555 Oscillation between 20 and 30KHz

Discussion in 'The Projects Forum' started by mushaba, Aug 23, 2010.

  1. mushaba

    Thread Starter New Member

    Aug 22, 2010
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    Hello to you all

    I'm studying the last days the timmer 555 and i have a difficulty finding something out..At first i use a soundcard based program for oscilloscope and spectrum analyser named "Daqarta" and i'm trying to measure the frequency of an output pulse at 25 KHz,but the software seems to limit at 22 KHz,how can i determine which is the max frequency my laptop's soundcard can measure (HP 6730s)??

    Also i'm using 555 Timmer V4.1 software and i wonder if there is a way to make 555 oscilate between 20 and 30 KHz range by one single trimmer resistor between pin 7 and pin 8

    Please excuse my bad English,here i hope to gethelp,provide help if i can and make some friends!

    mushaba
     
  2. tom66

    Senior Member

    May 9, 2009
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    A laptop sound card will have a cut-off frequency at half its sampling rate. The sampling rate is usually 44,100 Hz; half that is 22,050Hz and this is the maximum frequency your sound card can measure, which correlates with your readings. The reason for this cut-off band is that it is just beyond the range of hearing.

    A 555 timer can oscillate at 20 to 30 KHz, but you will have difficulty if you are breadboarding it. When I was breadboarding my 555 timer circuit I was able to go up to 300 KHz or so, that was without a capacitor: the breadboard's capacitance was acting as a parasitic capacitor, and the cap charging/discharging waveform was badly distorted by the inductance of the traces in the breadboard.

    The formula for a 555 timer frequency is:
    F = {{1.44} \over {C(R1+R2)}}
    Frequency in Hertz.

    I worked out C = 6.8nF, R1 = 6.8k, R2 = 1.5k to give 25,513 Hz.
     
  3. mushaba

    Thread Starter New Member

    Aug 22, 2010
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    tom66 thank you for your reply

    I don't have an oschilloscope or a spectrum analyser,so i think i am condemmed :) A USB oscilloscope module sold on ebay like this cannot take measurments like a "real" oscilloscope,do you think there is any chance with a frequency devider ic to take measurments over 22 KHz??

    I didnt have in mind at all the breadboards capacitance...I would like through a variable resistor to oscillate between 20 to 30 KHz and later on after i figure out the trimmer value in ohms to change it with a resistor network and connect a microcontroller to make this variable scaling change from 20 to 30 KHz(Most schematics with variable frequency i saw has a trimmer providing possitive voltage to pin7 of a 555) Do you think it will work?

    Thanx again friend!
     
  4. Wendy

    Moderator

    Mar 24, 2008
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  5. tom66

    Senior Member

    May 9, 2009
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    @Bill: It depends on the capacitance of the breadboard. The absolute maximum I got was 325 KHz on a breadboard. Tested with an oscilloscope. At this point, the slew rate of the output also became a factor. I was using LM555 chips in a DIP-8 package.

    @OP: If you are looking for a decent oscilloscope, avoid USB ones. They are usually garbage, unless you pay a lot for them, by that point you might as well buy a separate oscilloscope. You can pick up a decent analog oscilloscope for less than $100; but make sure it is dual-trace and has a bandwidth of at least 50 MHz (if not 100 MHz.) You get even more value for money if you buy a digital storage oscilloscope; new ones start at around $350, and you can usually find old/second hand ones for the $200 mark.

    The oscilloscope you linked to is even worse than your sound card! It only goes to 3 KHz - 3,000 hertz!! If that's a typo and they meant 3 MHz, that's still an abysmal bandwidth. It's also limited to 0-5V, so no negative measurements, and no chance of measuring anything higher (my 'scope measures up to 250V, I haven't used this yet but it's good to have some safety margin.)
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    I have measure as much as 8V/µS with op amps, and the 555 still leaves it the dust.

    When I get a chance, I'll actually test out the max.
     
  7. tom66

    Senior Member

    May 9, 2009
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    I was doing a test of my multimeter. The manual says the frequency counter is rated up to 200 KHz. I got it working at 340 KHz, about the highest I could generate with the parts on hand. However, it took at least 1 minute before it autoranged to the frequency; before, it just showed "0.000 Hz". (It was also very accurate: my oscilloscope (timebase ±0.005%) and it matched.
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    That usb oscilliscope has a sampling rate of 8 kHz and a max frequency of 3 kHz.
     
  9. tom66

    Senior Member

    May 9, 2009
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    If the sampling rate is 8 kHz its bandwidth is more like 800 Hertz, and it's probably got an even worse risetime, so won't even meet 800 Hz (3.5/freq, so ~4ms.) You've got to have about 10 samples per cycle (general rule) to get anything interesting. For example, the oscilloscope I have has only a 10 MS/a ADC (x2) because it is from 1988 (ADCs were expensive), so its one-shot bandwidth is only 1 MHz. But in equivalent time sampling mode it has a 10GS/s sampling rate, and this is past its rated 100 MHz analog bandwidth.
     
  10. mushaba

    Thread Starter New Member

    Aug 22, 2010
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    (What's OP by the way??)

    I looked out on ebay and some other sites for a decent oscilloscope but unfortunatelly the shipping costs are twice the price of the oscilloscope,do you have in mind any e-shops selling products in the money range you said?

    All this talking about oschilloscopes (very usefull information) and we forgot about the threads question? A setup for the 555 to oscillate between 20 and 30 KHz with a variable resistor.

    Thanx guys
     
    Last edited: Aug 24, 2010
  11. tom66

    Senior Member

    May 9, 2009
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    OP: Original poster.

    If you use the values I suggested, but instead R1 = 5.6k, then putting a 5k variable resistor or potentiometer wired as a variable resistor between R1 and R2 will allow you to adjust the frequency from around 17kHz to 29kHz. Is this good enough?

    What is your price bracket for a decent oscilloscope? If you look around you can find a CRO (cathode ray oscilloscope) for $100-$150; if you're prepared to spend $350, you can pick up a new Rigol DS1052E 50MHz dual-channel oscilloscope (digital) which can be hacked to work at 100MHz.
     
  12. mushaba

    Thread Starter New Member

    Aug 22, 2010
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    Fantastic price!!!! I plan to spent 300 Euros about an oscilloscope Tom (used or new one) i found this RIGOL DS1052E for 410 euros with free shipping to Greece,i ll try to save some more money to buy one...Is it really can be hacked to work at 100 MHz???Fabulous!!!!!
    Thanx again man!
     
  13. mushaba

    Thread Starter New Member

    Aug 22, 2010
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    even better 399 USD on ebay,free shipping!
     
  14. tom66

    Senior Member

    May 9, 2009
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    http://www.youtube.com/watch?v=LnhXfVYWYXE

    Remember to watch the follow up if you're buying a new scope; a few things have been changed:

    http://www.youtube.com/watch?v=R2dGKcMtAvg

    It turns out the DS1052E (50 MHz version) and DS1102E (100 MHz version) are identical in every way *except* the DS1052E has an artificial bandwidth limiter enabled which limits it to around 50 MHz, the software hack disables this.

    I'm actually considering spending some money on a new scope. But for now this DSO (HP 54501A) is more than suitable. I suggest you have a look around for stores selling used test equipment, where I bought mine.

    100 MHz bandwidth should be more than enough for a hobbyist.
     
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    OP is the original poster, in this case ... you.

    Look for amateur radio fests as there are vendors there that might have an oscilloscope or two.
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    Using a 555 Hysteretic Oscillator, a 9V battery, and the Oscope as the load I got the following results...

    R = 10Ω C = 100pf, F calculated to 700Mhz

    555 = 244Khz, duty cycle 98%; The fall time was less than 50ns and the rise time was less than 10ns. I suspect inductance, not capacitance, was the real frequency limiter.

    Swapping the IC with a TLC555 (TI CMOS chip) I got between 2.0Mhz to 2.27Mhz, it wasn't very stable, with a full peek to peek signal and nice edges. Duty cycle was 76%.

    Calculated frequency was 700Mhz, so I was taking these chips to their max.

    Just for the heck of it I put a 1KΩ and 100pf to see what would happen. Calculated F = 7Mhz.
    CMOS 555 = 1.5Mhz 57% duty cycle, 555 = 357 Khz 90% duty cycle

    10KΩ 100pf, F = 700Khz
    555 = 625Khz 50% duty cycle, CMOS 555 = 1Mhz duty cycle 50%

    100KΩ 100pf, F = 70Khz
    CMOS 59Khz, DC 50%
    555 57Khz, DC 56%

    My Oscope is new to me. I paid $75, and saved on shipping at a local ham con. It was for a LEADER 1041 40Mhz dual channel scope. Ham events are excellent venues for all test equipment, and as a group they tend to be more honest (and gabby, goes with the hobby).
     
    Last edited: Aug 26, 2010
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