# Oscillator, Voltage Divider, Help

#### cheddy

Joined Oct 19, 2007
87
I am studying a textbook, teaching myself.

This chapter asked me to create the following astable multivibrator using the 555 IC. According to the datasheet I am using, my 555 IC is capable of operating at 2MHz.

I have constructed this exact thing many times so I am sure that I am following the book correctly. I've built it from that design and I've also built it by copying the example breadboard layout included.

According to the book I should get the following values:

C1 = 10uF,.....Frequency = 5.8Hz,......Voltage = ~0V
C1 = 1uF,.......Frequency = 58Hz,.......Voltage = ~0V
C1 = 100nF,....Frequency = 580Hz,.....Voltage = ~1.5V
C1 = 10nF,......Frequency = 5.8kHz,....Voltage = ~4V
C1 = 1nF,.......Frequency = 58kHz,......Voltage = ~4.2V

I am supposed to come to the following conclusions:
1. Above a certian frequency, the "Corner Frequency" the output signal is constant at it's maximum
2. At low frequencies the output is zero
3. Between these two sections the output signal varies in size depending on the applied input frequency

MY DILEMMA:

When I do the experiment myself I arrive with the following data:

C1 = 10uF,.....Frequency = 5.8Hz,......Voltage = ~0V
C1 = 1uF,.......Frequency = 58Hz,.......Voltage = ~0V
C1 = 100nF,....Frequency = 580Hz,.....Voltage = ~1.5V
C1 = 10nF,......Frequency = 5.8kHz,....Voltage = ~4V
C1 = 1nF,.......Frequency = 58kHz,......Voltage = ~1.6V

I have been frustrated for 2 days trying to figure out what is the problem.

I am using a new digital multimeter, I paid \$40 at Radio Shack for it, It should be advanced enough to give me accurate readings.

I've tried 3 different 555 ICs from different manufacturers

I've checked my resistor values countless times and all my connections are correct.

I've used 2 different brands of 1nF capacitors (both ceramic) and get the same results.

#### ixisuprflyixi

Joined Sep 16, 2007
52
Is it possible that the new DMM you got doesn't have a high enough sample rate to accurately read the voltage when the frequency is at 58Khz? Just a thought, also in the schematic you drew it looks as though the capacitor has polarity and in that case did you try a 1nF tantalum cap? The tantalum caps are superior for temperature stability and frequency response.

#### cheddy

Joined Oct 19, 2007
87
Is it possible that the new DMM you got doesn't have a high enough sample rate to accurately read the voltage when the frequency is at 58Khz? Just a thought, also in the schematic you drew it looks as though the capacitor has polarity and in that case did you try a 1nF tantalum cap? The tantalum caps are superior for temperature stability and frequency response.
You are right there is a polarity on the C1 capacitor. I think this is because in the first two tests we are asked to use 10uF and 1uF electrolytic capacitors, for the other capacitors there is no specific type of capacitor it says to use. In the breadboard example layout there is a picture of a ceramic capacitor. Wiki states that ceramic capacitors are suitable for high frequencies.

How can I tell what the DMM sample rate is? Would most generic multimeters fail at 50kHz+ ? Google doesn't seem to have any answers for this question.

#### JoeJester

Joined Apr 26, 2005
4,126

C2-R3 form a high pass filter. I doubt you will see anything less than 1 kHz.

#### GS3

Joined Sep 21, 2007
408
Let's see. My calculations give different frequencies.
Rich (BB code):
   R      R         C          Ton        Toff         Hz   duty cycle
10000   4700   0.00001       0.101871   0.032571       7.4   75.77%
10000   4700   0.000001      0.010187   0.003257      74.4   75.77%
10000   4700   0.0000001     0.001019   0.000326     743.8   75.77%
10000   4700   0.00000001    0.000102   0.000033    7438.2   75.77%
10000   4700   0.000000001   0.000010   0.000003   74381.5   75.77%
At any rate, the duty cycle is always the same, 76% high, 24% low.

R3 * C2 = 0.000450 (450 us) so any states lasting longer than 1 or 2 ms will appear at R3 as spikes which rapidly decrease. As frequency rises and states become shorter the voltage at R3 becomes more rectangular but it is meaningless to discuss "voltage". What voltage? DC component will always be 0. AC voltage can also be analyzed as RMS, frequency components, etc.

Let us study RMS value of a rectangular wave, 9V pp with 76% up, 24% low and no DC component. It will be positive 76% of the time at 24% * 9V = 2.16 V and negative 24% of the time at 76% * 9V = 6.84 V. RMS is 3.84.

In summary, for low frequencies the RMS voltage at R3 is close to zero as the value of the spikes is very low. As frequency rises around 750 Hz we see the measured RMS voltage rapidly rise up to 3.84 V which is the maximum.

One has to be careful in making measurements, especially on two points, (1) make sure you understand exactly what is bein measured and how and (2) make sure the measuring process does not interfere with the quantity being measured or at least take this effect into account.

Some ascii art where the long horizontal line represents 0V:
Rich (BB code):
Low freq.
|
|
|
___________|\________________  _____________________
|/
|
|
|

High freq.
___   ___
___________|   | |   |  ___________
| |   | |
| |   | |
|_|   |_|

#### JoeJester

Joined Apr 26, 2005
4,126
GSM,

Your time off calculation is incorrect throwing off your other statements of frequency and duty cycle. The time constant for C2-R3 is 150 uS not 450u.

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#### GS3

Joined Sep 21, 2007
408
you mean GS3? (that would be moi)
Your time off calculation is incorrect throwing off your other statements of frequency and duty cycle.
Shoot! Right you are! I exchanged the values of R1 and R2 as I memorized them and went off to do my calculations. Oh well. never mind then.
The time constant for C2-R3 is 150 uS not 450u.
Again! I need to pay better attention. It *obviously* is 150 us, which I did mentally and then thought to myself, anything above about three times as much will give time for the pulse to decay to almost zero.... and then I wrote 450.

Okay, never mind me. Some days I'd do better staying in bed all day.

#### cheddy

Joined Oct 19, 2007
87
Thanks a lot for the PDF JoeJester it explains a lot. Should I conclude that the circuit is correct and the equations are correct and it's the multimeter that's giving me the wrong reading?

#### GS3

Joined Sep 21, 2007
408
Thanks a lot for the PDF JoeJester it explains a lot. Should I conclude that the circuit is correct and the equations are correct and it's the multimeter that's giving me the wrong reading?
While the exact values of my calculations are off, they are still in the ballpark and the points I was making are still valid. You need to understand wave shapes and characteristics and instruments limitations. The multimeter is not "giving you the wrong reading" but rather you are not understanding what you are measuring. The readings are "right" or "wrong", depending on how you look at it, over the whole range. The problem is not with the meter. The problem is between the meter and the chair.

#### JoeJester

Joined Apr 26, 2005
4,126
cheddy,

Errors notwithstanding, GS3's analysis is correct. You need to view the waveforms with an oscilloscope, not a multi-meter.

GS3 ... sorry about the GSM... I don't have a clue where that came from. Must be I'm suffering from CRS. Can't Remember S....t.