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I have build a tone generator and a sound amplifier with the same battery source. The sound from the tone generator is not only playing in the tone generator speaker but also in the amplifier speaker.

Is there a way to stop the tone generator from playing sound in the amplifier speaker? An additional battery source is not an option.

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Update 1

I did some changes but I still get the sound from the tone generator in the amplifier speaker. Any other idea why this happens?

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Update 2

I added some extra 10uf but I didn't have any 100uf as suggested in the comments. But I still have the main problem, is there is no audio in you can hear the beeping sound from the tone generator in the amplifier speaker:

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Erik Andershed
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  • What is the battery source, is it a small 9V battery or something else? Do the generator and amp share common wiring to the battery? – Justme Nov 28 '21 at 11:57
  • It’s a 6v battery. Yes both are connected parallel to the battery – Erik Andershed Nov 28 '21 at 12:04
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    Where are all your power supply decoupling capacitors? – Andy aka Nov 28 '21 at 12:09
  • I don’t think I have any – Erik Andershed Nov 28 '21 at 12:09
  • @ErikAndershed You can use ordinary capacitors, try with a 10 μF capacitor across the power supply pins of each IC and see if the situation is improved. If you haven't got any 10 μF capacitors, anything within an order of magnitude is a start. – Andrew Morton Nov 28 '21 at 12:42
  • Sorry but this is probably the stupidest question ever.. you mean between the battery plus and the plus on the IC? – Erik Andershed Nov 28 '21 at 12:47
  • @ErikAndershed Have a read of the answers to [What is a decoupling capacitor and how do I know if I need one?](https://electronics.stackexchange.com/q/2272/36731) for plenty of information ;) – Andrew Morton Nov 28 '21 at 12:57
  • Aaa I see :) I will try this. Thanks – Erik Andershed Nov 28 '21 at 13:02
  • Do you have Pin3 of LM555 connected to Pin3 of LM386 (just for make it clear)? Then you can disconnect this point or disconnect the LM386 Vcc with switch to stop playing. –  Nov 28 '21 at 16:13
  • No sorry missed that. The line out from my computer is going to the Pin 3 on LM386 – Erik Andershed Nov 28 '21 at 17:10
  • Your modified schematic won't work. You've put a 10µF capacitor in series with the power supply. Capacitors block DC. Your circuit won't get any power. – JRE Nov 29 '21 at 10:32
  • I discovered it too. Do you have any idea why I am getting the sound from the tone in the amp apeaker? – Erik Andershed Nov 29 '21 at 10:34
  • Try this to see where the problem is coming from: Short the audio in to ground. If the generator sound still comes in then it is through the power supply. If it gets quieter, then the LM386 is picking it up through the input. – JRE Nov 29 '21 at 10:43
  • You **must** connect 10µF from pin 6 to pin 4 of the LM386. That's the bypass capacitor every on has been telling you to use. Connect a 100nF capacitor between pins 6 and 4 as well (use both the 100nF and the 10µF.) Connect a 10µF capacitor from pin 1 to pin 8 of the LM555. Also install a 100nF capacitor from pin 1 to pin 8 - more bypassing. – JRE Nov 29 '21 at 10:47
  • The LM386 has lousy power supply ripple rejection (PSRR.) Even with the 10µF bypass capacitor, the datasheet says it has only 50dB of ripple rejection. Any significant amount of noise on the power supply **will** be audible on the output. – JRE Nov 29 '21 at 10:49
  • Do I need and 100nf? I only have 1,10 and 1000 at home. If so I will order some. – Erik Andershed Nov 29 '21 at 16:31
  • I did update the question with new wiring diagram as suggested. But I still have the same main problem, I can still hear the beep sound in the amplifier speaker. – Erik Andershed Nov 30 '21 at 07:16
  • Short the audio amplifier input to ground. Does the sound go away? – JRE Nov 30 '21 at 07:23
  • The ground is the minus poll on the battery, and if I short the audio amplifier to it AND have the tone using this as the ground also there is the sound. Is there a way to ground it without ground it to the battery? – Erik Andershed Nov 30 '21 at 08:20
  • @JRE Any idea on what I am doing wrong? – Erik Andershed Dec 02 '21 at 08:16

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Based on the schematics drawn, you have left out many important parts. There is not even a one single bypass capacitor at the chips, and also the capacitor to filter the internal biasing of the amplifier is missing. The 555 is famous for taking huge surges of current while switching, and you also have a large load in the form of a speaker that consumes significant pulses of current when driven with a square wave.

Justme
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  • You are probably right. I don’t know this things. Just looking at YouTube to find out how to do things. Can you help me? show what I need to do. Best – Erik Andershed Nov 28 '21 at 12:35
  • If you have a specific question, people at electronics.stackexchange can answer them, but bear in mind to search first, as most basic questions are already covered. – Justme Nov 28 '21 at 13:04
  • Yes I know.. I have been searching a lot and it’s here I am and don’t know what I shall do. – Erik Andershed Nov 28 '21 at 13:12
  • I have updated the question with more details. Hope this will fix the problem you seen. And maybe you can find out why I still can hear the beeping from the tone generator in the amplifier speaker – Erik Andershed Nov 30 '21 at 11:42
  • did you see my updated? any idea why this happens? – Erik Andershed Dec 02 '21 at 08:16