555 On-Off Switch using NO and NC Momentary Switches

Thread Starter

markmain

Joined Dec 28, 2008
21
I have a shop tool switch that has 2 momentary switches independent of each other. The NC momentary switch is for the red OFF button, and NO momentary switch for the green ON button. I want to use a 555 to make a dependable on-off switch that will power a DC to AC SSR.

Although I'm only running a shop vac with it, someone else may run shop equipment or CNC and so I would like to make sure that it's resistant to power surge and brownout failures. e.g. safely and reliably switches on/off when it's supposed to.

During a quick brownout I would like the switch to stay ON for maybe 250 to 300ms, but otherwise switch off.

If someone has an example schematic or URL that I'm unable to find so far, I'd appreciate the help. My biggest difficulty with this is how to use the NC for the Reset. Also, any suggestions to keep this safe and reliable is appreciated. E.g. resistor and caps used on the SSR and 555 pins.

I've never used an NTC Thermistor before. Any advice on best practice for the SSR output to a motor and/or surge protection on the DC line input is nice to know.

Thanks in advance for any help offered.
 

olphart

Joined Sep 22, 2012
99
I'll suggest a simpler way: a 2+ pole relay rated for the load, with one contact feeding the coil power through the NC PB and the NO PB directly feeding coil power. It's self latching, reliable and fault tolerant. It's even simpler if the relay coil is voltage is same as load.
 

AnalogKid

Joined Aug 1, 2013
9,493
Your question is confusing.

Are you talking about removing the two switches from the shop tool, and then using them in a constructed circuit?

Or, are you talking about piggy-backing on the two switches while they continue to switch whatever kind of power or signal they are switching in the shop tool?

Or, something else.

Simple before and after sketches would help.

Also, what power source is available for the new control circuit? Voltage, type (AC / DC), current - ?

With the right combination of answers, this might be done with one small SCR. No relay, no 555 ...

ak
 

Thread Starter

markmain

Joined Dec 28, 2008
21
I'll suggest a simpler way: a 2+ pole relay rated for the load, with one contact feeding the coil power through the NC PB and the NO PB directly feeding coil power. It's self latching, reliable and fault tolerant. It's even simpler if the relay coil is voltage is same as load.
Thank you. I can see the latching relay will work simpler. I found this link that explains it very nicely starting around 7 minutes into the video
 

Thread Starter

markmain

Joined Dec 28, 2008
21
Your question is confusing.

Are you talking about removing the two switches from the shop tool, and then using them in a constructed circuit?

Or, are you talking about piggy-backing on the two switches while they continue to switch whatever kind of power or signal they are switching in the shop tool?

Or, something else.

Simple before and after sketches would help.

Also, what power source is available for the new control circuit? Voltage, type (AC / DC), current - ?

With the right combination of answers, this might be done with one small SCR. No relay, no 555 ...

ak
Thanks for the question, no it's not an existing application, it's a new project. I'm turning on a shop vacuum from multiple locations using multiple buttons. I kept this simple because once I solve this with one of these buttons I can easily use multiple buttons in series for the NC's and parallel for the NO switches.

Attached is a typical switch that is used for shop equipment. They have 2 buttons, both are momentary switches, and are independent of each other. The red OFF button is always a NC momentary switch, and the green ON button is always a NO momentary switch. This seems to be the standard way they are made. I'm trying to learn how to use them in a real world application. An advantage of this is that you can also wire in a few emergency stop buttons, and perhaps 2 or more buttons to turn on/off equipment that is not dangerous. e.g. a shop-vac... i would only recommend 1 ON button for a table saw for example or other dangerous equipment.

Hope this helps.
NO-NC Momentary Switch.png
 
Last edited:

crutschow

Joined Mar 14, 2008
28,198
Below is the LTspice simulation of a latch circuit using a 12V DPDT relay.
It sets the relay ON (green trace) when the NO Set switch (yellow trace) is momentarily closed, and it turns OFF when the NC Reset switch (red trace) is momentarily opened.

For use with more the one switch, all the NC switches must be in series and all the NO switches must be in parallel.

What are you going to use to power the circuit?
A wallwort type supply would work.

For the 300ms brownout ON, you may need to add some large capacitor to the power supply output.


You could also use a relay with a mains (AC) powered coil.
In that case, you would remove diode D1.
But using AC to power the coil will make it difficult to provide the brownout delay you wanted.


1633625695151.png
 
Last edited:

crutschow

Joined Mar 14, 2008
28,198
The red OFF button is always a NC momentary switch, and the green ON button is always a NO momentary switch.
That's to make the operation fail-safe.
If continuity is accidentally broken anywhere in the ON/Start circuit, that will not cause the machine to start.
If continuity is accidentally broken anywhere in the OFF/Stop circuit, it will instantly stop the machine.
 

LowQCab

Joined Nov 6, 2012
1,334
Put this Circuit in series with the Relay-Coil for Under-Voltage-Shut-Down.
It has an approximately ~2-Second Delay to turn On or Off.
.
.
.
UVLO for Motor Starter 1 FLAT  .png
 

Thread Starter

markmain

Joined Dec 28, 2008
21
Below is the LTspice simulation of a latch circuit using a 12V DPDT relay.

What are you going to use to power the circuit?
A wallwort type supply would work.

For the 300ms brownout ON, you may need to add some large capacitor to the power supply output.
You could also use a relay with a mains (AC) powered coil.
In that case, you would remove diode D1.
But using AC to power the coil will make it difficult to provide the brownout delay you wanted.
Thank you for the lTspice. I was planning to use a 12V DC wall supply; regarding the brownout, I wanted to keep my circuit running for a brief flicker of the lights and otherwise letting it just die requiring the user to push start again. Delays up to 1 second seem safe, but then really at that point i want the switch dead so the user can decide when to safely power the equipment back on.

I prefer this can push through a minor light flicker, so I may go with a DC powered circuit.

Thanks
 

Thread Starter

markmain

Joined Dec 28, 2008
21
Put this Circuit in series with the Relay-Coil for Under-Voltage-Shut-Down.
It has an approximately ~2-Second Delay to turn On or Off.
.
.
.
View attachment 249791
If my circuit just dies after 1 second of brownout would this still be needed? Also, I've never used an NTC Thermistor for surge protection to AC motors or a DC circuit. If you or anyone has any background with this I appreciate it.
 

AnalogKid

Joined Aug 1, 2013
9,493
Thank you for the lTspice. I was planning to use a 12V DC wall supply; regarding the brownout, I wanted to keep my circuit running for a brief flicker of the lights and otherwise letting it just die requiring the user to push start again. Delays up to 1 second seem safe, but then really at that point i want the switch dead so the user can decide when to safely power the equipment back on.
With a DC-powered control circuit, that can be done with an additional diode and holdup capacitor big enough to power the relay coil for 1 sec.

In the first post, you say that the circuit is to drive a SSR. If so, the SSR need less power than the DPDT relay forming the latch. Consider swapping the relay for an all-solid-state solution.

ak
 

LowQCab

Joined Nov 6, 2012
1,334
A Negative-Temperature-Coefficient-Resistor is usually used for
Temperature related functions, not "Surge-Protection".
An MOV ( Metal-Oxide-Varistor ) is the most common part for this function,
although a TVS-(Transient- Voltage-Suppressor )-Diode
may also be used in certain applications.
For a Vacuum-Cleaner-Motor, You really don't need either one,
as a Motor can usually tolerate huge temporary Voltage increases,
or Voltage-drops for that matter.
.
.
.
 

Thread Starter

markmain

Joined Dec 28, 2008
21
I'll suggest a simpler way: a 2+ pole relay rated for the load, with one contact feeding the coil power through the NC PB and the NO PB directly feeding coil power. It's self latching, reliable and fault tolerant. It's even simpler if the relay coil is voltage is same as load.
Thanks again for your reply, I drew this schematic based on what you said. Do I have the correct understanding of what you are describing?
 

Attachments

Last edited:

Thread Starter

markmain

Joined Dec 28, 2008
21
Below is the LTspice simulation of a latch circuit using a 12V DPDT relay.
It sets the relay ON (green trace) when the NO Set switch (yellow trace) is momentarily closed, and it turns OFF when the NC Reset switch (red trace) is momentarily opened.

For use with more the one switch, all the NC switches must be in series and all the NO switches must be in parallel.

What are you going to use to power the circuit?
A wallwort type supply would work.

For the 300ms brownout ON, you may need to add some large capacitor to the power supply output.


You could also use a relay with a mains (AC) powered coil.
In that case, you would remove diode D1.
But using AC to power the coil will make it difficult to provide the brownout delay you wanted.


View attachment 249777
I just started studying your plan here after I just replied to olphart, and what you wrote is the DC version of what he was describing and I created. Well, at least I can see that I'm understanding this correctly. I like the DC version better because it gives me my brownout feature. For my vacuum I'll likely keep it running for 3 or 4 seconds, but if it were something else with added risk I'd shorten the time down to 1 second. A bad idea for a machine to appear dead from a long power outage, only to have it turn on when the power returns, so need to be careful sizing those caps. haha
 

Thread Starter

markmain

Joined Dec 28, 2008
21
A Negative-Temperature-Coefficient-Resistor is usually used for
Temperature related functions, not "Surge-Protection".
An MOV ( Metal-Oxide-Varistor ) is the most common part for this function,
although a TVS-(Transient- Voltage-Suppressor )-Diode
may also be used in certain applications.
For a Vacuum-Cleaner-Motor, You really don't need either one,
as a Motor can usually tolerate huge temporary Voltage increases,
or Voltage-drops for that matter.
.
.
.
I'd forgotten about the MOV, thanks for reminding me. Here is why I was thinking of the NTC resistor, I'd read a few places where it could be used for inrush current limiting and i seemed to see it talked about for motors, especially when used with SSRs.

https://amwei.com/how-to-select-ntc-thermistors-for-inrush-current-limiting/
https://www.newark.com/pdfs/techarticles/ametherm/Ametherm_PPT_FEB07_REV6DistyTutorial.pdf
https://www.homemade-circuits.com/using-ntc-resistor-as-surge-suppressor/
https://amwei.com/ntc-thermistors/inrush-current-limit-surge-suppress-power-ntc-thermistors/
 

crutschow

Joined Mar 14, 2008
28,198
With a DC-powered control circuit, that can be done with an additional diode and holdup capacitor big enough to power the relay coil for 1 sec.
You many not need the diode if the power supply has no bleeder resistor and the output voltage simply decays due to the load current.
 
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