/3 and /5 dividers as simple as possible (just for fun stuff). - Page 1
Forum section: electronics / projects

author: Benta
author_rank: Super Contributor
timestamp: June 23, 2023, 09:47:49 pm
content: In a couple of projects, I've needed divide-by-3 and divide-by-5 logic dividers, and being price- and space-restrained I needed to do this in the simplest way possible.On the interwebs, there are lots of circuits, but they're all based on D-flipflops, meaning extra gates.Everyone seems to have forgotten the power of JK-flipflops.I attach my own two circuits exploiting this power.1: a /3 circuit using two JK-FFs and no gates. Can't be done simpler.2: a /5 circuit using three JK-FFs and also no gates. I'd normally use a counter instead, but interesting is, that the final FF can be replaced by a D-type, of which there's very often a spare one present in the circuit. That makes it as simple to implement as the /3 circuit.Might be useful to someone...(Oh, dear, what else to do when waiting for your flight.)

author: tggzzz
author_rank: Super Contributor
timestamp: June 23, 2023, 10:54:23 pm
content: Too complexYou only need three transistors and a few passive components. See, for example, the Tektronix 184, which uses that circuit for /5 ratio, but it can be adjusted to other ratios.Basic principle: each input pulse deposits a glug of charge on a capacitor. When sufficient glugs have raised the voltage to a threshold, then an output pulse is generated and the capacitor discharged. EDIT: more accurately two transistors to do the division, plus one as a buffer to drive an output that isn't part of the divider chain.

author: Benta
author_rank: Super Contributor
timestamp: June 23, 2023, 10:56:48 pm
content: Quote from: tggzzz on June 23, 2023, 10:54:23 pmBut completely frequency independent.Your call

author: tggzzz
author_rank: Super Contributor
timestamp: June 23, 2023, 11:24:46 pm
content: Quote from: Benta on June 23, 2023, 10:56:48 pmSome people want everything at onceJust checked the 184 circuit. It is more accurate to state two transistors to do the division, plus one to buffer the output for an external stage.

author: themadhippy
author_rank: Super Contributor
timestamp: June 23, 2023, 11:56:30 pm
content: I'd just chuck in a 4017

author: Benta
author_rank: Super Contributor
timestamp: June 24, 2023, 12:08:34 am
content: Quote from: themadhippy on June 23, 2023, 11:56:30 pmNo.Very amateurish including set or reset inputs in a synchronous design (runt pulses galore). It'll expose you as a non-professional immediately.If you had suggested74HC161, 74HC191 or74HC193 I would have agreed with you.But like I said, this is just for fun created during waiting time.

author: ledtester
author_rank: Super Contributor
timestamp: June 24, 2023, 02:27:41 am
content: Quote from: tggzzz on June 23, 2023, 10:54:23 pmThis reminds me of how a divide by 3000 counter is implemented in this "All Transistor Clock" (starts at 10:00):All Transistor Clock - Leo's Bag of Trickshttps://youtu.be/Y5kkgO5MPIA?t=10m

author: tggzzz
author_rank: Super Contributor
timestamp: June 24, 2023, 07:49:58 am
content: Quote from: ledtester on June 24, 2023, 02:27:41 amYes, very similar.

author: Gyro
author_rank: Super Contributor
timestamp: June 24, 2023, 08:56:26 pm
content: Check out a copy of Don Lancaster's TTL Cookbook, he has a raft of simple odd number dividers.

author: Benta
author_rank: Super Contributor
timestamp: June 24, 2023, 09:36:30 pm
content: Interesting. Never had or read the book, but as it's free to download I took a look.Don Lancaster agrees with my /3 circuit (p.226). He's a genius!That aside, he's got an extremely irritating way of showing complex functions as "pseudo-PCB-layout" instead of using a functional symbol. I just hate that, it's intellectual laziness.

author: Gyro
author_rank: Super Contributor
timestamp: June 24, 2023, 09:52:31 pm
content: Yes, agreed, it makes it impossible to visualise the schematic without the datasheet. I haven't read it for years, the odd divides stuck in my memory though.

author: tggzzz
author_rank: Super Contributor
timestamp: June 24, 2023, 10:42:37 pm
content: Quote from: Benta on June 24, 2023, 09:36:30 pmIt is irritating, but there's an argument that it is worse than laziness.Designschematics are there to express intent. There are certain schematic "design patterns" that are used as shorthand to quickly convey the intent of how a circuit is intended to work. Examples: positive voltages at the top, negative at the bottom, normal signal flow left to right, feedback signals right to left, transistors in long-tailed pairs are horizontally level and facing opposite directions, inverting opamp amplifiers have the +ve input lower than the -ve input, non-inverting opamp amplifiers have the +ve input above the -ve input, etc, etc, etc. Avoiding using such design patterns indicates that the author is "ignorant of the art".Worse, arguably, is that they are making itunnecessarilydifficult for other people to understand the design. That is both selfish and rude.

author: Benta
author_rank: Super Contributor
timestamp: June 24, 2023, 10:50:05 pm
content: @tggzzz, a big thumbs-up from me! Can't be said better, you hit it 100%.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 25, 2023, 06:07:41 am
content: Quote from: Benta on June 24, 2023, 12:08:34 amEven the 74HC193 is short of ideal, as it has an async load.The smarter counters like HC/LVC169 and HC40103 seem to have faded, which is hard to understand as you can build a synchronous /N counters with no added parts with those.

author: Benta
author_rank: Super Contributor
timestamp: June 25, 2023, 08:18:27 pm
content: Quote from: PCB.Wiz on June 25, 2023, 06:07:41 amYou're right.Only the 74HC161 will do the job correctly, but needs an additional inverter (never understood why).

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 25, 2023, 08:49:31 pm
content: Quote from: tggzzz on June 24, 2023, 10:42:37 pmYes. That was pretty common in those 70s-80s "cookbooks" though. I guess they were more targetting tinkerers that were looking for quick solutions, with an immediate diagram they could use to wire up physical chips, rather than "textbooks" meant to teach people something. Not that those are mutually exclusive though.I'm more annoyed when I see this in product schematics - even these days, some people do this and it's horrific.With the "cookbook" term you kinda know what to expect - a list of quick recipes. Not that it makes it any better, but I would keep this in perspective when considering whether Don was disrespectful.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 25, 2023, 10:05:23 pm
content: Something that irritates me in multipage schematics, is when you have a net spread across several pages, and there is no comment on the schematic itself to what page or pages it jumps to. For a computer it is no problem to find the net, however, the poor chap which is unfamiliar with the circuit and is attempting to trouble shoot it, makes his/her life more difficult.

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2023, 10:40:35 pm
content: Quote from: SiliconWizard on June 25, 2023, 08:49:31 pm"Cookbook" is too close to "cargo-cult" for comfort

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2023, 10:44:45 pm
content: Quote from: schmitt trigger on June 25, 2023, 10:05:23 pmAgreed. Still difficult for the computer to find if it is on paper or in a pdfThen there's the trick of having all ICs connected to short "wires" each with a name. The PCB program can tell the connectivity, but it is bloody difficult for a human - even when searching locates all wires with the same name.

author: Benta
author_rank: Super Contributor
timestamp: June 26, 2023, 08:14:23 pm
content: Quote from: schmitt trigger on June 25, 2023, 10:05:23 pmFor my own schematics, I have two solutions for this.My favourite is using hierachical schematics. This also forces me to split a large design into logical blocks at the start, which is good. Hierachical schematics can in my case be used 95% of the time.For the remaining 5%, I use what I call "TV Service Manual" style. It's a flat set of schematics (picture a number of sheets spread on your desk). Every signal leaving a sheet gets one or more labels of the type "3-4C", "5-2D" etc. meaning that the signal goes to sheet 3, coordinates 4C and sheet 5, coordinates 2D. Those sheets have the same kind of label pointing to the originating sheet.It's not bullet-proof and has issues with bidirectional or bus lines.

author: tggzzz
author_rank: Super Contributor
timestamp: June 26, 2023, 09:16:13 pm
content: Quote from: Benta on June 26, 2023, 08:14:23 pmBoth those are useable.While I like hierarchy, sometimes the logical function and structure is not mirrored by the physical hierarchy. In such cases either the physical or functional dominates, or both are shown on the same diagrams.Old Tektronix scope schematics embodied that problem. The schematics are mostly "TV service manual" style, where functional representation dominates, and physical partitioning being shown by dotted lines and annotations.