the audio output signal.
For this test, you must operate the golf ball microphone using 5VDC. The proposed wiring scheme is shown in the table below.
|Arduino Uno/Nano||Golf ball Microphone|
The below Arduino Sketch can compute the difference after finding the minimum and maximum signal extents, convert it to volts and finally print the result out on the serial monitor window (Thanks to Adafruit). Not very exciting, but you can go further with this.
const int sampleWindow = 50; // 50 mS Sample window
unsigned int sample;
Serial.begin(9600); // Serial Start
startMillis = millis(); // Sample Window Start
peakToPeak = 0; // PP Level
signalMax = 0;
signalMin = 1024;
// Gather data
for 50 mS
– startMillis < sampleWindow)
if (sample < 1024) // Cast out spurious readings
= sample; // Save just the MAX levels
(sample < signalMin)
= sample; // Save just the MIN levels
signalMax – signalMin; // MAX – MIN =
double volts =
(peakToPeak * 5.0) / 1024; // Convert to
That’s all. Whatever your skill level, I hope this short post will inspire you to build your own surveillance microphone/audio projects 👍