transistor flip flop - Page 1
Forum section: electronics / beginners

author: Boschi
author_rank: Regular Contributor
timestamp: December 27, 2019, 01:40:36 pm
content: i have a 32khz signal, and i want to divide it without using any IC.tha final goal is to have a fully discrete clock.i found this:which does exacly what i want to do, the only problem is that it works with 60hz and with +12V and -6V, while i want to make it work at 5V.in the past few days i tried toying a bit with the circuit, but im not able to make it work.i attached the modified circuit.the signal that clock the circuit is a 32khz impulse, i tried with a pulse duration of 2us, 1us and about 300ns but the circuit wont change state, it remains always the same.also, while i understand how it works, i dont understand what resistors are influencing the time costant, just R42/43 or also R40/41?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2019, 03:18:35 pm
content: The main resistors to effect the time constant should be R42 and R43. However there is a certain ration of R42/R40 and R43/R41 needed. The ration of 10 looks about good. So the main adjustment of the time constant to the lenght of the trigger pulse would be the capacitors.The circuit should be OK to run with +5 V instead of 12 V. The trigger pulse may need a slightly negative voltage to really turn of the transistors. One could get this with AC coupling the trigger pulse. Alternative one could use a diode or similar to shift the emitters up a little.The rising slope of the trigger signal should also be fast enough.In the hand drawing the resistors parallel to the caps should be larger. I would consider some 2-3 K the minimum.

author: iMo
author_rank: Super Contributor
timestamp: December 27, 2019, 03:56:22 pm
content: For example

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2019, 06:14:40 pm
content: Alinkto one I designed while ago.Discrete div 2 counter.asc

author: atmfjstc
author_rank: Regular Contributor
timestamp: December 27, 2019, 11:13:09 pm
content: That sure is a lot of transistors you're gonna be using just for dividing the clock down to 1Hz. Think you're looking at 14 stages, that's 28 discrete transistors, diodes, and double the number of resistors. I wonder if there is a discrete component solution for dividing the clock in larger increments at a time (4, 8, 16 etc) with fewer components and acceptable precision. Maybe something like each pulse increasing the charge in a capacitor, and it could trigger a discharge and and output pulse when it accumulates the energy equivalent to, say, 8 input pulses?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2019, 11:21:19 pm
content: If the input frequency is more or less fixed, one can use a synchronization oscillator. So in the extreme case just 1 transistor. The weaker the coupling of the input, the more complicated divider ratios can be used. This can include simple fractions that in modern logic needs a PLL.

author: tggzzz
author_rank: Super Contributor
timestamp: December 27, 2019, 11:57:08 pm
content: Quote from: Kleinstein on December 27, 2019, 03:18:35 pmThe purpose of the capacitors is to speedily inject some current into the transistor base, to get the switching started faster. There is no relevant RC time constant per se, other than the capacitors should have completed the charge injection before switching in the other direction needs to occur.As for using "too many transistors", you can have a /5 or /10 stage using just three transistors. The basic operation is that each pulse injects a glug of charge onto a capacitor, which causes the voltage across it to increase. When sufficient glugs have been deposited the voltage reaches a value which causes the charge to be rapidly dumped, thus forming the pulse to the next stage.For an example, see the Tektronix 184 Time Mark Generator.

author: mikerj
author_rank: Super Contributor
timestamp: December 28, 2019, 02:31:54 pm
content: Quote from: tggzzz on December 27, 2019, 11:57:08 pmIs that an SI unit?

author: tggzzz
author_rank: Super Contributor
timestamp: December 28, 2019, 04:47:55 pm
content: Quote from: mikerj on December 28, 2019, 02:31:54 pmIt is standard terminology for high resolution DVMs, but the size of the glug varies between instruments

author: Tom45
author_rank: Frequent Contributor
timestamp: December 28, 2019, 05:13:36 pm
content: Quote from: tggzzz on December 28, 2019, 04:47:55 pmAnd there's also the issue of Metric vs. Imperial glug.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2019, 10:48:03 pm
content: Quote from: tggzzz on December 27, 2019, 11:57:08 pmI don't see how those capacitors will do much to speed it up. The base voltage already goes below zero, reversing the base current, when the input pulse goes low.