2 transistor asynchronous multivibrator driving logic chip - Page 1
Forum section: electronics / beginners

author: Dabbo56
author_rank: Contributor
timestamp: November 26, 2022, 03:08:49 pm
content: I have 2 circuits that work no problem:1. A traditional 2 BJT astable multivibrator generating square wave at 1 Hz (0v to 5v), and2. A counter 74HC393 chip driven by a function generator putting out square wave at 1 Hz (0v to 5v).So, I replaced the function generator with the astable multivibrator but getting nonsense output from the chip.Both square waves look similar on the scope. What am I doing wrong? What do I need to read up on? Thanks!

author: Ian.M
author_rank: Super Contributor
timestamp: November 26, 2022, 04:19:48 pm
content: I'd bet the risetime of the function generator is *MUCH* faster than that of your BJT astable, and that the latter is violating the 74HC393's max. input transition rise/fall time given in thedatasheet(∆t/∆v, page 3). The slow risetime keeps the clock input in its transition region where it is sensitive to noise pickup (possibly from its own output transitions) for too long, resulting in extra counts.Its very hard to get a clean enough squarewave with nice edges out of a BJT astable unless you emitter couple the output. See attached LTspice sim.

author: Zero999
author_rank: Super Contributor
timestamp: November 26, 2022, 08:55:28 pm
content: The transistors in an astable actually turn off very quickly, because the base voltage goes negative. It's the voltage dropped across the collector resistor which makes the rise time sluggish. Add a diode and an extra pull-up resistor to get a faster rise time. Here's an example showing the difference between the extra diode & resistor vs without.

author: Dabbo56
author_rank: Contributor
timestamp: November 26, 2022, 09:59:56 pm
content: Thanks Zero999 and I’ll definitely implement your modification. One question though, as I understand it the 74HC393 does its thing on the negative going edge which, as you say, happens quickly. Will improving the rise time help?

author: Dabbo56
author_rank: Contributor
timestamp: November 26, 2022, 10:04:35 pm
content: Thanks Ian.M I'll give it a try

author: Zero999
author_rank: Super Contributor
timestamp: November 26, 2022, 10:30:31 pm
content: Quote from: Dabbo56 on November 26, 2022, 09:59:56 pmIt might do. If the dv/dv of the input waveform is too slow, anything could happen. Operating an IC outside of its design specifications is asking for trouble.

author: David Hess
author_rank: Super Contributor
timestamp: November 26, 2022, 11:16:21 pm
content: Some companies like Hitachi or NEC put Schmitt trigger inputs on all of their HC logic clock and edge inputs.

author: Dabbo56
author_rank: Contributor
timestamp: November 27, 2022, 11:17:31 am
content: Quote from: Zero999 on November 26, 2022, 08:55:28 pmYes that worked and the logic chip is now working fine. The rise time which was 68mS is now 34nS.The fall time is less than 12nS.Datasheet states max 500nS so well within the limit

author: Dabbo56
author_rank: Contributor
timestamp: November 27, 2022, 12:30:53 pm
content: As a follow up question, there are 2 ways in which the astable multivibrator circuit can be drawn. In type A the resistor R2 is connected to the base of the first transistor.In type B the resistor R2 in connected to the capacitor.Both appear in textbooks and references.Is one better than the other?

author: Zero999
author_rank: Super Contributor
timestamp: November 27, 2022, 12:38:53 pm
content: Quote from: Dabbo56 on November 27, 2022, 12:30:53 pmWhat's the difference?Please take another look at the circuits, noting the connections between the components.

author: magic
author_rank: Super Contributor
timestamp: November 27, 2022, 12:39:31 pm
content: Type B is better because there is less line crossing and opportunity for confusion

author: wasedadoc
author_rank: Super Contributor
timestamp: November 27, 2022, 12:43:58 pm
content: Quote from: Dabbo56 on November 27, 2022, 12:30:53 pmThey are exactly the same circuit. Just a mater of preference. Some may prefer the second as it has only one crossover.BTW. Running the circuit from more than about 6 volts causes the negative excursion at the bases to exceed the reverse breakdown voltage of many silicon transistors. That does not necessarily damage them but does cause the circuit to run faster than the usual theoretical calculation.

author: Dabbo56
author_rank: Contributor
timestamp: November 27, 2022, 12:48:00 pm
content: Am I crazy? Surely not the same circuit

author: Dabbo56
author_rank: Contributor
timestamp: November 27, 2022, 12:49:42 pm
content: Oh I get it. R2 becomes R3

author: wasedadoc
author_rank: Super Contributor
timestamp: November 27, 2022, 12:49:59 pm
content: Quote from: Dabbo56 on November 27, 2022, 12:48:00 pmYes.

author: Zero999
author_rank: Super Contributor
timestamp: November 27, 2022, 02:49:00 pm
content: Quote from: wasedadoc on November 27, 2022, 12:49:59 pmCome on, go easy on the original poster. This is the beginners section.Quote from: Dabbo56 on November 27, 2022, 12:49:42 pmThere are always different ways of drawing the same circuit. Here's another example. It's exactly the same circuit, drawn a different way.Wikipedia has an excellent article about this circuit.https://en.wikipedia.org/wiki/Multivibrator#Astable

author: David Hess
author_rank: Super Contributor
timestamp: November 28, 2022, 10:14:08 am
content: There is also a single capacitor version, shown below.