DYI Start button, electrical help needed.

[crispybagel]

Hi, I'm working on a chassis design and I have a question regarding how a start button functions. (not sure if this is the right subforum)

Normally, the front IO start button cables are plugged into two motherboard connectors, plus and minus. But if there is no start button present, you can start the PC by making contact to both the + and - pins on the motherboard with an electrically conductive material and flipping the power switch on the power supply. 

So if i want to fabricate a custom power button, will it work like i've drawn in this picture? Sorry for the poor quality of the drawing, I hope it still conveys what i mean. The contact between the metal piece and wire ends essentially close the loop which starts the PC, is this correct?

[GuiltySpark_]

The motherboard power button is the simplest possible switch. As long as you momentarily short the two pins, the PC will be triggered to startup. 

 

Commonly the switches used are called "momentary switches" for this reason. Polarity also doesn't play a roll, this is why you can just short the pins with a screwdriver briefly. 

 

In your diagram, you've essentially drawn a momentary switch. 

[crispybagel]

2 minutes ago, rickeo said:

The motherboard power button is the simplest possible switch. As long as you momentarily short the two pins, the PC will be triggered to startup. 

 

Commonly the switches used are called "momentary switches" for this reason. Polarity also doesn't play a roll, this is why you can just short the pins with a screwdriver briefly. 

 

In your diagram, you've essentially drawn a momentary switch. 

Alright, so touching both pins with an electrically conductive surface will start the PC? E.g. pushing the start button (which is electrically conductive) onto the ends of the + and - cable will work as a start button. Or does the PSU also have to be flicked from off to on at the same time?

[GuiltySpark_]

Just now, Crispy Customs said:

Alright, so touching both pins with an electrically conductive surface will start the PC? E.g. pushing the start button (which is electrically conductive) onto the ends of the + and - cable will work as a start button. Or does the PSU also have to be flicked from off to on at the same time?

The PSU needs to be on of course but does not need to be done at the same time. 

 

These questions have me quite curious what you're doing and the backstory.

[Dutch_Master]

There's a bit more to this though. In principle the circuit is correct, but if you have exposed wires I'd highly recommend adding surge protection to both wires. Long story short: your body is essentially a capacitor in certain circumstances and you may zap the pins by touching them. Perhaps not the first (few) time(s), but eventually the pins will loose their function. If that happens, you have a problem. Not "a challenge", but a seriously defunct mainboard and it's not covered by your warranty

[crispybagel]

2 minutes ago, rickeo said:

The PSU needs to be on of course but does not need to be done at the same time. 

 

These questions have me quite curious what you're doing and the backstory.

Ok, thanks. Here are some in-progress renderings of the chassis. Maybe a week of work and the design is finished for prototyping and manufacturing. 

 

4 minutes ago, Dutch_Master said:

There's a bit more to this though. In principle the circuit is correct, but if you have exposed wires I'd highly recommend adding surge protection to both wires. Long story short: your body is essentially a capacitor in certain circumstances and you may zap the pins by touching them. Perhaps not the first (few) time(s), but eventually the pins will loose their function. If that happens, you have a problem. Not "a challenge", but a seriously defunct mainboard and it's not covered by your warranty

That sounds like agood idea, how exactly is surge protection implemented for a power button?

[Dutch_Master]

The simple version uses 4 Zener diodes in a "full bridge" configuration. The "safe" version uses an opto-coupler and transistor, with some resistors and capacitors and power from the Vsb pin of the 24 pin PSU connector.

[crispybagel]

8 minutes ago, Dutch_Master said:

The simple version uses 4 Zener diodes in a "full bridge" configuration. The "safe" version uses an opto-coupler and transistor, with some resistors and capacitors and power from the Vsb pin of the 24 pin PSU connector.

Sounds complicated, is this something that is implemented in current PCs? Can i avoid this by having a non-conductive surface as the one I physically press, while the underlying material that contacts the cables is still conductive?

[Dutch_Master]

Yes, if you ensure there's sufficient insulation between the contacts and the environment (like your finger) then there's a very low chance of interference. A 1mm thick styrene sheet will do, provided it's large enough said finger cannot accidentally touch the contacts.

 

But by then you've basically created a push button switch from scratch, which is orders of magnitude more expensive as simply buy one from Aliexpress and modify tour design to accommodate said switch. Just my 2 p worth

[GuiltySpark_]

People have been using Bulgin switches for years in custom builds or Amazon knockoffs. Really no need to re-invent the wheel if you don't have to.

 

https://www.bulgin.com/us/products/range/switches/push-button.html

[crispybagel]

6 minutes ago, Dutch_Master said:

Yes, if you ensure there's sufficient insulation between the contacts and the environment (like your finger) then there's a very low chance of interference. A 1mm thick styrene sheet will do, provided it's large enough said finger cannot accidentally touch the contacts.

 

But by then you've basically created a push button switch from scratch, which is orders of magnitude more expensive as simply buy one from Aliexpress and modify tour design to accommodate said switch. Just my 2 p worth

3 minutes ago, rickeo said:

People have been using Bulgin switches for years in custom builds or Amazon knockoffs. Really no need to re-invent the wheel if you don't have to.

 

https://www.bulgin.com/us/products/range/switches/push-button.html

Of course purchasing a switch would be the least troublesome option. I should look into this in more detail to see what kind of switches they offer. It's possible that I could simply modify the look of the button given it has the clearance required for the chassis. 

[crispybagel]

20 minutes ago, rickeo said:

People have been using Bulgin switches for years in custom builds or Amazon knockoffs. Really no need to re-invent the wheel if you don't have to.

 

https://www.bulgin.com/us/products/range/switches/push-button.html

I would assume that these buttons work just as well for a reset button since it's the same kind switch?

[H713]

Usually trying to fabricate your own custom switch is a bad idea, unless you have the means to do silver plating on the contacts. 

 

The easy (and usual) solution is to use a generic momentary switch (check Digi-Key - there's a million different options, most will work fine here) and fabricate a custom switch cap. 

 

 

[crispybagel]

6 hours ago, H713 said:

Usually trying to fabricate your own custom switch is a bad idea, unless you have the means to do silver plating on the contacts. 

 

The easy (and usual) solution is to use a generic momentary switch (check Digi-Key - there's a million different options, most will work fine here) and fabricate a custom switch cap. 

 

 

Thanks, I'll check them out.