Driving 555 IC with a pulse stream

The reset pin is active high but it doesn't need to be used. Just connect it to ground.

The 4017 pulse is probably positive going, and the

555 needs a negative going edge, so you will need a NPN transistor to invert the pulse and a cap to trigger the 555 on the leading edge. Something like the drawing, but you may also need a diode from the transistor base to GND if the input is more than 6 volts. +V | 555 / +------+ 5.1K | | | | 3|--> Out +--------|2 | C | | 0.01uF |/ +------+ In --///---| |--+--B| NPN 5.1K C | | E 5.1K / | | | | GND GND

-Bill

I have a decade counter (NTE4017B) that generates a pulse stream on each decoded output, a couple of which I will use. However, the width of these pulses are too wide at lower frequencies. So, I'd like to use a 555 IC operating in monostable mode to set the pulse width to what I want, regardless of the frequency and pulse width coming from the decade counter.

My guess is I have to use the decade counter output to both reset the 555 (pin 4) and trigger it (pin 2). Is that right? I know the triggering is by dropping pin 2 to ground, perhaps by switching with a transistor, but what does the reset want, high or ground?

TIA

Ed

You can just use it in monostable mode. Tie reset to high. Look at the datasheet (google 555 datasheet) for more information.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

555 > (pin 4) > and trigger it (pin 2). Is that right? I know the triggering is by dropping > pin 2 > to ground, perhaps by switching with a transistor, but what does the reset > want, > high or ground? >

Why not use a one-shot, like a 74123. Then you can use either a positive or negative going edge triggering. Brian

--- _____ It's active low; RESET

--
John Fields

Other respondents seem to have no trouble with your spec, so the odds are that I've missed something - but I'm darned if I can make sense of your description. Perhaps if you could describe your end objective?

Meanwhile, as I understand it, you have a 555 astable clocking a 4017, yes? You want the 4017's outputs to have a fixed duration? Then what's the problem with just fixing the 555 astable period accordingly, which then fixes each 4017 high output to the same duration?

In which case, where does the need for a 555 (or any other) monostable arise?

You don't mention any specific values, but FWIW here's a 5V circuit with a 555 clocking a 4017 so that each of its outputs flashes an LED for about 4 ms.

--
Terry Pinnell
Hobbyist, West Sussex, UK

Hey, Terry.

I think what he wants is to shorten the pulse coming out of the 4017. Thus, I believe he is asking how to set up a 555 as a one-shot, given a long pulse.

Now that I think about it, he probably wants an edge detector to drive the 555 trigger input.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

--- No, it's actually pretty slick!

What's happening is that when the 4017 output goes low it unconditionally resets the 555, discharging the timing cap, then when it goes high and it gets to about 0.7V it releases the reset and drags the trigger pin high along with it. Now, since the trigger pin will be at less than about 1/3 Vcc when the 555 comes out of reset, the output will go high and the 555 will start timing out. What's slick is that it's only going to take ever how long it takes for the counter's output to get to higher than 1/3 Vcc to not be triggering the 555 any more, and then the 555 will time out during the time the counter's output is high.

__________ __________ ______

4017 OUT __________| |__________| |__________| _____ __________ __________ ______ RESET __________| |__________| |__________| ____ __________ __________ ______ TRIG __________| |__________| |__________| _ _ _ 555 OUT ___________| |___________________| |___________________| |___

-->||

John,

I now have the input to the 555 going to BOTH TRIGGER and RESET, without differentiation (i.e., no capacitor in series), and it seems to work. I can definitely get pulses out of the 555 that are shorter than the inputs. As I see it, I am resetting and triggering at the same time. Is that antithetical?

Ed

Thanks for coming into the discussion, Terry.

The project was first introduced about a month ago, 10/10/04, as "Special dual pulse generator circuit." I'm implementing a circuit basically as proposed by Chris Foley. Briefly, I'm trying to achieve a tester for the EFI system on a Jaguar V12. the car has a trigger board in the distributor that generates two switch closures per revolution, 180 degrees apart, that drive the electronic control unit (ECU). I intend to plug the tester into the harness in place of the trigger board, thus allowing me to drive the ECU, amplifier, and fuel injector solenoids without the engine running. When the engine is running there are variations in speed etc. that make interpretation of scope traces difficult. My hope is this will allow me to test the various components under more controlled conditions.

So, with Chris' great design the a-stable 555 runs at 10 times the rotor speed, so to speak, and the decade counter outputs are ad one 10th that frequency, thus emulating the spinning distributor. By picking up two the the decade counter outputs, e.g. 1 and 6, I get a very good representation of what actually happens. Importantly, the speed can easily be varied while always having the two outputs exactly 180 degrees apart.

Chris' design meets my original description of the problem, but I then got ambitious. I noticed that by putting another 555 IC (actually 2, one for each output) between the decade counter and the output stage (optoisolators) I could have control over both speed and pulse width, allowing the same tester to be used to emulate the trigger board and drive the ECU OR to emulate the trigger board and ECU.

Meanwhile, the big cat lumbers in the garage!

Ed

---

Yeah, but now your timing will look like this:

__________ __________

4017 OUT _______________| |____||___| |______ || _____ _______________ ____||___ ______ RESET |__________| || |__________| ____ _______________ ____||___ _____ TRIG |__________| || |__________| _ || _ 555 OUT __________________________| |__||______________| |___ ||

That is, the 555 will trigger on the trailing edge of the 4017 output instead of on the leading edge.

-- John Fields

Thanks, John. I may have misled you though, as I actually have an NPN (NTE

123AP) transistor between the 4017 and the 555. E.g., pin 2 (a decoded output) of the 4107 goes a 10K resistor, thence to the base of the transistor. Another 10k resistor connects the base to ground. The collector goes to pin 2 of the 555, and the emitter is grounded. Pin 2 is also connected through a 10k resistor to +12. This is to present pin 2 with a negative pulse, which is the way I've triggered 555 in other projects.

So, does the analysis still hold?

Ed

Because that would make sense. You're confusing logic with your thinking. (a line by Gene Burrows from ABC TV Hollywood.) gg

OK, thanks, understood. A more complex spec than I'd thought!

From your separate thread 'Special "dual" pulse generator circuit (follow up)', I see you're sorted, apart from one residual issue.

FWIW, simulating an opto-isolator fed with a 0-12V fast rise time pulse via a 10k base resistor gave me much faster output rise times than those you observed. With both models in its library, a 'generic' and the OP4N25, CircuitMaker gave the following results (which square with Chris's point about lowering the resistor value).

Rc Rise time

---- ---------

10k 300 us 1k 50 us

So I'd suspect some other cause. Is your circuit, or at least that section, posted anywhere?

--
Terry Pinnell
Hobbyist, West Sussex, UK

Thanks for your interest, Terry.

I just uploaded a scan of my circuit (thanks to Chris Foley) to:

The diagram shows a 400/500 ohm resistor and an LED in the 2N3094 collector circuit, in the path from pin 2 of the 4N32. At the moment the resistor is a 5% 400 ohm, measuring about 380 ohms, and the LED has been removed.

BTW, what version of CircuitMaker are you using, student of full? The full version is kind of pricy for someone not making their living in this stuff.

Ed

Robert,

Would a small capacitor in the input to the 4017 serfe as an "edge detecter?"

TIA

Ed

Jag:

If you are worried about the length of the pulse coming out of the 4017, you can use a capacitor and resistor to detect either a rising or falling edge of the output.

To detect the rising edge, connect an output to the input of the next stage through the cap, and use a resistor to ground on the far side of the cap, as so:

.---------. | | | | C | | | | || | 4017 Qx|-----||--o---- Out | | || | | | | | | | | | | | / R | | '---------' / | GND created by Andy´s ASCII-Circuit v1.25.250804

Then, when without a transistion for a while, the output will be grounded. When the Qx output goes from GND to 5V, however, the output will momentarily go up to 5V, and then will decay back to ground depending on the values of R, C, and the impedance of the output. The voltage at Out will fall to about 37% of the input pulse in t = R*C.

When the 4017 output goes from 5V to 0V, the output will drop below GND. However, you can add a diode from GND to Out to prevent this.

For a falling edge detector, you can use this (with the diode drawn in)

VCC o--o | | .---------. | | | R / - Diode | | ^ | | / | | | || | | | 4017 Qx|-----||--o--o- Out | | || | | C | | | | | | Pulse length is ~RC | | '---------' created by Andy´s ASCII-Circuit v1.25.250804

For your problem, I'd say you can use the second circuit between your

4017, and the trigger input of a 555. It will generate a nice negative pulse. Then, you can set the output pulse width using the forumlas given in the 555 datasheet for the monostable configuration.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.