Electret microphone noise

[hoghunter]

I have an electret microphone as part of a device I have designed. The system is 12v and does have a step down to 5v for other components. The microphone is connected to 12v pos & neg. The mini DVR that is receiving the audio signal is on the 5v circuit. I am getting audio, but it has a lot of noise on the recording when played back. Any advice?

Thanks for any help.

 

[MikeMl]

What kind of noise? Hiss? Hum? Static?

Post your circuit.

 

[atferrari]

Try grounding the metallic case. It worked for me.

 

[hoghunter]

What kind of noise? Hiss? Hum? Static?

Post your circuit.

When the laughing has subsided, please read on.....
I am using EAGLE to layout the circuit the best I can. I do not have much electrical knowledge, obviously and as I am using the FREE version of eagle, I cannot build a monitor or camera to place in my sketch. Everything is working well at this point except the recorded audio from the electret mic. What I have copied here may not help at all. Power is from (3) 18650 li-ion batteries. 11.1 is bucked down to 5v for the mini-DVR. The electret mic is like the one shown below. There are 3 wires... black, red & white. You cannot see the white wire in this photo. I originally connected the POS red lead from the mic to 5v and (-) to the 5v loop. I got a hum with no type of signal at all. I checked and the mic needs 9-12v so I moved the POS lead to 12v.
I tested it again and got my recorded voice this time, but very noisy, not necessarily a 'hum', just dirty noise. I'm thinking about adding a 9v battery and letting one of the (-) leads from the DVR trigger it on via a mosfet while isolating the mic . I would only have the white wire going to AUD in on the DVR. Or, it may be as simple as adding a resistor or capacitor in the right place. Your guidance would be appreciated. Thank you. I will work on a breadboard to test any recommendations. Also, the case is PVC plastic.

 

[audioguru]

A microphone must use shielded audio cable where the shield is connected to the circuit ground or 0V and it blocks mains hum and other interference.
Most electret mics are powered from only a few volts. I have used many electret mics that have only two wires and I have heard about but I have never seen one with 3 wires.

It is impossible to see details on your fuzzy schematic. Save it and post it as a very clear PNG file type.

 

[AnalogKid]

The noise probably is because the DVR expects line level audio, and is trying to increase the mic signal to line level and bringing up the noise. What is the DVR device, what are its specs and pinout. Link to datasheet? No info, no answers.

ak

 

[hoghunter]

I will get that info posted asap for you guys, thanks...

Schematic is attached...

 

[hoghunter]

The DVR is here. The yellow one is the same with different label and color. Still looking for spec sheet.

 

[hoghunter]

Getting the noise out of the audio is one issue, but I do have another question for you: This DVR will be put in a unit that is activated by a motion sensor. I want to be able to trigger recording when motion is sensed. The DVR records individual video files in 10 min segments. There are cables attached to ports that would activate 'record', 'playback', etc.. You can see that in the pic above. I'm wondering how that trigger would be activated? I tried to activate it by connecting the signal wire from the motion sensor to the record wire from the DVR, but nothing happened. Maybe this function will require an Arduino or Digispark microcontroller?

 

[hoghunter]

A microphone must use shielded audio cable where the shield is connected to the circuit ground or 0V and it blocks mains hum and other interference.
Most electret mics are powered from only a few volts. I have used many electret mics that have only two wires and I have heard about but I have never seen one with 3 wires.

I have wondered about that. All electret mics available in the EAGLE libraries have only two leads. Some of these pre-fab mics sold on amazon have two and some have three wires coming from them. They have some circuitry built into them under the shrink tubing where the mic attaches to the wires. Yes, I'm confused. Ultimately I would like to be able to buy the electrets and mount any required op amps, capacitors, resistors, etc, to a PCB in my unit and just solder two leads to the mic. I have done extensive searching on the web for specifics on how to do this, but am not finding the info or I am not recognizing it. This particular mic I have tested has three wires and the description states that it is supplied by 12v. I think I am using the wrong mic?

 

[AnalogKid]

To activate any of the three functions, connect that function's wire to GND. For a completely dry interface, have the motion sensor drive a small reed relay, and connect the relay contacts to the DVR control wires.

ak

 

[MikeMl]

I would like to see the specs for the electret mic.

 

[hoghunter]

To activate any of the three functions, connect that function's wire to GND. For a completely dry interface, have the motion sensor drive a small reed relay, and connect the relay contacts to the DVR control wires.

ak

Ok, I will have to visual this and sketch it up to understand. Thank you so much for the info !! Can I use a 2N3904 transistor for this? If I send the motion sensor signal to the 2N3904 to switch the gate and allow the current from the RECORD wire to flow to ground? Is that the idea?

I don't understand what a 'dry interface' means.

 

[hoghunter]

I would like to see the specs for the electret mic.

This comes under the description on amazon:

• CCTV Microphone to apply to Recorders like DVRs, Linemak, Dahua, Hanbang , and many other Brands.
• Voltage: Linear power supply (transformer) DC 9-12V
• Electric current: 15ma
• Output impedance: 600ohm non equilibrium
• Connecting mode: red (power), black (ground), white (audio)

 

[MikeMl]

This comes under the description on amazon:

• Connecting mode: red (power), black (ground), white (audio)

Your schematic only shows two pins connected?

 

[audioguru]

You need a microphone that is made by a reputable microphone manufacturer and sold by an electronics distributor like Digikey who has its datasheet available. Amazon knows nothing about it and does not even know who made yours.

 

[hoghunter]

Your schematic only shows two pins connected?

Yes I know. I stated that I was confused by that in post #10. No electrets in EAGLE library have 3 pins..... I also have a camera and monitor that are not shown as I am using the FREE version of Eagle and can only create one module (one sheet) which I used for the DVR.

 

[hoghunter]

You need a microphone that is made by a reputable microphone manufacturer and sold by an electronics distributor like Digikey who has its datasheet available. Amazon knows nothing about it and does not even know who made yours.

I appreciate that. Any suggestions as far as part#? I would suspect there are many different configurations? I will check out what is available on DigiKey.
Thanks again!

 

[hoghunter]

As I expected, the options are many. This will be for picking up normal voices anywhere from 2' to 10 ft away. I would also like to have one that will be super sensitive that will pick up sounds such as rustling in brush up to 20'+ away if possible.

 

[AnalogKid]

Can I use a 2N3904 transistor for this? If I send the motion sensor signal to the 2N3904 to switch the gate and allow the current from the RECORD wire to flow to ground? Is that the idea?

I don't understand what a 'dry interface' means.

That is the idea, but it depends on what the DVR circuits want to see externally. They are designed for a simple SPST switch closure, but might be ok with pulling the control wire to GND through a saturated transistor switch.

A dry circuit is one with no voltage or voltage-variable devices in the loop. Examples are mechanical relay contacts, pushbutton switches, etc. are simple metal-to-metal contacts where current can move in either direction with basically zero voltage drop or resistance between the contacts. A bipolar transistor passes current in one direction only, and introduces a voltage drop (the Vcesat saturation voltage) between the "contacts" (the collector and emitter). Because of the names of the four wires available, I am assuming that the control lines have a current-limited positive voltage on them, such as a transistor gate or logic chip input with a pullup resistor, and pulling the line to GND activates the circuit. It is a reasonable assumption, but...

I think it is safe to try controlling the DVR with a small NPN transistor such as a 2N3904, 4401, 2222, etc. or a small MOSFET lid a 2N7000 or 7002.

ak