Anyone experimented with minimum pulse with 555?

[chadj2]

I am preparing to build a project with several 555 timers producing pulses of various durations between 10 and 20 microseconds. The 555 I am using is actually one of the dual 556 ICs. Here is the datasheet https://www.st.com/stonline/books/pdf/docs/2183.pdf. Now, in the datasheet it claims to be capable of a frequency greater than 500Khz that would imply that this IC is capable of a on time of less than 2 microseconds by just deviding 1 by 500,000. However, I have also read several places on the web about a minimum pulse duration of 10ms or something like that. I was wondering if anyone has had difficulty producing very short pulse durations with the 555 timer IC.

Thanks

 

[Nigel Goodwin]

555's aren't really very fast - it's more common to use simple CMOS or TTL gates for such purposes - either configured as oscillators or monostables. You can even buy specific monostable logic chips, but you can figure a couple of gates as a monostable anyway.

But moving out of the 1970's and 1980's, a modern solution would be to use a micro-controller, PIC's and AVR's are the most popular - this would make it simpler, and far more accurate.

 

[philba]

You should be able to get the durations you want from a 555. However there are a number of drawbacks. Accuracy is the biggest one - since the 555 uses an RC time constant, you are at the mercy of the accuracy of the Rs and C you use. I also recall having trouble with short pulses and the trigger.

The suggestion of using a uC is a good one, you can get pretty accurate pulses down to the microsecond level (or lower if it's a multiple of the clock frequency).

A non uC based approach is to use a counter/divider (like the 4060) and a crystal for the timebase. Much more accurate than a 555 approach though it takes a little more ingenuity to get the exact length of pulse you want.

 

[Speakerguy]

I have run 555 timers up to 1MHz successfully in astable mode with very respectable waveforms. I think they get a worse rap than they deserve. I was using a CMOS version though (LMC555). I can't vouch for the quality of any TTL parts.

ETA: Texas Instruments has dual CMOS parts they claim will do >2MHz astable. They also have them available for free sample. Make up 'Your Name Inc.' and order a few.

https://focus.ti.com/docs/prod/folders/print/tlc556.html

Caution: at these frequency ranges, you will be using a very small (pF) cap. Keep the traces to/from the cap short, small, and away from other traces/ground planes. Parasitic capacitances will throw off the accuracy of the standard 555 equations. Use a pot instead of a resistor in the 555 ckt so you can tune it by hand to get the right delay and account for the effect of any parasitics.

(I speak from experience, as I stupidly ran two very small traces to a 22pF cap that I made sure was right next to the pins, and totally forgot about the ground plane underneath My calculations were 25-50% off or something).

 

[chadj2]

Thanks for the responses guys. I dont think I have the skill level to use programmable chips yet. I am happy to hear that 10 microsecond pulses are at least possible with the 555. I guess I may have to reorder new parts in the CMOS versions.