Smart hearing aid circuit that doens't work

[guillotf]

Hi I am a Mechanical Engineer student and I am learning electronic so I selected a <<Smart Hearing Aid>> project for school that can be found at

http://therohanaurora.com/top-5-easy-mini-projects-for-biomedical-engineering-students/

I have also added the DC voltage measurements as an attachement

That circuit had inherent problems even if it says TESTED:
First when the circuit was in operation the 555 and the T5 were burning. The 8 ohm impedance of the earphone was draining so much current that Pin 3 of the 555 was only at 3.5 V. At the electronic store they suggested to add an audio transformer with 1.2 kOms. It seems to have solve that problem.

Now the main problem is the condensor microphone that doesn't work. there is almost no current flowing form R1 down. The microphone measurement resistance at 1.2 KOhm. I ordered two from future Electronic. There is also T2 collector at 0 Volt I don't get.

I did checked the circuit, soldering, did check all transistors with dynamic transistor checker are all fine.

I removed R6 and was able to drive the output with noise and hiss when I touch with my finger the collector/emitter of T2, then the voltage at collector seem to bias correctly and so for the rest of the stages. That 680 kOhms seems to set the collector to 0V.

Still the microphone drive no current. Put the microphone directly on my PC speaker... no hope.

Not familiar the way those mini-mic works how do they get in operation if not biased? I have no scope to see the AC side of the circuit

The specs can be found at http://www.icc-intervox.com/inc/Intervox_catalog/page33.pdf

MEO-63PN-02-663 (643) Pin

Sensitivity -66,-64 20 ~ 16,000

Operating Voltage 2.0V 10V

Max current 0.5mA

within -3dB @ 1.5V >38dB

Since R1 = 2.2 KOhm and microphone = 1.2 KOhm should be a current of 0.25 mA?


Thanks your your help !

 

[OLIVE2222]

Hi,

Nice to see detailed post and annotated schematic !
Electret microphone's include a fet transistor for amplification purpose. It' s described here under.

http://en.wikipedia.org/wiki/Electret_microphone

You can't get it working by connecting directly to a loudspeaker. That what the amplifier chain is making for. A small utility amplifier can be used to test it when you don't have a scope.

alike:
http://www.ebay.com/itm/Low-Voltage...pt=Vintage_Electronics_R2&hash=item5d31083d4a

You can also carefully connect it to the mic input of a PC.
Those mikes can be a little bit fragile when overheated. If yours is dead you can use one from a cheap PC headset.

Olivier

 

[duke37]

The op amp driving the audio amp is connected directly to the base of the transistor. This destroys any action of the biasing resistors. Add in a capacitor and check the transistor is OK.

 

[sharealot]

I had done the same for my miniproject and i am happy to inform that i got the output after making some small changes to the circuit...

First of all I added a capacitor to the base of T2 as suggested by 'duke37' which not only corrected the bias but also solved the problem of burning of transistors and 555. Smaller the value of the capacitor the better.. dont go for values such as 0.1uF,0.01uF etc... an optimum value of either 2.2uF, 3.3uf is sufficient

I removed the resistor R5 2.2k which was making the collector voltage of T2 zero.. it was not the 680k making the collector voltage 0.

 

[rasiyasultana]

hello sharealot...
i am doing this as my miniproject and followed the corrections you made.but still i cant get any output.

 

[john monks]

What is the voltage across the microphone?
What is connected to the collector of T5?
You might have to make some adjustment to the value of R10 to get T5 to operate in the linear region.

 

[CDRIVE]

As of this post I count 4 members building this same project. Are you guys all classmates?

Besides the coupling issue on the base of T2 that Duke found, there are other issues with this circuit. In my opinion this amplifier is a very poor design. There may be other issues that I've missed but this statement is based solely on the BJT stages. The design is poor because the bias techniques for the transistors are too basic. They don't employ resistive current dividers and no emitter resistors either. Without either of these features no two amplifiers will perform the same. In order for this circuit to work as the author drew it he would have to state required beta for each transistor. Even then the temperature stability would still be awful. The circuit may work at room temp but crap out with high or low temperatures.

Chris

 

[rasiyasultana]

voltage across condenser mic is 1.14v and i have connected an earphone to the collector of T5

 

[CDRIVE]

My opinion of this design hasn't changed but if you guys insists on pursuing it you would be well advised to divorce the 555 from the audio circuit. This will only be for testing purposes. You can do this by lifting the connection to pin 3 of the 555. Then connect the + node of C3 directly to the 9V rail. Doing so will supply Vcc to T2 thru T5, thus eliminating the 555

Chris

 

[john monks]

I agree with you CDRIVE that this is a terrible circuit and for the reasons you explain. The problem is that this is a homework assignment and he is stuck with it unless he decides to go with another project. But short of that he needs to select a high impedance earphone, maybe 1K then adjust R10 to get about 4 volts across the earphone.

I agree that this circuit will be so unstable that it will probably never work. But the interesting thing is that the output stage is off until sound comes into the microphone.

Guilloft, this is not a good project for a mechanical engineer. maybe you should select another project unless you can get permission to greatly modify the circuit.

If you want the entire output circuit can easily be replaced with an operational amplifier circuit.

 

[CDRIVE]

John, I don't disagree with anything you said. The instructor should at least let them tweak the values of the bias resistors or supply the students transistors with the identical beta that was used in the original circuit.

OpAmp....YES!

Chris

 

[davenn]

..........Guilloft, this is not a good project for a mechanical engineer. maybe you should select another project unless you can get permission to greatly modify the circuit.
If you want the entire output circuit can easily be replaced with an operational amplifier circuit.

you do realise that Guilloft's post was 2 years ago and it was his one and only post ?

Dave

 

[CDRIVE]

you do realise that Guilloft's post was 2 years ago and it was his one and only post ?

Dave

I didn't catch that but on the up side this gentleman pulled the thread out of moth balls.

hello sharealot...
i am doing this as my miniproject and followed the corrections you made.but still i cant get any output.

Chris