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The Problem with Microswitches

 

 

I've got a few things I'd like to say about micro-switches as there's a whole lot of misunderstanding and misinformation surrounding them.

 

Micro-switches used as trigger units are bad.

 

That's the short of it (see what I did there).

They are used by manufacturers because they are cheaper by an order of magnitude to buy and manufacture than traditional trigger mechs.

 

They are great little devices if used for the purpose they were designed for - low current electronics.

They are definitely not suited to Airsoft!

 

Let take a little look at the inside of a micro-switch:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

That's almost identical to the ones used in every Ares gun.

 

There are 4 main parts inside:

- The input line on the bottom left

- The two output lines on the bottom middle and bottom right

- The spring copper plate in the middle

- The actuator (black button) on the top & its lever arm

 

When the button is pressed the sprung copper plate jumps from one output line to the other. Of course, in airsoft we only use one of the output lines but that doesn't really matter here.

 

As you can see, the copper plate in the middle thats actually transferring all the power is pretty thin.

You might imagine that it can't handle much power and you'd be absolutely right!

 

They are typically rated at a pitiful 13A.

Most stock airsoft guns will draw between 15 - 20 Amps!

 

Of course, there's a little bit of lee-way but you get the idea, putting a dirty great big LiPo through there is not a good plan.

And before you ask, Yes they are rated for 240V @ 13A so if you step down the Voltage you can step up the Current but the distance between the contacts is tiny to you'll still get Voltage arcing, even at around 10V.

Even putting high-capacity NiMhs through it is not ideal because the battery chemistry doesn't matter in this case - it's all about how much current you can provide.

 

For example. to work out how many Amps a battery can give you multiply the capacity (mAh) by the discharge rating (C).

In this case, you convert the capacity from mill-Amp-hours to Amp-hours before hand.

So a 1600-mAh battery would become 1.6-Ah.

If your battery has a 15C discharge rating you would multiply 15 x 1.6 to give you 24Amps!

 

And that example is of a fairly standard NiMh battery used in the majority of stock airsoft guns.

Some battery and gun combinations can draw upwards of 40-50 Amps!

So thats problem number 1 with micro-switches

Problem number 2 is the issue of micro-bounce.

 

Have a look here for some more info and actual Oscilloscope read-outs:

Extreme-Fire Micro-Switch Bounce Tests

 

That thin spring copper plate in the middle is very susceptible to vibration.

When the gearbox cycles the micro-switch vibrates causing the copper plate to bounce. This is exacerbated by the weight on the end of the copper plate that gives you the snappy action.

This causes a Voltage Arc between the contacts every time this bounce happens which can very quickly build up enough excess heat to melt the whole unit.

When you hook up a computerised MOSFET or one with Active Braking however; bad things can happen.

The bouncing contact sends potentially hundreds of pulses a second to the FET causing all sorts of electrical issues ranging from unusual firing patterns to massive heat build-up and complete component failure of your expensive MOSFET unit.

You can buy a professional de-bouncing unit that solves the problem but at an extra cost of £60+ it really is the expensive solution to a problem that shouldn't exist in the first place.

But why would you want to hook up a MOSFET to a micro-switch, surely you don't need to as they give you a better trigger response already?!

 

Yes and No is the simple answer to that one.

 

Yes, a micro-switch does indeed give you a snappier trigger response but that is at the cost of reliability as I've mentioned above.

They are very prone to failure - especially with LiPos that tend to have higher discharge ratings - and almost always have poorly implemented selector systems.

 

A MOSFET however, is not just something that gives you a better trigger response. It is primarily there to handle all the power from your battery to your motor so the trigger unit doesn't have to.

That means the micro-switch is back within the operating parameters its been designed for :)

A simple MOSFET with no computerised functions and no Active Braking (which isn't really needed anyway) actually works very well with micro-switches and solves all the power-handling problems I've mentioned above.

 

So in actual fact, not only is a simple MOSFET advisable when using a micro-switch, if you're using higher power batteries it is absolutely ESSENTIAL!

 

This also solves Issue number 3.

 

The micro-switches lack the nice, self-scrubbing action of our traditional trigger mechs.

When the trigger block on a normal mech moves in and out of the contacts it acts as a self-cleaning mechanism - removing any carbon build-up caused by Voltage arcing.

Of course, that's not an issue if you're using a MOSFET but it quickly becomes one if you're not; causing an unreliable connection.

 

If you've managed to get to the end of this rambling essay, I hope that's cleared up some of the myths regarding Micro-Switches.