Page 1 of 1

### #9 The 555 Timer [Post Homework Here]

Posted: Thu Dec 20, 2012 7:06 pm
A new lesson was posted today:
http://www.pyroelectro.com/edu/basics/555_timer/

### Re: #9 The 555 Timer [Post Homework Here]

Posted: Fri Dec 21, 2012 3:29 am
1.
What are two uses of the 555 timer?

Metronome, stable timer for time critical circuit

2.
How many pins does a standard LM555 timer IC have and what are the names of each pin?

8 pins in total: (details given for astable mode, which was what was shown in the tutorial)

1) Ground - Most negative potential of the IC
2) Trigger - Connected to Threshold (pin 6) and between R2 and the capacitor
3) Output - Outputs a High (5-15v) or Low (0v) digital signal (square wave) depending on the timing set in the IC
4) Reset - Voltage (0.1-0.4V) applied to this pin resets the output to Low and restarts on-off cycle. Tied to Vcc when not in use.
5) Control Voltage - Used to vary the timing of the device independently of the RC values.
6) Threshold - When given a voltage above 2/3 of Vcc (the normal voltage of the Control Voltage pin), it resets the cycle and sets output to Low just like the Reset pin.
7) Discharge - Connects timing circuit to ground. Vcc (+) - Input voltage

3.
What R1/R2/C1 values could be used for a 10 milliseconds On-Time pulse from an a-stable
555 timer configuration? (use Frequency = 100 Hz)

I got a strange answer for this -- probably completely wrong.

F = 100Hz = 1/period = 1 /(Ton + Toff)
Ton = 0.01s
100Hz = 1/(0.01s + Toff) => Toff = 0s

Ton = 0.693 * (R1 + R2) * C1 = 0.01
Toff = 0.693 * R2 * C1 = 0
and Ton != 0, so the only component of Ton that can be 0 and is also in Toff's equation is R2. Therefore R2= 0.

0.01/0.693 = R1 * C1

R1 and C1 are factors which multiply to give 0.01443001

### Re: #9 The 555 Timer [Post Homework Here]

Posted: Fri Dec 21, 2012 9:47 pm
coresnake wrote:....
....
...
3.
What R1/R2/C1 values could be used for a 10 milliseconds On-Time pulse from an a-stable
555 timer configuration? (use Frequency = 100 Hz)

I got a strange answer for this -- probably completely wrong.

F = 100Hz = 1/period = 1 /(Ton + Toff)
Ton = 0.01s
100Hz = 1/(0.01s + Toff) => Toff = 0s

Ton = 0.693 * (R1 + R2) * C1 = 0.01
Toff = 0.693 * R2 * C1 = 0
and Ton != 0, so the only component of Ton that can be 0 and is also in Toff's equation is R2. Therefore R2= 0.

0.01/0.693 = R1 * C1

R1 and C1 are factors which multiply to give 0.01443001

Hi Coresnake,

Good answers, I like that you looked in detail to what each pin of the 555 timer actually does. Like the 741 op amp there are more complexities in these integrated circuits then I let onto since we're introducing them.

Question #3 was a trick question. You can't have a Ton = 10mS with frequency @ 100 Hz because the period would be 10mS, thus the signal is 'always on'..sorry for making you the test student on this one. I realize now that it's a bit mean. I went ahead and changed the question as I feel students will benefit more from solveable homeworks:

What R1/R2/C1 values could be used for a 10 milliseconds On-Time pulse from an a-stable 555 timer configuration? (use Frequency = 5 Hz, 20 Hz and 50 Hz for 3 sets of R1/R2/C1 Vaues)

Don't be mad at me! ### Re: #9 The 555 Timer [Post Homework Here]

Posted: Mon Dec 24, 2012 12:18 pm
Haha, you got me!

Well technically I solved it right though, since I calculated Toff as 0!

That was a good learning experience nonetheless

### Re: #9 The 555 Timer [Post Homework Here]

Posted: Thu Feb 07, 2013 1:02 am
HOMEWORK QUESTION 1
What are two uses of the 555 timer?
Proximity alarm and music keyboard
HOMEWORK QUESTION 2
How many pins does a standard LM555 timer IC have and what are the names of each pin?
eight!
1=Ground, 2=trigger, 3= output, 4=reset, 5=control Voltage, 6= Threshold, 7=discharge, 8= Vcc (+)
HOMEWORK QUESTION 3
What R1/R2/C1 values could be used for a 10 milliseconds On-Time pulse from an a-stable 555 timer configuration? (use Frequency = 5 Hz, 20 Hz and 50 Hz for 3 sets of R1/R2/C1 Vaues)

HOMEWORK QUESTION 4
If a 555 timer’s output signal has a period of 25 milliseconds, what is the frequency of this signal?
f=1/0,025=1600Hz

### Re: #9 The 555 Timer [Post Homework Here]

Posted: Sat Feb 16, 2013 5:09 am
jonnyquin wrote:HOMEWORK QUESTION 1
What are two uses of the 555 timer?
Proximity alarm and music keyboard
HOMEWORK QUESTION 2
How many pins does a standard LM555 timer IC have and what are the names of each pin?
eight!
1=Ground, 2=trigger, 3= output, 4=reset, 5=control Voltage, 6= Threshold, 7=discharge, 8= Vcc (+)
HOMEWORK QUESTION 3
What R1/R2/C1 values could be used for a 10 milliseconds On-Time pulse from an a-stable 555 timer configuration? (use Frequency = 5 Hz, 20 Hz and 50 Hz for 3 sets of R1/R2/C1 Vaues)

HOMEWORK QUESTION 4
If a 555 timer’s output signal has a period of 25 milliseconds, what is the frequency of this signal?
f=1/0,025=1600Hz

Hi Jonny,

Answers 1 and 2 are correct. #4 should be 25 mS = 0.025 = 1/0.025 = 40 Hz, it looks like you were on the right track but got mixed up at the end.

Is Question #3 too complicated? Let me know if you guys want, I can post a solution for it or make it a tad easier to solve.

### Re: #9 The 555 Timer [Post Homework Here]

Posted: Mon Nov 25, 2013 2:04 am
HOMEWORK QUESTION 3
What R1/R2/C1 values could be used for a 10 milliseconds On-Time pulse from an a-stable 555 timer configuration? (use Frequency = 5 Hz, 20 Hz and 50 Hz for 3 sets of R1/R2/C1 Vaues)

this one has me at a stand still I've tried changing the final formula but all answers result to 100 so is R1/R2/C1 values all 100 ohm and farads?