Single shot with a VERY long trigger - Page 1
Forum section: electronics / beginners

author: escapekit
author_rank: Contributor
timestamp: May 22, 2020, 06:59:53 am
content: Hello!I've been lurking in the forum for about a year or so, learning all kinds of interesting things on the way, but now I need to ask for your help.I am designing a device that needs to trigger an MP3 player (chinese YX5200). There is a line that at some point will change from low to high and at that point I want to trigger the MP3 player with a pulse no longer than a second. I thought about using a 555 in mono-stable config. but I read somewhere that the trigger pulse should be shorter than the output pulse or it could damage the 555. In my case the trigger pulse may be as long as 20 or 30 minutes, in some cases it can even last hours. How can I turn this long trigger into a short pulse?I thought about using a 74121 but I think I will run into the same problem the 555 had. Reading from the datasheet it states that "input pulses may be of any duration relative to the output pulse". English is not my primary language, but if I understand it correctly, it means the input pulse can be of any duration inside the duration of the output pulse? Does it mean that it can't be longer than the output pulse?I usually like to come up with my own solutions by doing research but this time I am really stuck, I've been looking at this problem for about a month on and of and I don't seem to stumble into a solution. Furthermore, I need to be sure the solution is very reliable because it is for someone else and I don't want them to have problems with it. I could use a micro as I've done plenty of times, but the challenge was to try and do it without a microcontroller. I've managed to design the whole circuitry without it (learning a few interesting things in the way) but this final step has me really frustrated.Thanks in advance and sorry for the long and boring post!

author: Benta
author_rank: Super Contributor
timestamp: May 22, 2020, 07:34:05 am
content: CD4538/MC14538 can be edge triggered, which is what you're looking for.

author: tautech
author_rank: Super Contributor
timestamp: May 22, 2020, 07:36:26 am
content: 4013 as a single shot.

author: jfiresto
author_rank: Frequent Contributor
timestamp: May 22, 2020, 09:47:21 am
content: I would also suggest an old-fashioned, metal gate CMOS 4538/14538 dual one-shot. They are the second refinement of the original 4528 and are meant to be as accommodating as possible. The dual, schmitt trigger inputs of Texas Instrument's CD14538, for example, tolerate slowly rising inputs. If I make no mistake, their part also has an internal network that resets it at power up. On a clean PCB, you could produce a one-second pulse with a 4M7 resistor and a 220nF film capacitor.

author: nali
author_rank: Frequent Contributor
timestamp: May 22, 2020, 11:03:26 am
content: There's no reason you can't use a good old 555, just Google "Edge triggered 555 monostable"e.g.https://www.electroschematics.com/edge-triggered-555-monostable-multivibrator/

author: escapekit
author_rank: Contributor
timestamp: May 22, 2020, 11:23:03 am
content: Thanks everyone for your fast reply!I think I am going to be using the CD14538 or the regular CD4538. They look better fitted for the job than the 555 solution (less components at least).Again, thank you very much! I'll let you know how it works when I get the boards assembled.