Detect an Acitve Low signal

Thread Starter

anishkgt

Joined Mar 21, 2017
447
Is there a better way, with minimal components to blink LED L1 when control pin pulled LOW via an Arduino ?

It can be byredrawing it to be active HIGH which i don't wish to. I was thinking of using this Logic NOT gate to invert the signal and with a feedback from pin 4 via the resistor and cap C3 i could blink the LED. First time with logic gates so, does logic gate default to LOW when powered or is it HIGH at the output ?
 

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Thread Starter

anishkgt

Joined Mar 21, 2017
447
Are you suggesting to omit the optocoupler ? The relay i plan to use draws about 84mA and it will be driving a mains load so with safety in mind i had put it.
 

djsfantasi

Joined Apr 11, 2010
6,338
Is there a better way, with minimal components to blink LED L1 when control pin pulled LOW via an Arduino ?



It can be byredrawing it to be active HIGH which i don't wish to. I was thinking of using this Logic NOT gate to invert the signal and with a feedback from pin 4 via the resistor and cap C3 i could blink the LED. First time with logic gates so, does logic gate default to LOW when powered or is it HIGH at the output ?
Why can’t you change the sketch to pull the control pin high? Why don’t you wish to? It’s probably a simple change on one line...
 

AlbertHall

Joined Jun 4, 2014
9,642
Why can’t you change the sketch to pull the control pin high? Why don’t you wish to? It’s probably a simple change on one line...
A transistor will be needed to drive the relay whichever way the logic works. It is the same circuit either way, just the other way up.
 

Thread Starter

anishkgt

Joined Mar 21, 2017
447
Why can’t you change the sketch to pull the control pin high? Why don’t you wish to? It’s probably a simple change on one line...
Its like this;
The relay acts as a backup to switch-on a load (for now lets consider a DC Motor) and keep it ON even when the power is lost. So, driving it active LOW would be ideal as it would remain ON with load connected through NC (Normally Closed). if active high then it would require the arduino to source current continuously to keep the relay in NO (Normally Open). Hence, in active low it does not source but connects to GND and that would consume less current.
 
Last edited:

Thread Starter

anishkgt

Joined Mar 21, 2017
447
Turns out that the Logic part is useless here. A simple flasher when the relay is activated via the control pin is what am trying to achieve here. Since the Logic part is ommited and with the inverted signal from the opamp. it should be simple with some transistors.
Annotation 2020-03-28 145303.jpg
 

AlbertHall

Joined Jun 4, 2014
9,642
With this circuit the control pin would have to be able to sink the 80mA for the relay coil. If the optocoupler is going to switched on all the time then it serves no purpose and can be removed but that still leaves 80mA sink from the control pin.
 

Thread Starter

anishkgt

Joined Mar 21, 2017
447
So when is an optocoupler useful then ? I was told it was there to protect anything that handles HIGH voltage ? so is it used to protect from the coil kickback when pulsing ? would a 10Ohm resistor in series with the control pin be ok ?
 

Alec_t

Joined Sep 17, 2013
11,141
I think this does what you want.
1585418013841.png
When the Control signal is high the LED flasher operates and the relay is energised.
When the Control signal is low the flashing stops, the relay drops out and current drawn is virtually zero.
 

danadak

Joined Mar 10, 2018
4,057
I would advise not tying directly to a CMOS logic pin a large cap w/o limiting
discharge current thru internal protection diodes when supply collapses. This
can cause a die hot spot, even Si melting if current too high. Simply place a
series R at CMOS input to limit the discharge current. Some vendors spec injection
current, some not. In latter case a few mA should be ok.

Regards, Dana
 
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