Generating this negative pulse?

C on output going to R pulled up to supply. Possibly upwards pointing diode across R, but doubt that's needed. CMOS 555s I guess?

Cheers

--
Clive

A time axis might be helpful to know how critical the relative timing might be...how fast do you have to reset the timing components? How accurate/repeatable do the delay and pulse outputs have to be. Are the delay and width fixed? Or vary over what range?

Have you tried an XOR gate with an RC on one input?

If you wanna have some fun, consider the emitter follower. If you put a resistor in the base and drive current into the emitter, the emitter impedance looks like the base resistor in parallel with an inductor. The time constant of that LR is the Tsub-t of the transistor. I'm too sleepy to remember, but it's something like 1/2pi*ft. Works great as a fast differentiator. The cool part is that the L was never really there, so when you turn off the current, all you have to wait for is the charge in the base junction to dissipate. The width of the pulse is relatively independent of the rep rate.

Thanks Clive, appreciate that fast reply. Haven't bread-boarded it yet but simulation looks good.

I rather think the diode is needed as I suspect the +ve pulse might cause trouble?

Terry, East Grinstead, UK

Thanks Mike. I'll experiment with that later but Clive's ultra-simple suggestion looks good!

Terry, East Grinstead, UK

Also meant to also reply to your questions about the time scale. Initial delay mono trigger pulse not critical, but typically 100 ms. Delay mono output user-settable between 1 and 30 s. Main mono user-settable between

5s and 7 mins. I've amended my illustration accordingly.

Nothing deserving much rigour, it's just hobby stuff. I'm using main mono to light an external lamp as part of my project to get video of an occasional overnight garden visit by a local fox and cub.

Terry, East Grinstead, UK

It's likely that the 555 input will have a protection diode, but I haven't looked, that's just a guess, so yes, use one. A Schottky diode would keep the pulse down to maybe 300mV above rail.

Cheers

--
Clive

One method would be a couple of NAND gates and an RC, forming a "half monostable".

0-*-------RRRR-------*-----------N * | A___________0 | CCC N | CCC *-----D | | | | GND | | | *------N | | A-----------------* | N *------D

When the input is sitting high, the second NAND sees 10 so it stays high.

When the input goes low, initially the the second NAND sees 11, so its output goes LOW. The RC charges up to the logic threshold, at which point the second NAND sees 01 and brings its output high.

A Schmitt-trigger NAND such as a 74HC132 or 4093 would be best.

An RC and an XOR will generate pulses on both edges of its input.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 

On similarly minimalist lines, I'll test if this also works in practice.:

(BTW, this old batch of 555s are not CMOS.)

Terry, East Grinstead, UK

Thanks Phil, neat, I'll get around to trying it, but see my earlier reply to Mike.

Terry, East Grinstead, UK

A 555, as the second one-shot, needs a brief pulse. That means you need

3 monostables instead of 2.

The 2 halves of a 4538 would do this without needing to shorten the pulse in the middle. It can be retriggerable or not simply by feeding an output back to an input. (It has Q and /Q, and 2 inputs where 1 is inverted and then they are ORed.)

I'm weird because I never liked the 555. It should have been 2 chips better optimized for astable and monostable. There's enough demand for

2.

7 minutes is a long time for a 555. Save yourself a lot of grief by using a CPU. These are dirt cheap and trivial to program with nothing more than a usb port and free high level language program. Demo example programs to do (almost) what you want are on the website.

More expensive if you want 'em quick.

+1

Aren't there also gadgets on ebay that do exactly this job? I guess they might have a relay on the output though, which may or may not be what you want.

--
  Rick C. 

  - Get a 1,000 miles of free Supercharging 

Thanks, but two extra monos seems a tad OTT? I was hesitating even about my second mono. Hopefully Clive's simple RC-derived suggestion will do the job when I get back to the workshop later this weekend.

Great chip IMO; compact, versatile and a relatively unfussy trigger. (Although I'd prefer +ve going.) And lots of hobbyist circuits to play with ;-)

BTW, very little space inside the case for this add-on. The delay mono is on a small rectangle of veroboard with just a few holes left.

Terry, East Grinstead, UK

Not yet breadboarded but its sim leaves little doubt that it would work fine:

Terry, East Grinstead, UK

In place of the 555s, not in addition to it!

Sure it's preferable to just add another RC when you've already built it, but I was suggesting why I don't like to use the 555 in the first place. Because the 4538 responds to the edge and only to the edge and doesn't care how long the pulse lasts, it wouldn't need an extra RC to shorten the middle pulse.

Is the delay mono to debounce the trigger for the main mono? You only need one mono. Put an RC delay on its trigger such that the trigger delay is well in excess of your bounce time.

No. See earlier posts.

Terry, East Grinstead, UK