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I have designed a SIM900 circuit on a zero pcb board for testing purpose. It is working fine. In my application I need to make a call and then talk to the other person on call. So I have used PIC MCU to initiate a call whenever a person press the switch. The circuit is working as expected. SIM900 requires voltage between 3.3v-4.8v and PIC MCU works on 5V and my input voltage to the pcb is 12v so I have used a LM317 for SIM900 to give it 4v and have used LM7805 for PIC MCU to give 5v.

When on a call I need to listen to the other person so I have used this speaker but the output audio was very low so I though of designing audio amplifier circuit using LM386. The circuit is also working fine and giving loud audio. As the input requirement of the LM386 is 5v and because I have designed it on another pcb so I have used the 5v from the LM7805. I connected everything and pressed the switch. Speakers were making call connecting sound and as soon as the call is connected, there was no sound. I tried again and again and checked the circuit but nothing was wrong. Then I used external 5v power supply and gave it to the LM386 circuit and now everything started working. There was clear and loud audio. But as I cannot use two different power supplies( 12v, 5v) I need the complete circuit on same 12v supply.

I don't know why its not working on 5v from LM7805 but its working on external power supply. Does anyone have any idea about it. Please help.

Here is the schematic of power circuit:

enter image description here

Below is the schematic of LM386:

enter image description here

S Andrew
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  • Schematic where? – Ignacio Vazquez-Abrams Jul 28 '16 at 11:06
  • @IgnacioVazquez-Abrams I have updated the schematic for power circuit which I am using in my PCB board. – S Andrew Jul 28 '16 at 11:13
  • Do either of your regulators get hot? Have you tried measuring the current through them? Or measuring their output voltage in operation with the SIM900? – Steve G Jul 28 '16 at 11:17
  • @SteveG No none of them get hot. I haven't measured the current but yes output voltage is fine. LM7805: 4.97v and LM317: 3.9v – S Andrew Jul 28 '16 at 11:18
  • The LM386 should be able to take 12V. Could you try powering the LM386 from the 12V rail? – Buck8pe Jul 28 '16 at 11:38
  • @Buck8pe Yes I have tried giving it 12v but the problem is same. When I press the call, the call connecting sound and other little noise comes but as soon as the call is connected, speakers act like they are disable. No sound. but its working on external 5v – S Andrew Jul 28 '16 at 12:01
  • My suspicion is that the act of connecting is putting some kind of interference (oscillation, spikes) onto your power rails knocking out your LM386 or (most likely) your PIC. Make sure your decoupling is spot on (see datasheets) and it would help if you could scope the rails. – Buck8pe Jul 28 '16 at 12:16
  • @Buck8pe What I have done to test it is I have connected the 12v to another LM7805 and then given this 5v to LM386 circuit. But still its seems not to working. Its only working on external supply. – S Andrew Jul 28 '16 at 12:27
  • When the amp and the pic share the 12V line you have issues and it works at the start, but then fails. That suggests some event that triggers oscillation and I think it's caused by the LM386 and the afflicted part is the pic. Try rebuilding the amp on the breadboard and use a 100uF decoupling cap in parallel with C5. – Buck8pe Jul 28 '16 at 12:43
  • Also, try increasing C6 to, say, 220uF or 470uF. – Buck8pe Jul 28 '16 at 12:45
  • Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/43136/discussion-between-andrew-and-buck8pe). – S Andrew Jul 28 '16 at 13:10

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The problem is resolved. Thanks to @Buck8pe . I removed the c4 capacitor and increased the power supply to 12v so now I am getting loud audio and both of my circuits are working fine with the same supply.

S Andrew
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