Are there variable threshold schmitt trigger ICs on the market - Page 1
Forum section: electronics / projects

author: Cicada
author_rank: Regular Contributor
timestamp: April 10, 2015, 09:39:47 am
content: HelloI can build avariable threshold schmitt triggerusing op-amp, resistors and variable reference voltage. The low (Vth_l) and high (Vth_h) trigger points (see attached picture) is then moved by changing the reference voltage.Do you get such circuits in a all inclusive IC?So that you must only apply the variable reference voltage. I have search but cannot find anything.My ApplicalionI want to build a movable voltage window circuit by using 2 x variable threshold schmitt triggers offset from each other as shown in the attached picture. There will be only one variable reference voltage which is directly fed to the first trigger and a fixed offset version fed to the second trigger circuit. It should be clear from the picture how the window will work. I will probably have the Vth_h trigger point of the lower schmitt trigger overlap with the Vth_l of the higher schmitt triggerCircuit useIt is important to note that the voltage to be monitored will be set to a certain voltage. The comparitor window will almost certain not overlap with the new voltage to be monitored.The outputs of the 2 schmitt triggers will then indicate via LEDs in which direction the trigger points of the 2 triggers needs to be moved by adjusting their reference voltage.Thank you for any helpCicada

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 10, 2015, 09:45:59 am
content: Yes, it's called a comparator.

author: Bassman59
author_rank: Super Contributor
timestamp: April 10, 2015, 04:21:43 pm
content: Quote from: mikeselectricstuff on April 10, 2015, 09:45:59 amZing!

author: ajb
author_rank: Super Contributor
timestamp: April 10, 2015, 05:06:40 pm
content: It sounds like what you want is a window comparator. You can get window comparator ICs with built-in references that just need three resistors to set the window, or build one yourself with a pair of normal comparators.

author: dom0
author_rank: Super Contributor
timestamp: April 10, 2015, 05:15:41 pm
content: Integrated Schmitt triggers usually don't let you adjust anything, although you can always influence hysteresis. Generally speaking you usually don't care for the exact high/low thresholds, but rather set a middle point and the hysteresis for a design.That being said, you want a window comparator. [And if your input is really noisy, you can use Schmitt triggers for the comparators]

author: dannyf
author_rank: Super Contributor
timestamp: April 10, 2015, 05:31:20 pm
content: QuoteYes. It is called opamp.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 10, 2015, 06:18:38 pm
content: Quote from: dannyf on April 10, 2015, 05:31:20 pmAn opamp is a linear amplifer. It is often a bad idea to use it as a comparator, as it can suffer from issues like slow recovery when the outputs hit the rails.

author: rdl
author_rank: Super Contributor
timestamp: April 10, 2015, 07:00:42 pm
content: CMOS Cookbook shows how to make a Schmitt trigger from a non-inverting buffer or gate (example was a 4050). Put a 22k resistor on the input and anywhere from 47k to 470k feedback resistor from the output to the input. In that range, upper and lower trip point can be varied from 3.7v / 1.3v to 2.6v / 2.4v (with 5 volt power supply).

author: free_electron
author_rank: Super Contributor
timestamp: April 10, 2015, 10:48:41 pm
content: the question is : how fast do you want this thing to be ?analog devices an linear have specialistic schmitttriggers where you can set the two levels. they ain;t cheap ( 12$ and more) but they are very fast ( 1.2 nanosecond domain )

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2015, 01:37:23 pm
content: Micrel make them.http://www.micrel.com/_PDF/mic833.pdfhttp://www.micrel.com/_PDF/mic841.pdf

author: retrolefty
author_rank: Super Contributor
timestamp: April 12, 2015, 07:09:14 pm
content: Adafruit sells a nice 12 bit ADC I2C module that has a programmable window comparitor digital output. They also offer a 16 bit version.http://www.adafruit.com/products/1083

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2015, 07:42:03 pm
content: Quote from: retrolefty on April 12, 2015, 07:09:14 pmThat's good but it needs an MCU.It would be nice if there was a flash option which could be programmed, then would work without an MCU.

author: Cicada
author_rank: Regular Contributor
timestamp: April 24, 2015, 02:48:44 pm
content: Quote from: EmmanuelFaure on April 10, 2015, 09:02:41 pmI had to divert my attention to other work so that's why there is a 2 week delay in my reply.I am fully aware of windows comparators. I guess my lack of understanding my won problem made me express my problem in a bit inaccurate and incomplete.Well after reading the data sheet in "Emmanuel Faure" reply I found this phrase.An important feature of the LTC1042 is the non-interactionof the inputs. This means the center and width of thewindow can be changed without one affecting the other.Also note that the width of the window is set by a groundreferred signal WlDTH/2).This is exactly what I was looking for. I want my voltage window width to be fixed and independent of the window center as I change it. I will only change the center.Thanks EmmanuelHow does this forum work? Is there a place to mark this thread as solved? Maybe give you some points for not posting a reply that just push up your post counter?

author: Cicada
author_rank: Regular Contributor
timestamp: April 24, 2015, 02:50:40 pm
content: Also thanks to all the other poster that made a contribution

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 24, 2015, 03:05:42 pm
content: Look at logic analyzer input comparator type chips.