555, monostable, making an output pulse shorter than the triggering step? - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: January 12, 2023, 03:56:30 am
content: I've been looking for a method to get an output pulse from a 555 timer in monostable mode which is shorter than the length of the triggering waveform. I think I've found a way, it works on breadboard, but I want to check if it is bad practice for the long term health of the chip, or if there is a better way.555's it seems, normally, will keep the output high until the triggering waveform has gotten back above the 1/3 level, so normally will not be able to give an output pulse shorter than the triggering low square pulse.So, I'm taking the input (square) waveform and putting it through a capacitor with a pull-up resistor to Vcc on either end of the cap. This acts as a differentiator and makes a sharp downward spike on the 555's input pin end of the capacitor whenever the waveform on the other end of the capacitor drops. However it also makes a sharp spike above Vcc on this end which inputs in to the 555 whenever the trigger waveform rises again. This spike is pretty big, it was up to 2Vcc when i first tired it, but I've added a diode and current limiting resistor from the 555's end of the capacitor to vcc and this has brought it down to about 6.5V peak as compared to a 5v Vcc.The 555's datasheet isn't clear whether Vinput>Vcc is ok, and there are so many different versions of the 555 (and I'm not even sure exactlly which one I have, let alone whether ones I have for breadboarding are the same as those I have for SMD boards) that this is even toughter to check.Are there alternative ways I can get a 555 to give an utput pulse shorter than the low period of the triggering waveform? Anything clever with diodes or transistors, as I'e seen done for ways to make a monostable 555 retriggerable during its outputting period? Otherwise, is Vinput>Vcc ok for a 555, so long as Vinput is still < the 18V maximum which the datasheet mentions?I am looking to do this with the very minimum of components, resistors, capacitors, diodes... not extra IC chips.Thank you

author: Ian.M
author_rank: Super Contributor
timestamp: January 12, 2023, 04:40:28 am
content: You need to rearrange where your current limiting resistor is, and to avoid all risk of catastrophic latch-up, use a Schottky clamping diode so the over-voltage is clamped below the turn-on voltage of any parasitic BJT structures present. Here's a LTspice sim which should give you the general idea.

author: Infraviolet
author_rank: Super Contributor
timestamp: January 12, 2023, 04:58:29 am
content: I don't have LTspice, can you post a picture of the schematic which the asc file contains? Thanks

author: Ian.M
author_rank: Super Contributor
timestamp: January 12, 2023, 05:09:47 am
content: Get LTspice! Its free, (unless you are a competing semiconductor manufacturer) and runs natively on Windows, and under WINE on LINUX. There is an OSX release, but it has some UI issues, so many prefer the Windows release under WINE.

author: Infraviolet
author_rank: Super Contributor
timestamp: January 12, 2023, 05:14:25 am
content: Might need to fiddle with wine settings...Until I get chance to install it, is there any way I can see the schematic of that asc file, if not the full simulation contents of it? to see what you were specifically advising about the resistor and diode.Thank you

author: bdunham7
author_rank: Super Contributor
timestamp: January 12, 2023, 05:55:44 am
content: Here it is:

author: Zero999
author_rank: Super Contributor
timestamp: January 12, 2023, 10:48:08 am
content: Quote from: Infraviolet on January 12, 2023, 05:14:25 amLTSpice is designed to run under WINE and I can confirm it works perfectly.Quote from: Ian.M on January 12, 2023, 04:40:28 amIs the diode really necessary? The bipolar 555 is fairly robust. The data sheet even has an example with 1nF capacitors, 33k pull-ups and no diodes. Refer to figure 22 on page 17: Sequential Timer. Going by the plot, the supply voltage is only 5V, so I wouldn't have any concerns in that case, but it might be a problem with higher supply voltages.https://www.ti.com/lit/ds/symlink/ne555.pdfLooking at theinternal transistor level schematicof the 555, the trigger is one input of a Darlington differential pair, connected to VCCvia a current source. When the trigger is taken above the supply voltage, the emitter of the differential pair will sit close to VCCand the base-emitter junctions will be reverse biased. The reverse breakdown voltage of a typical BJT is typically around 5V and there are two reverse biased base-emitter junctions in series, so it should be able to withstand around 10V more than the supply, at least briefly.I wouldn't bother with a diode for supply voltage under around 9V and for higher voltages, an ordinary silicon diode will do. Using a smaller capacitor, 1nF maximum will also reduce the possibility of damaging it. I've made this circuit before and I recall using a 100pF capacitor and 100k pull-up resistor. I don't recall whether I used a protection diode, or not.

author: Ian.M
author_rank: Super Contributor
timestamp: January 12, 2023, 11:04:17 am
content: That depends on whether or not your 555 closely follows the original Signetics bipolar NE555 schematic, and if you respect abs. max. ratings. e.g. T.I.s bipolar 555 have a max input voltage limit of VCC. However the original Signetics NE555 datasheet doesn't, and I suspect you are correct, and if you use a small enough cap or limit the current enough with a series resistor, can get away without the diode or with a jellybean silicon signal diode.With a CMOS 555, you'd be pushing your luck to omit it, even with a resistor to limit the input current.

author: Zero999
author_rank: Super Contributor
timestamp: January 12, 2023, 11:21:38 am
content: Quote from: Ian.M on January 12, 2023, 11:04:17 amAnother solution is to bias the trigger to half the supply voltage and put a resistor, equal to half the resistor values, in series with the input. This will ensure the trigger input swings between the supply rails and never exceeds them.EDIT:Added the plot.

author: ledtester
author_rank: Super Contributor
timestamp: January 12, 2023, 11:35:22 am
content: I'd recommend learning how to use LTspice - you'll get accurate results and I find it perhaps the easiest simulator to get started using.That said, here's a Falstad simulation of the first neg. edge triggered circuit:(Falstad simulation)I changed the timing cap from 10n to 100n to make the output blip stand out better in the graph.In the graph the green trace is the input signal and the output is the red trace.

author: Ian.M
author_rank: Super Contributor
timestamp: January 12, 2023, 11:43:13 am
content: While we are on the subject of the 555, this page is worth a look:http://www.righto.com/2016/02/555-timer-teardown-inside-worlds-most.htmlHalf way down, the second image of the NE555 die is interactive, and will tell you how each section relates to the schematic just below it.

author: BillyO
author_rank: Super Contributor
timestamp: January 12, 2023, 04:02:06 pm
content: Are you somehow tied to the 555?A 74LS121 will do what you want without complex circuitry.1 IC, 1 cap and 1 resistor. In fact , if the internal resistor meets your needs, you only need a single capacitor.

author: ledtester
author_rank: Super Contributor
timestamp: January 12, 2023, 04:43:30 pm
content: The "C005" timer module is also a possibility -- low power consumption and conveniently it is set up to trigger on a negative edge:https://www.instructables.com/The-Best-Little-Delay-Timer-in-Arduinoville/

author: Benta
author_rank: Super Contributor
timestamp: January 12, 2023, 06:53:57 pm
content: I'd just use a CD4528/MC14528.But on this forum, the 555 seems to be the only timer in the world.

author: Infraviolet
author_rank: Super Contributor
timestamp: January 12, 2023, 07:51:13 pm
content: The reason I'm choosing to use a 555 is because I have a bunch of them so can work on this now, ordering any other chip will take 2 days or more to arrive. And I like giving myself a bit of a challenge sometimes rather than just going for the "chip that does exactly this but is only available if you're lucky".Thanks for the various ips of alternative ways to establish an RC differentiator, I'll have a go and see which are positive (>Vcc) spike free.Good to have a clear explanation of how some 555 types can happily take >Vcc on the triggger pin, but as I'm not sure which 555 type I have (RS often doesn't give the datasheet for the more common chips on sale there) and as what I prototype on breadboard has to work with SMD chips too for later PCB designs I'm going to try to minimise those >Vcc spiking events.Thanks everyone for so many suggestions