I am selecting relay for A micro-controller project and I am electronic beginner so I faced an issue when I had to choose between low-level triggered and High-level triggered

As far as I understood :

  • Low-level triggered will allow the current to go through the power line when the control signal is below a certain voltage.
  • High-level triggered will allow the current to go through the power line when the control signal is above a certain voltage.

Am I right?

  • are you selecting solid state or mechanical relays? can you post what part you are looking at? – Wesley Lee Nov 25 '15 at 10:04
  • won't help a lot I guess : world.taobao.com/item/… seems to be mechanical – Memes Nov 25 '15 at 10:13
up vote 8 down vote accepted

A mechanical relay needs current to pass through its bobin to trigger. If you wire one end of the bobbin at V+ then you trigger it by driving the other end low, if you wire the first end av 0V/GND, then you trigger it by driving the other end High. So the relay itself doesnt care if its High or Low trigger (thats why I asked if it was a mechanical or solid state, and the part link).

The link that you posted however, shows a relay module with a transistor driver.

enter image description here

In such cases, depending on the transistor and configuration you use you can have specifically high or low level triggers.

However, there is another catch. That relay has both NO (Normally Opened) and NC (Normaly Closed) pins.

This means that when a relay is not triggered, the NO pin is opened (not connected) and the NC is shorted to the common.

When a relay is triggered, the switch bounces and NO becomes closed (connected) and NC becomes opened (disconnected).

Which means that.. when that relay is triggered, depending on how you wire it, you can have it conducting or not.

So.. answer is: you are partially correct and the rest can be correct depending on how you wire it..

Did I make sense..?

  • Your reasoning was correct, I'm just being thorough. – Wesley Lee Nov 25 '15 at 10:27
  • that is a perfect answer that sums up what I found/understood after asking the question. which is basically : The relay switches when there is current, the control circuit (included in the module I am considering) will define High or Low trigger and some relay are not single line but double, one NO and one NC (this part I knew already as I bought the water valve already for my watering system project :) ) – Memes Nov 26 '15 at 2:53
  • what you said about "single/double line": take a look at SPDT, SPST, DPDT switches, it adds even more configurations possible – Wesley Lee Nov 26 '15 at 7:09
  • aren't you suppose to simplify my problem? :) Yes, I'll have a look at that. – Memes Nov 27 '15 at 11:27
  • was still a question though. Where is the limit between high and low level on a 5V relay? if I have 4V, will that be high? as far as I remember from school, it was halfway with tolerance so below 2V would be low, above 3V would be high and in between would be... well, in between :) – Memes Nov 27 '15 at 11:28

You are essentially correct.
You can state the meaning somewhat more generally so it applies to more situations.

In general terms, in a micro-controller environment when digital signals are involved then:

  • Low = ground level = logic 0 = logical false

  • High = supply voltage (usually Vcc or Vdd) = logic 1 = logical true.

You use the (sensible enough) "current goes through the power line" as a indication of triggering. Other results may equally qualify.

So:

  • "low level trigger" means "desired action occurs or is commenced or is triggered or ... when the control signal is 0 / false / ground.

  • "high level trigger" means "desired action occurs or is commenced or is triggered or ... when the control signal is 1 / true / Vcc or Vdd.

This is a reasonably pedantic change but allows the meaning to cover complex outcomes (eg a Bluetooth message is sent) or ones with no current involved (MOSFET gate signal is applied (or removed))

And also allows for the desired outcome being a negative one. eg motor is turned off by safety switch when a high level signal is received. - ie here the removal of voltage and current occur with the application of a high level trigger signal.

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