Go Down

Topic: Several TTL outputs in paralel (Read 837 times) previous topic - next topic

qwertysimo

Hi again,

I was using Sparkfun PIR motion sensors with open collector output. I have used multiple motion sensors connected to a single input pin (pulled up to 5V) on Arduino and if any of the motion sensors triggered, the input pin was pulled low.

Recently I bough couple of these eBay PIR motion sensors with TTL output. I need to hook more than one sensor to a single input pin on Arduino.

What can I expect I if connect several TTL outputs together? Smoke, floating output, working as with open colector outputs, something else?


Thanks.


Best regards
qwertsimo

Runaway Pancake

The outputs can be diode-OR'ed into one input.
(If they're O.C. then pull them up.)
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

Grumpy_Mike

Quote
What can I expect I if connect several TTL outputs together?

You can expect to burn out one or more outputs. The open collector type of outputs are fine to connect together, that is known as a wired OR arrangement. If you have several TTL outputs and you want to combine them use a NOR or NAND gate depending on what level you want to see as the "winner".

Quote
floating output

No such thing, if it is floating it is not an output.


Docedison

Pull up's on all outputs and diodes cathode to output anode to common "Controlled" input, put a 4K7 pull up there too.
@ Mike an OC output is open... until it's activated or it's inverted and low until activated when it becomes 'open'.

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

dc42


Pull up's on all outputs and diodes cathode to output anode to common "Controlled" input, put a 4K7 pull up there too.
@ Mike an OC output is open... until it's activated or it's inverted and low until activated when it becomes 'open'.

Bob


Judging from the example schematics they provide, the output is active high (so not NPN open collector) and possibly current-limited. I would use a 74HCT27 triple 3-input NOR gate to combine up to 6 of those PIR outputs into one 5V Arduino signal.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

dhenry

Runaway Pancake gave you the right answer: run one resistor from the rail to the output bus; and then run 1 diode / module from the bus to the module's output pin. The output is low if any of the module outputs a low.

dc42


Runaway Pancake gave you the right answer: run one resistor from the rail to the output bus; and then run 1 diode / module from the bus to the module's output pin. The output is low if any of the module outputs a low.


What makes you think the output is active low, when the schematics they show turn the connected equipment on when the output is high?
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

qwertysimo

Thanks a lot for replies. This eBay sensor has active high output. I used quad 2-input OR gate to get two independent groups with 3 sensors in each group.

dhenry

Quote
This eBay sensor has active high output.


In that event, run each output, through a diode, to a common resistor to ground and take your signal from that resistor.

The output is high if any of the output is high.

dc42


Quote
This eBay sensor has active high output.


In that event, run each output, through a diode, to a common resistor to ground and take your signal from that resistor.

The output is high if any of the output is high.



Since the high level is only 3.3V, that will give a high level on the 5V Arduino input of only 2.65V or so. Not enough for reliable operation IMO.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

MarkT

Firstly I don't think they mean "TTL", its a 3.3 module, no doubt CMOS (everything is CMOS now!).  Looking up the chip
part number from the photo might give chapter&verse.

But back to the plot - it is possible to use diode gating, but it gets more complex(*) (2 diodes and one resistor per input),
and using logic ICs is simpler.

(*) diode to ~10k resistor to +5V on each input, then parallel diodes from the diode/resistor junctions to form a diode
or-gate with 3.3V output - level shift one diode drop up before the diode or gate.
[ I won't respond to messages, use the forum please ]

Go Up