suitable microphone to trigger an adjustable level

danrogers · Feb 25, 2010

hi all, i need to find a microphone for my project. I want the microphone output to be compared with an adjustable level, and if the microphones output is higher i want that to trigger my rf transmitter.

Ive spoken to someone on here breifly about it before and they suggested using some T072's for amplifying and comparing the signal from the microphone.

I have looked but im really not sure what type of microphone i will need? it must be fairly small.

thanks in advance if anyone can help

Nigel Goodwin · Feb 25, 2010

Everyone and every thing uses an electret mike - nice and small as well.

audioguru · Feb 25, 2010

I used very small electret mics from broken cell phones for two of my projects.

danrogers · Feb 26, 2010

thanks ive just been looking on farnel and they have quite a few. how would i know what will work with my application?

Im not sure how thwy work to be honest. Do you feed a voltage in then the vibrations they pick up correspond to vary the output voltage?

audioguru · Feb 26, 2010

An electret mic has a microphone diaphragm and a backplate with a 48V charge attached permanently. The small capacitance between them creates a small variable voltage divider that is buffered by a Jfet transistor. The Jfet transistor must be powered from a few volts at about 0.5mA.

danrogers · Feb 27, 2010

thanks very much audio guru

so basically i would need something like a 3v supply for it?

audioguru · Feb 28, 2010

danrogers said:
so basically i would need something like a 3v supply for it?

A TL071 single opamp and a TL072 dual opamp are low noise, have low distortion and are wideband. they were used in many hi-fi systems for the last 30 years. their minimum supply voltage is 7V.
there are newer low noise, low distortion and wideband opamps that work with a supply as low as 3v.

Here is my electret mic preamp circuit:

Nigel Goodwin · Feb 28, 2010

audioguru said:
A TL071 single opamp and a TL072 dual opamp are low noise, have low distortion and are wideband. they were used in many hi-fi systems for the last 30 years. their minimum supply voltage is 7V.
there are newer low noise, low distortion and wideband opamps that work with a supply as low as 3v.

Here is my electret mic preamp circuit:

Hi AG,

This is another excellent circuit you often have reason to post, so I've added it to your other opamp circuit in the sticky at the top of this forum.

audioguru · Feb 28, 2010

Thanks, Nigel.
I will link to the sticky for the next time.

danrogers · Feb 28, 2010

ah i see thanks for posting that circuit

so i dont have to feed a power supply into the electret mic?

Nigel Goodwin · Feb 28, 2010

danrogers said:
ah i see thanks for posting that circuit

so i dont have to feed a power supply into the electret mic?

Yes, R1 is the power feed to it.

Thanks to Hero999 for pointing out I've messed up the circuit in the sticky, I'll go and cure it now

danrogers · Feb 28, 2010

of course, sorry im really inexperienced with these types of audio circuits!

so how does the feedback come from the mic? or does it measure the voltage drop getting pulled to ground through it?

i hope that makes sense

thanks

audioguru · Feb 28, 2010

danrogers said:
so how does the feedback come from the mic? or does it measure the voltage drop getting pulled to ground through it?
thanks

The mic has a metalized membrane that is vibrated by sounds. The membrane or the backplate is permanently charged with 48VDC using electret material and the membrane forms a variable capacitor voltage divider. The variable voltage from the membrane feeds a Jfet transistor inside the mic that is powered by the 10k resistor in the preamp circuit.

The mic will produce acoustical feedback howling if it hears its output from a speaker nearby.

danrogers · Mar 1, 2010

Thanks alot, they really helps me to understand it better.

what is the purpose of the VR in the circuit?

also, I want to be able to use a adjustable level output to trigger an RF module to send. what other bits of audio processing might i need to make that work?

audioguru · Mar 1, 2010

VR is the gain control. It adjusts the sensitivity to sound level.

danrogers · Mar 2, 2010

i see, thanks thats really helpfull.

Do you happen to have any idea how much power in volts the circuit would output?

thanks again

Nigel Goodwin · Mar 2, 2010

danrogers said:
i see, thanks thats really helpfull.

Do you happen to have any idea how much power in volts the circuit would output?

thanks again

Fairly obviously it depends low loud the input is, and how the gain is set.

audioguru · Mar 2, 2010

With a 9V supply, a TL071 opamp clips (severe distortion) when its output is about 3.3V peak or 2.3V RMS.

danrogers · Mar 2, 2010

ok thanks, im wondering if that would be high enough to trip a relay or something similar, to trigger my rf module.

any suggestions on that?

thanks for your help and time

audioguru · Mar 2, 2010

A relay is turned on and turned off, not tripped. A relay is driven with DC but the output of the preamp is AC audio.
Your RF module also might be turned on (not triggered) with DC, not AC audio.

The output of the preamp can be rectified, filtered and amplified to produce DC.

suitable microphone to trigger an adjustable level

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