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I have this circuit from the image below that I am designing myself. part of this circuit is cut so 2 78xx is present. This circuit is being designed and tested in parts. This part of the circuit has to work basically by amplifying the signal from the electret microphone, as it will also work by sounding a siren sound generated by the 555 when the button is pressed. The part of lm386 is ok, without noise. the problem is when I connect output 3 of the 555 to one end of the button and the other to input 3 of the op amp; the siren sounds very low in the background when I do this. already tried to solve even with transistor and nothing solved. does anyone have any tips on how can i solve this ?.

Thanks in advance

obs. I've tested it with other buttons, it seems that the buttons continue to drive even in cut enter image description here

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  • \$\begingroup\$ one possible mitigation is to have the switch also halt the 555s (eg by grounding the reset pin) \$\endgroup\$
    – Jasen Слава Україні
    Commented Jan 20, 2020 at 4:09
  • \$\begingroup\$ also add 100nf across the 555 power supply. \$\endgroup\$
    – Jasen Слава Україні
    Commented Jan 20, 2020 at 4:11
  • \$\begingroup\$ As you suggested and unfortunately ir Didi`t work \$\endgroup\$
    – gabriel
    Commented Jan 21, 2020 at 12:57
  • \$\begingroup\$ The lm386 is very noisy, even if doing this \$\endgroup\$
    – gabriel
    Commented Jan 21, 2020 at 12:59

1 Answer 1

Reset to default
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Pin 5 of a 555 is usually 2/3rds of the 12V supply which is +8V. You can force it up to 11.6V and force it down to 4.4V. The output of the left side 555 driving yours forces it up to +10.7V and down too far to almost 0V. So attenuate the 0V to +10.7V down to 0V to +7.2V and capacitor-couple it into pin 5 of the frequency shifting 555.

Then the output level from the siren 555 will be WAY TOO LOUD and it will need its own volume control.

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  • \$\begingroup\$ Hello audioguru, As you suggested and unfortunately ir Didi`t work \$\endgroup\$
    – gabriel
    Commented Jan 21, 2020 at 13:01
  • \$\begingroup\$ The datasheet of an LM555 or an NE555 shows what I said. maybe you are using a Cmos LMC555, TLC555 or ICM7555 instead? Please show the schematic that failed. \$\endgroup\$
    – Audioguru
    Commented Jan 22, 2020 at 14:21

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