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### Topic: Schmitt Trigger. (Read 3989 times)previous topic - next topic

#### cjdelphi

##### Nov 20, 2012, 08:46 am
Other than using a 555 as a schmitt trigger...

I bought 5 off these off of ebay, I was looking for a simple case of, feed the analogue voltage signal in, 1v, 2v, 4v, 5v, etc, and simply have two states, high or low, so in my head I had this

Voltage DC In > Voltage DC 5v High, Voltage 0v LOW.

So the best i could find on ebay was the 74HC14 7414 IC CMOS 14 PIN HEX SCHMITT TRIGGER INVERTER.

Nor is the voltage threshold of when High becomes high and Low becomes Low (what voltage? is that depending on Voltage in?) - anyway I got 5 of these i'm expecting any moment
could i simply feed it into 1 logic gate, get the inverted signal, and feed that into a 2nd gate, giving me a nice Schmitt Trigger ?

#### Grumpy_Mike

#1
##### Nov 20, 2012, 08:52 am
I am not sure what you are trying to do but there is no advantage in pitting two gates in seriese. The threshold is arround 2.4V from memory with a 0.5V or so hysteresis.
If you want a variable threshold then use a comparator like an LM399.

#### cjdelphi

#2
##### Nov 20, 2012, 09:28 am
you answered half my question, inbuilt set voltages (cool, perfect thanks)

I want to feed a voltage in > 1v = low 5v = high.

but this is a Hex Inverted, if i feed in 1v, i'll get a High, i want a Low, so i'm asking would it be perfectly legit to simply feed it back in and get a low from 1V and a high from 5v

1v in > out (high as it's inverted) > back in ....in out > LOW

using 2 logic gates out of the 4 to get a schmitt trigger LOW/HIGH without an inverted signal.

#### Grumpy_Mike

#3
##### Nov 20, 2012, 09:31 am
Yes you can invert the signal like this, but if it is feeding into an arduino there is no point.

#### cjdelphi

#4
##### Nov 20, 2012, 09:36 am
No arduino, i bought a selection of logic chips, just to get a better understanding of logic gates and well computers in general, building 1 bit storage registers using flip flops, how to convert an analouge into a digital signal etc etc... it's just fun right now

#### Grumpy_Mike

#5
##### Nov 20, 2012, 10:25 am
OK, yes just wire them up any way you want input to output.

#### cjdelphi

#6
##### Nov 20, 2012, 11:45 am
I have several flip flops (d type), and soon some and/or/nand (only one that are schmitt triggered) gates and I hope to create myself 8 bits of memory out of pure logic gates and use Arduino to retrieve / store

i'm a long way but you have to start somewhere lol

#### Grumpy_Mike

#7
##### Nov 20, 2012, 11:51 am
Good luck.

Have you seen the 74LS81 ?

#### cjdelphi

#8
##### Nov 20, 2012, 01:07 pm
74LS81

can't find any info on it, got a datasheet?

#### runaway_pancake

#9
##### Nov 20, 2012, 01:58 pm

... I hope to create myself 8 bits of memory out of pure logic gates ...

From 2-input NORs or NANDs you can make RS Latches (a/k/a SR Latches).
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

#### cjdelphi

#10
##### Nov 20, 2012, 02:32 pm
cool, i'm just waiting on the the logic chips to arrive to have to have some fun

#### dhenry

#11
##### Nov 20, 2012, 03:06 pm
You may think about a 2-input chip like hc132: they are flexible enough for you to configure other logic easily from.

#### Grumpy_Mike

#12
##### Nov 20, 2012, 04:10 pm

74LS81

can't find any info on it, got a datasheet?

No it's a bit old, try the 74LS89.

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