a few microphone questions

[SeanHatch]

Hello, I've just a few q's about microphones.


First off, any suggestions on small microphones? Size wise, I'm thinking about the size of a phone microphone. Quality wise, I'm not too concerned since I'll be using the microphone's signal to light some (see next question.)

I'm going to amplify the signal recieved from the mic, and then drive some lights. I'm not sure if I should use LED's or some kind of small bulb. Right now I'm leaning towards LED's; any opinions on this?

Lastly, about how many volts does a microphone (unamplified) produce maximum? I don't want to need only a few volts for my LEDs but wind up with 20V from the amp.

EDIT: last lastly this will be a battery powered circuit so a low impedance for the mic would be nice.

 

[chemelec]

Hello, I've just a few q's about microphones.


First off, any suggestions on small microphones? Size wise, I'm thinking about the size of a phone microphone. Quality wise, I'm not too concerned since I'll be using the microphone's signal to light some (see next question.)

I'm going to amplify the signal recieved from the mic, and then drive some lights. I'm not sure if I should use LED's or some kind of small bulb. Right now I'm leaning towards LED's; any opinions on this?

Lastly, about how many volts does a microphone (unamplified) produce maximum? I don't want to need only a few volts for my LEDs but wind up with 20V from the amp.

EDIT: last lastly this will be a battery powered circuit so a low impedance for the mic would be nice.

Microphones in general put out VERY LITTLE Voltage and Amperage.
Milli-Volts, Some only Micro-Volts.

You will MOST Definately need an Amplifier to get power to light Any type of bulb or LED.

And the Impedance of the Microphone does Not change this.
Actually a Crystal (Pizo) Microphone is a Very High Impedance and probably put out More voltage than most Low Impedance ones.
(There are some of these for sale on my website)

 

[audioguru]

Hi Sean,
Telephones are all different and use nearly all the different types of microphones. You probably want an electret microphone that is very good, small and inexpensive. It needs a resistor to provide its internal jFET with a small amount of power. Then a preamp amplifies it and some LED driver transistors or an LED driver IC drives the LEDs.

I made and posted a project that is somerthing like what you are talking about. It is very sensitive so it picks up and displays the sounds of people talking in a pretty big room, TV sound, music or a pin dropped. It has 10 steps of LEDs (I used them in pairs) for a logarithmic loudness indication and has automatic gain control to shift its range to display loud sounds. I have it powered by a wall-wart and an internal rechargable 9V battery. Its display can be switched between being a bar graph or a moving dot.
https://www.electronics-lab.com/projects/motor_light/009/index.html

 

[SeanHatch]

Thanks, I don't know much about electrets, but I've been flipping through some datasheets. Is the specified 'operating voltage' a voltage used to bias the internal JFET circuit? If so, is the voltage #mV across the electret or #mV across the whole package (including fet amp). I'm curious because I'm thinking about using an emitter follower for the mic, will this still work, or be needed?

That automatic gain control is a good idea. thanks.

This is the first thing I've ever sort of designed on my own.

 

[audioguru]

Is the specified 'operating voltage' a voltage used to bias the internal JFET circuit?

The FET is a constant current sink of about 0.5mA. It works with a supply voltage for its load/powering resistor of from 2V to many volts. A 10k resistor from a 9V supply works well.

is the voltage #mV across the electret or #mV across the whole package (including fet amp). I'm curious because I'm thinking about using an emitter follower for the mic, will this still work, or be needed?

Its output level is about 5mV RMS with a talking voice about 1/2 meter away. It doesn't need an emitter-follower since it can easily drive an opamp or a transistor as its preamp.

 

[SeanHatch]

so this is the first thing i've ever designed, so take it easy on me......

The resistors at the bottom of the LEDs will be chosen to form voltage dividers such that each of the 5 diodes is harder and harder to turn on.

I put the rectifier in there but I think its making things unnessecarily complex. I think the lowest someone could talk is about 300hz, so after half wave rectification that is 150hz which is still twice as fast as the average human eye.

The microphone is represented by the circle. Most of the electrets I've been researching have an input and an ouput. Where do I power the jfet? which end is ground?

Any help would be appreciated.

EDIT: ooops i missed the op amp's feedback resistor. P.S. what op amp should I use? can I power it with just one 9v battery?

 

[audioguru]

Hi Sean,
1) Your electret mic doesn't have a resistor powering it so it won't work. It needs a resistor to the power supply to power it then a coupling capacitor to the opamp.
2) Why is there a FET between the microphone and the opamp? It should be a coupling capacitor.
3) Why is the opamp an inverting circuit that has a very low input impedance instead of a non-inverting circuit that has a high input impedance?
4) What is the rectifier bridge for? 3 of its diodes don't do anything and the one diode that rectifies is creating a 0.7V voltage drop. You don't need a rectifier because the opamp rectifies by itself if it has a single supply.
5) An opamp doesn't have much output current so it can drive only a single LED brightly. You need a circuit with a comparator/LED driver for each LED instead of all the resistors.
6) The RC that tries to keep the LEDs lighted for long enough time to be seen is overloaded by the LEDs. The RC should be at the input of comparators.
7) The resistor ladder at the LEDs will reduce their brightness while reducing the level they will turn on.

You need an IC that is made to do most of your circuit. An LM3914 has 10 steps with equal voltages. An LM3915 has logarithmic steps and I used it in my project. An LM3916 is a VU meter but I prefer the LM3915. They have 10 comparators fed from a resistor ladder inside. They drive 10 LEDs. The ratio of 2 resistors program the amount of input voltage needed to light the highest LED. The total value of the two resistors sets the LEDs brightness.

I used MC33172 dual opamps in my project because their inputs can work at 0V when they have a single supply. The MC33171 is a single opamp and is available at Digikey (they are sold out of the duals). They work with a supply voltage from 3V to 44V.

A 9V battery doesn't have much power to light 10 LEDs for much time. The LM39xx ICs can be set to light a single LED as a "moving" dot to save power instead of many LEDs in a bar. My project has a switch to select which mode.

 

[SeanHatch]

ok, just looked up the datasheet for the LM3915 and man, its like 25 pages long. I think I got the general idea. I didn't know they had chips to do this.

I'm going to set up the LEDs so that as the sound level increases, it gets brighter (ie more leds light up), so the bar thing you suggested isn't an option. This in mind, should I use a different battery, more of them maybe? I'll need some power for the electret to.

Also, is the electret signal large enough to connect right up to the 'signal source' of the LM3915?

 

[audioguru]

I'm going to set up the LEDs so that as the sound level increases, it gets brighter (ie more leds light up), so the bar thing you suggested isn't an option.

That is what the BAR does. The bar of light gets longer (and with more LEDs lighted up it looks brighter) when the sounds are louder.
In the DOT mode only a single "moving" LED is lighted.

This in mind, should I use a different battery, more of them maybe? I'll need some power for the electret to.

10 LEDs lighted use a lot of current. If an average of 5 LEDs are lighted brightly then a 9V alkaline battery will last 4 hours. My 9V rechargable Ni-Cad battery lasts about 1 hour. 6 AA alkaline cells will last about 20 hours.

Also, is the electret signal large enough to connect right up to the 'signal source' of the LM3915?

No, the mic needs a preamp and a half-wave peak detector is needed. They can be made with a dual opamp.