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  #1  
Old 11-15-2012, 09:33 AM
SNVHKC SNVHKC is offline
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Exclamation 555 timer 40khz 50%duty cycle

Could someone help me to generate a 40khz square pulses(Astable multivibrator) using 555timer IC,with 50% duty cycle.
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Old 11-15-2012, 10:50 AM
JDT JDT is offline
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This is not the correct forum for this!

A 555 cannot generate 50% duty cycle. Also I'm thinking that a 555 is not the best device for operation at this frequency.

If a 50% duty cycle is important I would have an oscillator that runs at twice the required frequency and follow it with a divide by 2 flip-flop. Easily done in 4000 series CMOS.

Edit: In fact have a look at the datasheet for a HEF4047B. It's all in there!
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Old 11-17-2012, 07:01 PM
BillO BillO is offline
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Here is link to the data sheet. Page 10 shows one method of doing 50% duty cycle.

You can also google "555 50% duty cycle" and you'll get a few hundred thousand hits.
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Old 11-19-2012, 03:51 PM
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In the attached schematic, make R1 = 910 Ω, C1 = .001 μF, and R2 = 17545 Ω. (You may use a 15 or 17 kΩ fixed resistor and a series trimpot for R2.) Mathematically, this should produce a 40 kHz output with a 51.26% duty cycle. Of course, component tolerances will cause variation.
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Old 11-19-2012, 07:00 PM
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Formula: T = 0.7 * (R1 + (2 * R2)) * C1 (without diode)
f = 1.4 / ((R1 + (2 * R2)) * C1) (without diode)

T = 0.7 * (R1 + R2) * C1 (with diode)
f = 1.4 / ((R1 + R2) * C1) (with diode)

Symbols: T = Time period (Units: Seconds)
f = Frequency (Units: Hertz)
Mark = High time of output waveform (Units: Seconds)
Space = Low time of output waveform (Units: Seconds)
Duty Cycle = % of period T in which output is high (mark)

Data: Mode: Astable
Calculate component values
Frequency = 40 Kilohertz
Duty Cycle = 50 %

Results: Period = 25 Microseconds
Mark = 12,5 Microseconds
Space = 12,5 Microseconds
C1 = 10 Nanofarads
R1 = 1 Kilohms
R1 nearest preferred value = 1 Kilohms (E24 / 5%)
R2 = 1,786 Kilohms
R2 nearest preferred value = 1,8 Kilohms (E24 / 5%)

Diode D1 should not be fitted
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Old 11-20-2012, 01:09 AM
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Don't use pin7, drive the cap from pin3 which is bipolar (through one resistor), and you just need 1 cap and one resistor and it automatically makes a 50% duty oscillator.

I have not used pin7 in a while, unless I specifically need a 555 with a duty that is NOT 50%.
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Old 11-10-2013, 01:41 PM
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Quote:
Originally Posted by THE_RB View Post
Don't use pin7, drive the cap from pin3 which is bipolar (through one resistor), and you just need 1 cap and one resistor and it automatically makes a 50% duty oscillator.

I have not used pin7 in a while, unless I specifically need a 555 with a duty that is NOT 50%.
Hi, there. I have been looking for a way of using ne555 as a 50% duty oscillator with only one resistor tuning for frequency. Your method sounds awesome, but it's still not very clear to me how to implement it. Could you please do me a favor to show me an example circuit that can do exactly what you said? Many thanks.
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Old 11-10-2013, 02:39 PM
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Here is an example of using the 555 to generate a square wave. If doing this at 5V Vcc or lower, use the CMOS version of the 555. Take the output as shown so as not to screw up the timing. Note that V(rc) turns around at 1/3Vcc and 2/3Vcc.
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Old 05-02-2014, 11:23 AM
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spark8217 can you please post the circuit diagram relative to which you are providing the values.
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Old 05-02-2014, 11:25 AM
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Quote:
Originally Posted by spark8217 View Post
Formula: T = 0.7 * (R1 + (2 * R2)) * C1 (without diode)
f = 1.4 / ((R1 + (2 * R2)) * C1) (without diode)

T = 0.7 * (R1 + R2) * C1 (with diode)
f = 1.4 / ((R1 + R2) * C1) (with diode)

Symbols: T = Time period (Units: Seconds)
f = Frequency (Units: Hertz)
Mark = High time of output waveform (Units: Seconds)
Space = Low time of output waveform (Units: Seconds)
Duty Cycle = % of period T in which output is high (mark)

Data: Mode: Astable
Calculate component values
Frequency = 40 Kilohertz
Duty Cycle = 50 %

Results: Period = 25 Microseconds
Mark = 12,5 Microseconds
Space = 12,5 Microseconds
C1 = 10 Nanofarads
R1 = 1 Kilohms
R1 nearest preferred value = 1 Kilohms (E24 / 5%)
R2 = 1,786 Kilohms
R2 nearest preferred value = 1,8 Kilohms (E24 / 5%)

Diode D1 should not be fitted


Can you please post the circuit diagram relative to which you are provoding the values.
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