# 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
2,613
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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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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!

Mar 24, 2008
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5. ### tom66 Senior Member

May 9, 2009
2,613
214
@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.

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
2,613
214

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

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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1,354
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