# Schmitt Trigger Help

Discussion in 'General Electronics Chat' started by sjrobinson, Feb 5, 2014.

1. ### sjrobinson Thread Starter New Member

Feb 1, 2014
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0
So redplaya suggested I look into Schmitt Triggers. I think I have the basics figured out but I'm hoping to get a verification on a few things.

I'm trying to make a switch that would activate a relay when 4.5v or greater is applied to the circuit.

From what I read it seems that a non-inverting trigger is best for what I need. This would consist of an op-amp (any basic op-amp?) a 1k resistor before the ground, and 10k resistor.

Its that simple?

Thanks

2. ### MrChips Moderator

Oct 2, 2009
18,208
5,722
No, it's not that simple. But it could be.

You are lacking a lot of information.

Where is the 4.5V coming from?
What is the part number or specs on the relay?
What is supplying power to the relay?

3. ### Papabravo Expert

Feb 24, 2006
11,994
2,574
The basic point you are missing is that a Schmitt Trigger has not just one, but TWO(2) switching thresholds. Given an initial state, the Schmitt Trigger will switch to the opposite state when the input crosses the threshold that is the gratest distance (in voltage) from the initial state. After the output changes state, the switching threshold is changed so that is farther away (in voltage) from the new output state.

This is a circuit with a digital output and analog inputs. An opamp is normally unsuitable for this application. The good news is that a circuit called a comparator is just the ticket for you.

Checkout the following:
http://en.wikipedia.org/wiki/Schmitt_trigger

Look for the comparator circuits along the right hand side of the page.

4. ### #12 Expert

Nov 30, 2010
18,076
9,678
As usual, there are about 6 ways to do this, 12 ways if the only thing you give us is, "4.5 volts". Get real pacific about what you have and what you want and an answer will be tailored to your needs.

ps, I know how to spell. I'm just having some fun because this guy is from New York and would know how to interpret the joke.

5. ### sjrobinson Thread Starter New Member

Feb 1, 2014
14
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Sorry, I gotta get used to what to specify and how to word these.

I did see that comparators could be used as well. Looking up voltage switch gave me a bunch of plans that kinda confused me.

This is a DC circuit. So basically it would be 4.5 volts coming from a pot controlled 5v source. That would be tee'd into the + voltage wire. So the voltage can be 2 volts or it can be 4 at any given time. Once it reaches that threshold (4.5V+) I would like it to send a current through the 5v SPST relay to switch on another circuit. Ill draw up a quick idea of what I mean...

6. ### crutschow Expert

Mar 14, 2008
21,422
6,137
You need to answer the question about the current the coil of the relay requires. What is the relay coil resistance?

7. ### sjrobinson Thread Starter New Member

Feb 1, 2014
14
0
Very little current. 5 amps would suffice.

8. ### MrChips Moderator

Oct 2, 2009
18,208
5,722
What do you mean "Very little current"? .5A or 5A is a lot of current for a relay.

Give us the part number of the relay, please.

9. ### sjrobinson Thread Starter New Member

Feb 1, 2014
14
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Excuse my ignorance, I'm hoping to learn here. But doesn't a 5v SPST relay work the same regardless of its capacity for 5 amps or .5 right? Isn't it triggered by the voltage?

I suppose up to this point I'm used to seeing schematics for circuits that work for the particular application even when a few parts are slightly different than the specs. I have a rather large tolerance to work with as this is all experimental.

Heres a link to a relay that I could use. I see 55 ohms at the coil.
http://www.ebay.com/itm/1-pc-Relay-...862?pt=LH_DefaultDomain_0&hash=item23358082a6

Last edited: Feb 5, 2014
10. ### MrChips Moderator

Oct 2, 2009
18,208
5,722
You need to know the characteristics of the relay coil, not the contacts.
If the DC resistance of the coil is 55Ω then it will draw 90mA @5V
You need this information in order to design your relay coil driver circuit.

11. ### sjrobinson Thread Starter New Member

Feb 1, 2014
14
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Ok. Thanks.

So with the information provided, what options do I have for a switch to be activated by a >5 circuit (preferably adjustable)? Or is there anything else I need to provide?

I'm trying not to break the forum rules... So for this link, I don't want to copy it exactly and don't mean to display this as an automotive mod. But its the best thing I can find.
http://www.autospeed.com/cms/gallery/article.html?slideshow=0&a=110548&i=12

I see something as complex as that. It seems like something I would like to build because of the convenience of the pot. But there's also this thing that I can't figure out how to build because of whats labelled K1 FSMR which I assume is a relay. However, I fail to see how it would work since the relay should have at least 4 connections. Otherwise its not doing anything.

http://www.zen22142.zen.co.uk/Circuits/Switching/vcs555.htm

Last edited: Feb 6, 2014
12. ### MrChips Moderator

Oct 2, 2009
18,208
5,722
One NPN transistor with 1kΩ resistor in series with the base of the transistor should do it. Connect the emitter to ground and the collector to the relay coil.

13. ### sjrobinson Thread Starter New Member

Feb 1, 2014
14
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Ok. Thanks for your help. Much appreciated

14. ### sjrobinson Thread Starter New Member

Feb 1, 2014
14
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I just realized... Why wouldn't a relay with a resistor to drop the voltage down to a specific voltage that the relay would switch at work for this?

I'm trying the NPN transistor and I also tried a schmitt trigger with a 555. Problem is (what I think is called) the hysterisis is too wide of a range (?). Is there a way to narrow it down so that the tolerance is within .5v?

What I found was that the relay in the circuit would only switch on at 7v but switch off at 4. Thats a 3v gap.

15. ### MrChips Moderator

Oct 2, 2009
18,208
5,722
You are conflicting what is known as hysteresis.

Before we get into that I had asked what power supply is powering the relay? You have not provided that information.

Mechanical relays have an inherent hysteresis where the latching voltage is much higher than the holding voltage. 7V and 4V respectively is not unusual for that type of relay.

The hysteresis on the input voltage of an NPN common emitter driver circuit is much different. This is the hysteresis you need to focus on.

16. ### sjrobinson Thread Starter New Member

Feb 1, 2014
14
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I'm switching between using a 9v battery and 4 AA's to see what voltage works. The NPN Transitor didn't work at all though. I'm not sure if I missed something but the relay was acting a little funny too. I think it may just be the hysteresis of the relay.

Is there another type of relay with a closer latching and holding voltage? Or a component that I can put in the circuit to help?

17. ### MrChips Moderator

Oct 2, 2009
18,208
5,722
You are bouncing around too many issues all at the same time.
Focus on one question at a time.
The relay on the link you have provided indicates that it operates at 5VDC.
According to the DC resistance measurement you have made, the relay coil would require 100mA @ 5V.
The relay should work with 4AA's or one 9V. Note that a 9V battery is not designed to supply 100mA for a very long time.
So your first goal is to get the relay working with a suitable power source.

The next goal is to place an NPN transistor to function as a switch to turn on the relay. Let's not deviate from this second goal.

18. ### sjrobinson Thread Starter New Member

Feb 1, 2014
14
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Ok. I understand. It would make sense that the relay does need a certain amount of current.

Ill try the Transistor circuit again then update.