555 monostable mode with an external trigger

[thaintrinh]

Hi,
I am working on a circuit of generating a squarewave (f = 40kHz) with a HIGH time of several nanoseconds ( i.e t = 10ns) using 555 ICs. (In this case, the duty cycle is 0.04%)
I'm using a 555 monostable mode (Pic 1) with an external trigger (Pin 2).
.
Here's my plan: 1st, I generate a 40kHz squarewave with a very high duty cycle(i.e 99%) (with another 555 - astable mode) and then put it in the PIN 2 of the monostable one. In this article, he's using an arduino for the 1st part.
I tested both; however, if the HIGH time is too small (according to calculation - i.e 500ns) the circuit will not work, it can only work, i'd say, bigger than 100us - this is not enough.
I also tested an astable mode one with an diode, i can only achieve a 300ns HIGH time which, is not enough...

I want to comfirm these:
_ According to theory,
_ The HIGH time : t = 1.1*R*C (R,C are from monostable part) - is this correct???
_ The 1st square wave will decide the overall output frequency and the 2nd (the monostable) will decide the HIGH time of that wave - is this correct also???
Help me pls, thank you. Finals are coming

P/s: If anyone has an idea of a squarewave with the HIGH time of several nanoseconds, pls share
Thank you guys.

 

[thaintrinh]

Btw, this is the article: https://www.instructables.com/id/555-Timer/?ALLSTEPS from STEP 4, thanks guys

 

[BradtheRad]

Two simple methods to shorten the duty cycle from a single 555 IC. (No guarantee they can make a 555 operate for a few billionths of a second, however.)

Left: When output goes high, it conducts through the diode to charge the timing capacitor quickly.

Right: Makes use of the 'ctl' pin. Apply a range of voltage, and you alter the duty cycle.



The above are modified from circuits included in the menu of Falstad's interactive animated simulator.

- - - Updated - - -

Send a pulse through a capacitor to produce a spike. Adjust resistor value until you obtain a usable waveform. Feed the result to a logic gate to clean it up.



Only the positive-going signal is shown. When the signal falls to 0V, the output goes into the negative. This should be prevented by installing a diode in a strategic position.

 

[thaintrinh]

Thanks,
Can you provide me the formula to calculate the frequency & duty cycle from your 2 methods, thanks.
I havent tested yet but can this provide me a nanosec high time? i mean, in theory.
Thanks

 

[BradtheRad]

Thanks,
Can you provide me the formula to calculate the frequency & duty cycle from your 2 methods, thanks.
I havent tested yet but can this provide me a nanosec high time? i mean, in theory.
Thanks

A real 555 is not likely to perform that fast. The simulation works only in theory, of course. You'll probably have better luck with simple components, and a low parts count.

As for how to calculate RC values, the normal formula is invalid due to the unusual operation. Your simulation is as good as my simulation.

A small capacitor needs tiny current to charge/discharge quickly. Therefore you should probably start with the smallest values in your parts bin. Say 33pF.