# Need Help Explaining: 555 Timer's Circuit.

Discussion in 'The Projects Forum' started by InfinitelyFinite, Dec 3, 2015.

1. ### InfinitelyFinite Thread Starter New Member

Dec 3, 2015
7
1
Hello Everyone,

I am having a hard time seeing what is going on in this circuit in the sense that from my perspective(I know this is probably a bad way of looking at a circuit), the 12 Volt is connected to a switch/button, which then goes into the 555 timer as an input. The 555 timer in astable mode makes the output into a type of AC ripple that pulses over the transistor? I am not to sure what happens to the output when looking at the NPN power transistor. It is unclear to me how the transformer is connected and how it gets its input. I know that when the battery is on and the timer is on that should mean that the transistor is on making the current across CE able to flow, but I am not sure of the rest. Any and all help is very appreciated. Thanks!

-Kenyon Smith
Moderator Note: 555 discussions ok. Stun guns violate TOS.

Last edited by a moderator: Dec 3, 2015
2. ### UsagiCurry New Member

Nov 17, 2015
9
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Just a warning that this topic is against the rules of this forum, because it is used to shock someone.

3. ### InfinitelyFinite Thread Starter New Member

Dec 3, 2015
7
1
Hello UsagiCurry,

This will not be used to shock someone I am only trying to analysis this circuit from a physics perspective. We could take out the voltage multiplier step which would make this curcuit not able to harm anyone if that better the situation?

-Kenyon

4. ### InfinitelyFinite Thread Starter New Member

Dec 3, 2015
7
1

So the output from the timer is then fed through the NpN transistor? How does this reach the transformer? Sorry having trouble seeing it?

- Kenyon

5. ### #12 Expert

Nov 30, 2010
16,665
7,313
The point labeled, "C" is connected to one winding of the transformer labeled, "1K:8R". The other end of that winding is connected to the 12 volt supply which is controlled by the "on" button.

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6. ### InfinitelyFinite Thread Starter New Member

Dec 3, 2015
7
1
Thank You #12!
I understand that part of the circuit pretty well now. I guess I would have to say I am not sure how they picked out their resistors and capacitors that are connected to the 555-timer, making it in the astable mode we want. Again Thank you for helping me grasp some concepts about this circuit.

-Kenyon

7. ### InfinitelyFinite Thread Starter New Member

Dec 3, 2015
7
1
Thank you, Dodgydave! So I am guessing the resistors and capacitors connected to the timer have to deal with the speed of the 555's output?

8. ### #12 Expert

Nov 30, 2010
16,665
7,313
Designing an astable 555 circuit is covered in the, "book" associated with this site. Look at a dark blue bar across the top of this page and click on the word, "Textbooks".

I see about 2.75 KHz as the speed of this circuit.

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9. ### ISB123 Well-Known Member

May 21, 2014
1,240
531
R1 and C set the frequency of the timer.

4.3 KHz accordingly to online calculator.

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10. ### #12 Expert

Nov 30, 2010
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From the formula: T = 1.1 RC...
Have I calculated only half the time by mistake?

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11. ### ISB123 Well-Known Member

May 21, 2014
1,240
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I'm getting 4,36 when I calculate it via the calculator.

This is the formula.
1.44 / ((R1 + R2 + R2) * C)

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12. ### #12 Expert

Nov 30, 2010
16,665
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I see only 33k and 10 nF. You seem to have found a second resistor.
I just noticed, pin 7 is not connected in this circuit.
Suddenly I realize I was just reciting from memory with brain in neutral.

Edit: I also see that finding the "Textbooks" is a new adventure because the Orange Team reorganized the site.

Last edited: Dec 3, 2015
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13. ### InfinitelyFinite Thread Starter New Member

Dec 3, 2015
7
1
Thank you all for you helping me analyze this circuit. As for pin 7, it is not shown but the line going out of the pin should be connected to the northern side of the line coming out of the 33k resistor. I am studying up on the 555-timer in astable mode. Thanks again guys!

-Kenyon

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14. ### ISB123 Well-Known Member

May 21, 2014
1,240
531
We are both wrong but you were close.

Proper formula when pin 7 is not used is:
f=0.72/R1 x C

Now I'm getting 2.18 KHz.

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15. ### #12 Expert

Nov 30, 2010
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All things considered, these variations in speed calculation don't seem very important.
2KHz, 3KHz, 4KHz...not awfully significant in this case.

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16. ### hp1729 Well-Known Member

Nov 23, 2015
2,086
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Another creative way to get a 50/50 duty cycle. Initial power on the cap is discharged, trigger is activated and the cap charges through the resistor and the output high side driver. When threshold is hit the output goes low and the cap discharges through the same resistor. 50% duty cycle.

Last edited by a moderator: Dec 3, 2015
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17. ### Kenyon Smith New Member

Dec 8, 2015
1
0
Hello,
I need some help with the calculations going through my each stage of the voltage multiplie if any knows? I know that each diode doesn't have much current going through. But not sure how to calculate? Thanks for any and all help.

18. ### InfinitelyFinite Thread Starter New Member

Dec 3, 2015
7
1
Can anyone explain to me what the 100nF capacitor in pin5 of the 555 timer is doing? I know it is the control pin and that it will affect the frequency of the timer, but I am not sure where the physics for the timer is. Also, I am unsure about what the 1k resistor connected to pin 3 actual does? My first thought it is a load protection resistor just in case the voltage gets too high?

19. ### hp1729 Well-Known Member

Nov 23, 2015
2,086
232
The capacitor is just a noise filter for the voltage divider inside the LM555. Inside the LM555 there are three resistors, all about 5K as I recall. They set the voltage limits at 1/3 of VCC for the trigger level and 2/3 VCC for the threshold level. The timing capacitor charges and discharges between these two points. The 1K on the output limits the base current to the transistor it drives.

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20. ### #12 Expert

Nov 30, 2010
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The capacitor on pin 5 is a voltage stabilizer for the internal voltage divider. The LM555 has a nasty Vcc glitch when the output changes state. The newer models have that fixed, as far as I know, but putting a tiny filter on a voltage reference doesn't do any harm, even if it is just an old habit.