Hi all,

I think this is the third little project I have asked for help with on this forum now and so far have been very impressed with the help I have recieved. Hope this time is no different!...

This time, I have been building a little LM386 based amp to go inside of my toolbox so I can plug in my cellphone when out on job sites and listen to the radio through it. I am using a 3" 1 watt 8 ohm speaker and 7.2v Lipo battery (with Microscream alarm to alert me when battery needs charging). So far so good. I was really impressed with the volume this little circuit can do with such small components, and while testing with a CD Walkman I had full control over the volume using the 10K ohm pot recommended on the datasheet. (I built it using the "Amplifier with gain = 20/ minimum parts" diagram initially and then modified slightly to get gain = 200 which worked just as well.

Now, however, I have hacked into the headphone cable from my Nokia cellphone and wired that in directly to the input but now there is no volume control at all. It just plays loud at maximum volume level. It seems that the 10k pot has no control over the input level. I set the volume on the cell phone at bare minimum, and I can confirm that the 10k pot is correctly wired, since cutting a wire on it all but kills the sound.

I have tried wiring in resistors in series with it up to a value of 240K ohm but they do nothing.

It's got me bafled, but hopefully someone here knows what is going on and what can be done!

(by the way, the nokia headset had a microphone in line with the stereo earplugs. I dont know exactly how it was wired up since there was only four seperate wires in the cable, but I opened up the earbuds to try and findout which two were common ground and joined the 4 into 2 pairs accordingly into the input of the amp. Not sure that I got ground correct - perhaps this is the cause of the problem?

Thanks for reading if you got this far! Hope you can help me too

 

Nokia's basic headsets are 4 wire with a N/O micro switch across mic pins to act as a answer button..
Your description indicates the maximum possibility that the ground is not available to the pot... Either you might have taken L & R wires and wired them directly mistaking the GND.. Otherwise the 3rd unused pin of the pot u r using is not GNDed..
Easiest way to check all this is to connect your cell phone to some other device and see if you can control the volume. Evn your tv is enough. In short, re-check the headphone wirings is the first thing to do..
I have wired both type of headsets succesfully sometime back and worked well..

Also, Nokia's headset wires are L, R, mic and GND.. last one is necessary..

 

Ah, thankyou for that. Yes, I see now after opening up the mic/switch housing that the headset I have is not stereo after all. I will have to rewire to the LM386 board tonight after work.

Another related question:
Which wire is for the radio antenna? Is it one of the earpiece wires or one of the mic wires? Should the ground for the earpiece be connected to the battery -ve connection in the back of the phone? How else can I tell which is which?

Thanks again

 

You're most welcome..
Abt the antenna, its the speaker wire as mic's will cause interference to the whole circuitry and there are headsets with no mic but just earphones and they work well.. You can just connect a wire one by one to each and see for yourself. GND is not necessary for the antenna of FM decoders which work with dedicated chips, and so is a phone.. Just the ANT is necessary.. Older radios like crystal ones which dont have an RF amplifier section only need them.. Experiment yourself wit a single wire and sway it and see... But dont ground the antenna wire ever.

 

Went over the Nokia headset wiring again, and still couldn't get the volume control working. Then I reread your first post and realized that I had not connected the third pin of the variable resistor to ground

So, after sorting that out it all goes great

For the size of the components and speaker (3") I am quite impressed with the results.

Anyway, here's a few pics.

Thanks again for your help.

 

You're most welcome. .This is a place to take and give help.. Glad that you got it running..