Using open input schmitt trigger CMOS logic as "soft latch"? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: September 24, 2023, 03:05:21 am
content: I'm looking for the simplest circuit to latch an output to be pulled low, or be open (or pulled high)With the circuit keeping it's previous state when the input is open circuit.Then to switch states just pull the input pin low or high.Like the behavior of the circuit I attachedWould a CMOS Logic inverter with a schmitt trigger have this behaviour? If the input is open will it retain it's previous state?For multiplexing LED's without needing to drive tons of peak current trough them to retain apparent brightness

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 24, 2023, 03:15:58 am
content: Quote from: ELS122 on September 24, 2023, 03:05:21 amBut then you need a latch for each LED, so you're basically back to direct drive.ie: Just use a LED driver IC with a serial interface. Or make your own with a shift register/latch combo. (74HC595)

author: soldar
author_rank: Super Contributor
timestamp: September 25, 2023, 07:53:02 am
content: Quote from: Kim Christensen on September 24, 2023, 03:15:58 amMaybe I am missing something but this does not make sense to me.If each LED has its own latch then it can be set and will stay in that state until changed.In a matrix row after row can be set and will remain in state indefinitely.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 25, 2023, 04:20:50 pm
content: Quote from: soldar on September 25, 2023, 07:53:02 amYup... It makes no sense to me either. It reads like the OP wants to multiplex LEDs without multiplexing them.See quote below:Quote from: ELS122 on September 24, 2023, 03:05:21 am

author: soldar
author_rank: Super Contributor
timestamp: September 25, 2023, 06:23:10 pm
content: Quote from: Kim Christensen on September 25, 2023, 04:20:50 pmIt seemswhat we have here is failure to communicate.What I mean is that the OP makes perfect sense to me. I understand what he is trying to do.

author: langwadt
author_rank: Super Contributor
timestamp: September 25, 2023, 07:32:47 pm
content: http://xilinx.pe.kr/_xilinx/html/tip/sixeasypieces.htm

author: Zero999
author_rank: Super Contributor
timestamp: September 25, 2023, 08:06:34 pm
content: Use an SCR/thyristor, as that's what's shown in the original post. The problems are the forward current needs to be over the holding current and it will stay latched, until the power is cycled.

author: Benta
author_rank: Super Contributor
timestamp: September 25, 2023, 09:28:20 pm
content: TI has (or had?) a family of bus buffers with "memory" that worked that way.Basically it comprised a Schmitt-trigger buffer (notan inverter!) with feedback through a resistor. This would hold the output at the last logic level applied at the input.Schmitt-trigger operation is not even necessary, a basic CMOS buffer like CD4050/MC14050 should do the job.

author: soldar
author_rank: Super Contributor
timestamp: September 25, 2023, 09:47:15 pm
content: Some decades ago I used latches extensively, at first the TTL 7400 family and later the CMOS 4000 family.

author: langwadt
author_rank: Super Contributor
timestamp: September 25, 2023, 09:51:36 pm
content: Quote from: Benta on September 25, 2023, 09:28:20 pmI've seen many FPGAs with what they call a weak keeper on input buffers

author: Benta
author_rank: Super Contributor
timestamp: September 25, 2023, 09:57:04 pm
content: Quote from: langwadt on September 25, 2023, 09:51:36 pmYeah, same principle.

author: rstofer
author_rank: Super Contributor
timestamp: September 25, 2023, 10:21:18 pm
content: I have read that floating a CMOS input will result in destroying the gate because the output buffers are fighting over whether to pull high or pull low. Specifically the CD4000 series. Other families have some form of weak pull up or pull down but that probably doesn't meet the OPs desires.Does Dave's push-on, push-off switch fit the needs?

author: langwadt
author_rank: Super Contributor
timestamp: September 25, 2023, 11:20:25 pm
content: Quote from: rstofer on September 25, 2023, 10:21:18 pma resistor from output to input of a non-inverting buffer will

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 25, 2023, 11:20:59 pm
content: Quote from: soldar on September 25, 2023, 06:23:10 pmI just don't understand why anyone wouldwantto drive LEDs that way when ICs like the 74HC595 exist.ie: You could daisy chain x2 74HC595 and control 16 LEDs with a 3 wire interface. Or you could go the OP's route and create a 4x4 matrix (8 wires) to drive 16 LEDs. OP's method would need even more wires when if they want to control more LEDs, but with the 74HC595 you just chain more chips and use the same 3 wire interface. (Interfacing a 4x4 matrix with 16 latches as described by the OP would be an "interesting" design challenge)If he only wanted a latch, then there are chips like the 74HC373, 74HC374 (Octal latches) or the 74HC75 (Quad latches). Why build one with a buffer IC and resistors, or multiples with discrete components and then figure out how to trigger them via a matrix connection? Seems like a lot of pain for very little gain.

author: langwadt
author_rank: Super Contributor
timestamp: September 26, 2023, 08:12:35 am
content: Quote from: ELS122 on September 26, 2023, 07:08:48 amso don't multiplex them, use a 74HC595/74HC373/74HC374why multiplex only to add some Rube Goldberg un-multiplex contraption

author: langwadt
author_rank: Super Contributor
timestamp: September 26, 2023, 09:15:18 am
content: Quote from: ELS122 on September 26, 2023, 08:41:42 amenlighten me ...

author: soldar
author_rank: Super Contributor
timestamp: September 26, 2023, 09:58:57 am
content: Quote from: Zero999 on September 25, 2023, 08:06:34 pmThat's a clever solution! I wonder if it has ever been used in real life.It has to be noted that all these different solutions do not allow for LED brightness modulation. It is only on/off. This could work for text and simple signs but not for video.

author: macboy
author_rank: Super Contributor
timestamp: September 26, 2023, 12:31:52 pm
content: Quote from: ELS122 on September 26, 2023, 08:41:42 amI think that you are not understanding multiplexing. If you are multiplexing 100 LEDs (as below), you don't have one on at a time as you suggest. You probably arrange them into a 10 row by 10 column matrix, and drive 10 LEDs (an entire row or column) at a time. So you need 10x the peak current, not 100x. LEDs will easily tolerate high current pulses at low duty cycle, exactly what you get when multiplexing. They must do so by design, because multiplexing and PWM brightness control is the norm rather than the exception.Quote from: ELS122 on September 26, 2023, 07:08:48 am

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 26, 2023, 03:11:13 pm
content: Quote from: ELS122 on September 26, 2023, 07:08:48 amNo, I'm saying there are better ways to drive the LEDs with steady current (74HC595) than using a multiplex matrix to drive a bunch of latches to drive the LEDs with a steady current.I can understand not wanting to multiplex because it can introduce noise due to the high peak currents, but like macboy pointed out, it's the average current through the LED that counts. But there will be a limit to the peak current through each individual LED. So there will be a maximum number of LEDs that can be multiplexed in a single "square" array. (More like a limit to the number of roworcolumn drivers.)Another reason for not multiplexing is if the driver halts unexpectedly and one row of LEDs ends up with 100% duty cycle. Then yes, some 3mm LEDs could be damaged in a poorly designed circuit.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 26, 2023, 03:26:49 pm
content: You can use a buffer (or two cascaded inverters) with a feedback resistor as a "level keeper". The resistor will always provide positive feedback, and thus resist a level change. If you then open the input (from some other tri-state buffer) then it will stay at the level it was before the "other circuitry" was disconnected.Sometimes these level keepers are even built into IC's. For example if you have a databus with a lot of IC's, then such buffers will prevent oscillations on the bus when it is not driven by any signals, and without the disadvantage of the current draw of pullup resistors.