I have a schematic where I'm using an LM741 op-amp as a voltage comparator when its Vout is HIGH/+Vcc it "allows it to operate the 555 timer" and therefore activates the buzzer, and vice versa when the Vout of the op-amp is LOW/-Vcc.

This is all I know base on how in general my circuit works (I tested it on a breadboard and it works). note that, I got this schematic on the internet that's why I don't know much

But I want an in-depth understanding of how does this "turn on" the 555. Can you provide an explanation as well as some equations to help me understand and in the process can you answer these?

  1. What do the 33k resistor and 100k Pot do here exactly? (I know that the Potentiometer 2 varies the frequency of the buzzer and if I attach a fan it varies the speed of the fan)
  2. Why is the Vout of the op-amp connected to the RST pin of the 555? (what does the rst pin do exactly?)
  3. Maybe a bit explanation on trggr and thrshld pin contributions?

enter image description here

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    \$\begingroup\$ For the 555 oscillator, google "555 astable multivibrator", lots of articles available. Those will give you the operation of RESET pin as well. \$\endgroup\$ Commented Jun 9, 2023 at 7:16
  • \$\begingroup\$ The lm741 really is another component to avoid. Using it as comparator is an especially bad idea due to its potential for latching up. \$\endgroup\$ Commented Jun 9, 2023 at 7:46
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    \$\begingroup\$ Also, if you're learning electronics with the ne555 and 741 opamps: maybe find a different book. Your book hasn't been updated since the 1970s. If you're learning from websites: don't use websites that copy stuff from the 1970s without mentioning how obsolete things are. \$\endgroup\$ Commented Jun 9, 2023 at 7:47
  • \$\begingroup\$ @RohatKılıç can you give a bit of insight? when I want to activate a 555 should the Vin always be connected to the reset pin? \$\endgroup\$
    – geekyhez
    Commented Jun 9, 2023 at 9:49
  • \$\begingroup\$ @HezekiahDerilo - Hi, You said: "I got this schematic on the internet". In order to comply with the Stack Exchange rule for referencing, please edit the question & add a link to the webpage which was the original source for that image. Thanks. \$\endgroup\$
    – SamGibson
    Commented Jun 9, 2023 at 13:25

1 Answer 1

  1. The pot and resistor control the 555 frequency by defining how quckly the capacitor charges and discharge.

  2. The 555 RST pin is reset. When it is high, 555 runs, when it is low, 555 does not run and it is stopped. Basically, the output if the op-amp controls if tone is generated or turned off.

  3. Those are the input pins to toggle the state of output.

You will find detailed NE555 operation in the data sheet which will cover these basic and more advanced questions about it. There should be plenty of resources available for lighter reading such as blogs and videos, as the chip is over 50 years old already.

  • \$\begingroup\$ Can you describe what does the 555 timer do here exactly? like does it delay or anything like that? \$\endgroup\$
    – geekyhez
    Commented Jun 9, 2023 at 9:50
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    \$\begingroup\$ @HezekiahDerilo Sorry if I misunderstood something, but somehow the question you ask does not correlate with at what level you want to know something. So what details you need to know? It's just a generic 50-year old 555 oscillator circuit that is controlled to produce a tone at frequency set by pot/cap and being turned on/off by reset pin. The details how the 555 itself works is in the datasheet. What kind of explanation you want and at what level? \$\endgroup\$
    – Justme
    Commented Jun 9, 2023 at 11:27
  • \$\begingroup\$ the loudspeaker translates electrical signals into noise, the frequency of the electrilcal signal correlates directly to the frequency of the tone that comes out ot the speaker. the 555 makes an uneven square wave that the loudspeaker will translate into a tone, \$\endgroup\$ Commented Jun 10, 2023 at 0:51

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