# Astable 555 500Khz 20ms

Discussion in 'General Electronics Discussion' started by nebooder, Mar 9, 2020.

1. ### nebooder

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Mar 9, 2020
Hello,
is it possible to made astable 555 circuit that will have 500Khz with 20ms time period with duty cycle close to 50%.
I used various calculators online, but cannot find values for resistance and capacity to suit this kind of circuit, I must inform you that my understanding of electronics is basic.

2. ### Alec_t

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Jul 7, 2015
Welcome to EP!
500kHz has a period of 2uS, not 20mS.
So what do you want; 500kHz or 50Hz continuous, or 500kHz interrupted for 10mS every 20mS?

3. ### Harald KappModeratorModerator

11,284
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Nov 17, 2011
500 kHz is equivalent to a period of 2 µs, not 20 ms. Or do you mean you want the 500 kHz signal to be active for 20 ms, the stop? A similar question is being discussed in this thread, although for much lower frequencies and longer periods. It requires 2 × 555 timers.
The standard 555 application circuit is not able to achieve close to 50 % duty cycle. You'll need a modified circuit.

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Nov 8, 2019
5. ### nebooder

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Mar 9, 2020
Or do you mean you want the 500 kHz signal to be active for 20 ms, the stop?
[/QUOTE]

That is exactly what I am looking to achieve, 500 kHz signal to be active for 20 ms, to work like a pulse generator.
I am trying to set left and right side in resonance, 555 timer and coil in resonance of 500KHz. Coil have 2.5mH with capacitor of C2 40pF, that should be 500KHz. What puzzle me is how to chose R1 C1, to get 500 KHz signal to be active for 20 ms on 555 ic.
Also because my knowledge is limited, does this values in this circuit have any scenes.

Last edited: Mar 9, 2020
6. ### nebooder

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Mar 9, 2020
500kHz interrupted for 10mS every 20mS?[/QUOTE]

Yes that's what I am looking for.

Thank you all on input.

7. ### PETERDECO

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Dec 19, 2019
Way way back I used a CMOS 555 in the standard astable circuit. I left pin 5 floating with no capacitor, used 10 megohms between pins 6 and 7 and a 1 megohm from 7 to 8 (B+). I got a very stable 1 MHZ square wave out of it.

8. ### Alec_t

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Jul 7, 2015
According to its datasheet the LMC555 can generate up to 3MHz, but can source only 10mA and sink only 50mA, so would need a buffer on its output to drive the resonant circuit.
I can't see anything in the datasheet of the common BJT version of the 555 to indicate that it could generate a frequency as high as 500kHz.

9. ### nebooder

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Mar 9, 2020
Initial plan was to be used LMC555.

10. ### duke37

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Jan 9, 2011
I am not sure what you want to do but to get a 500 kHz (not KHz) square wave I would use a 1MHz crystal oscillator and divide by two.
The 555 would be used to turn this on and off.

You have two 1N4004 diodes in series. You only need one or none..
The 1N4004 is a slow speed diode, you would be better using a single 1N4148.

11. ### nebooder

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Mar 9, 2020
Thank you all an any input.

12. ### WHONOES

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May 20, 2017
Try the attached circuit. It uses a 4093 which is a Schmitted 2 input Nor gate. The first stage produces the 500KHz whilst the second stage produces 50Hz pulses. The Second stage is used to gate the first stage on and off giving you 500KHz pulsed at 50Hz. The pulses of both sections will have as close to 50 / 50 mark space as makes no difference.
The two remaining gates in the package (they come as 4 in a chip) are then use to drive the totem pole pair of transistors that can be use to drive your resonant circuit directly or drive a larger transistor if more current is required.
The bottom trace (red) of the graph shows the pulsed output.
To fine tune the frequencies of each stage, a potentiometer may be put in series with each of the resistors.

#### Attached Files:

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• ###### 500KHz pulsed at 50Hz graph.pdf
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iimagine, nebooder and Alec_t like this.
13. ### nebooder

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Mar 9, 2020

Thanks a lot Mister WHONOES.

14. ### WHONOES

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May 20, 2017
BTW, the simulation was performed on Simetrix which is I think the best of the bunch.

15. ### nebooder

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Mar 9, 2020
Thanks a lot I will go with this design, do you maybe have suggestions for which voltage on capacitor before coil to go, coil is 2.5mH, and which diods will work best with this design.

16. ### WHONOES

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May 20, 2017
What are you trying to do with the coil. Are you asking about a power supply for the circuit? Why do you think you need diodes in your circuit?

17. ### Alec_t

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Jul 7, 2015
Good question. 500kHz was an international distress frequency but is now in an allocated Amateur Radio frequency band, and driving the coil with a square wave is likely to result in a lot of unwanted RF interference from the harmonics too.

18. ### WHONOES

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May 20, 2017
Yup, you are right. At 500Khz square wave, the harmonics will extend high into the MHz region. Didn't know it was the International distress frequency. We learn something every day eh.

19. ### Alec_t

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Jul 7, 2015
I only learned that today .

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May 20, 2017
Ha Ha.