Very small switch

[Hikerdude81]

Howdy all!
I'm a new member here, although I've been lurking for a while.
I've been looking in the archives and haven't found an answer to a question I have...Do any of you know of an extremely small switch? Preferably it would be a pushbutton (or maybe even a rocker). I have looked at the Kroll Clickie (I think that's what it's called...), and it's to big. Does anyone know of a smaller one?
Thanks for any help you may have on this.

 

[JSWrightOC]

Welcome to the Forum!!

Are you looking for a tailcap switch, or a switch in general (PCB mount, panel mount, etc.)? I think the Kroll is one of a very few tailcap drop-in replacement switches available on the market, and, depending on opinion, is the only one worth getting. (even at that, some people still have issues with it.)

Just trying to help clarify things for other readers.

Perhaps some measurements or other technical requirements might help explain what you need. /ubbthreads/images/graemlins/smile.gif

 

[Hikerdude81]

Sorry about posting in the wrong forum, I wasn't sure where this topic would best fit.

The switch is going to be in the tailcap.
The style (PCB Mount, panel mount, etc.) doesn't really matter, it just needs to fit into a space approximately .50"L X .50"W X .35"D. These dimensions are not completely set in stone, but they need to be pretty close to this.
I know this is very small, especially in the depth. I have been looking all over for one (Mouser, Digi-Key, Newark, Jameco, and elsewhere online), and haven't found anything that fits anywhere close to this. Most of the push buttons I have found have been over an inch Deep, and all the rockers are over an inch Long.
Even just a point in the right direction would be a help, I'm completely stumped as to where to look next.

 

[JSWrightOC]

Have you looked at any surface mount switches, esp. in the Digi-Key or Newark catalogs? I know C&K makes some absolutely tiny switches...even if you had to epoxy-glue the unit in place, I know of several that would most certainly fit. The only concern there is current handling capability in something that small.

You can "cheat" and tack-solder wires to the tiny leads on the surface mount switches. I've done that for several projects.

Switches being related to electronics, this is the right forum to post your question in. /ubbthreads/images/graemlins/tongue.gif

 

[MrAl]

Hello there,

Can you get away with a momentary contact switch or
do you need a switch that stays closed once pressed?
DigiKey has some extremely small ones, but i think
they are all momentary contact.

An interesting idea to get small switches to drive a
high powered LED is to use the switch to drive the base
of a low sat transistor through a resistor. This
allows the use of very very tiny switches to control
a large current such as when driving the 1 or 5 watt
LS's. Using a low Ron Mosfet you dont even need a
resistor, just a switch and the transistor!

Good luck with your LED circuits,
Al

 

[Hikerdude81]

I have looked at those extremely small switches (called Tactile, I think), and they are all momentary, I do need it to latch.

If these really small latching switches don't exist, does anyone know of a circuit that draws little to no current that latches on/off when you push a momentary switch?

MrAl, that is a good idea to use a MOSFET for the high current. I was wondering how to get around that.

 

[BrianC]

Sorry for the slightly off topic reply, but where do you guys get your Kroll switches?

 

[FreeBSDboy]

You might consider a relay with a small switch instead of a MOSFET. Although the MOSFET is nice, it will very slowly drain your batteries with leakage when it is off.

You'd probably still need the switch to latch though.

 

[InTheDark]

Have you tried the RAM switch. Inside is a pretty small clickie switch, I think it's close to those dimensions you mentioned.

 

[Hikerdude81]

RAM switch? I'm afraid I've never heard of that one. If you have a link to some information, that would be great.

BrianC, Dat2Zip's Sandwich Shop (http://home.attbi.com/~theledguy/cpf_store/cpf_store.htm) has the Kroll. There may be others, but this is where I found them.

 

[InTheDark]

http://shop3.outpost.com/product/3407283

 

[flashlightlens]

I bought a small switch like the one you're looking for from Digikey a while back. I'll have to double check on it when I get back home (in Vegas right now) and I'll get the part # for you. It's less than .5" square and about .25" high. Contacts come out the side and right angle down. If I forget, email me Monday night or Tuesday and I'll get the info for you.

 

[JSWrightOC]

A MOSFET, when operated correctly, has extremely little leakage current. In fact, I would be more concerned about the self discharge of the batteries than the leakage current through the MOSFET.

MrAl, when driving any kind of a field-effect transistor with a contact closure, you need a pull up or pull down resistor (depending on if you are using an enhancement mode (N-type) or depletion mode (P-type) transistor) to keep the gate from floating around and causing erratic operation when it's supposed to be off. This resistor can be quite large (1Mohm, even) and will not pass any current when the contact is open--only when it is closed (the light is operating anyway--the resistor's current of 6 microamps is relatively insignificant!).

As for a latching circuit, a debounced momentary switch driving a CMOS J-K flip-flop would do the trick--but good luck fitting something like that in a size that small. There are several micro-mini slide and toggle switches that are available from Digi-Key, you can browse their PDF catalog and see illustrations and drawings with measurements.

 

[LED_ASAP]

The idea of using NMOS or PMOS FET as the switch is practical only in higher voltage (4.5V or higher) lights. At lower gate voltage, the FET can not go into full saturation and you will waste significant energy to the resistance. That's why all the boost IC's that use MOS-FET as the driving transistors draw their power from the OUTPUT pin instead of the BATTERY IN pin during normal operation. If you really want to impliment this design, you will likely have to use either the output voltage or some auxiliary power source to drive the FET Gate.

 

[IsaacHayes]

I'll post a link to a circuit to make a momentary switch work as a push/on push off latch. I do belive the circuit was small and used no IC's. I'll have to find the link when I get home.

 

[IsaacHayes]

http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm
should have something that might possibly work..

 

[Hikerdude81]

Thank you all for the tips and schematics, some of those will come in handy. For this project, I only have enough room for one small latching switch. I think I may have found it at Action-Electronics. For anyone else in my same position <a href="http://www.action-electronics.com/">here</a> is the address. It's made by GC Electronics.
I will let you all know what I think of it when it arrives. I'm going to order it first thing Monday morning.

 

[MrAl]

[ QUOTE ]
JSWrightOC said:
A MOSFET, when operated correctly, has extremely little leakage current. In fact, I would be more concerned about the self discharge of the batteries than the leakage current through the MOSFET.

MrAl, when driving any kind of a field-effect transistor with a contact closure, you need a pull up or pull down resistor (depending on if you are using an enhancement mode (N-type) or depletion mode (P-type) transistor) to keep the gate from floating around and causing erratic operation when it's supposed to be off. This resistor can be quite large (1Mohm, even) and will not pass any current when the contact is open--only when it is closed (the light is operating anyway--the resistor's current of 6 microamps is relatively insignificant!).

As for a latching circuit, a debounced momentary switch driving a CMOS J-K flip-flop would do the trick--but good luck fitting something like that in a size that small. There are several micro-mini slide and toggle switches that are available from Digi-Key, you can browse their PDF catalog and see illustrations and drawings with measurements.

[/ QUOTE ]

Hello there,

I agree that the leakage of the MOSFET wont be significant
but you dont really need a resistor to drive the MOSFET
gate if you use a SPDT switch. One contact to V+ and
one contact to V- drives the gate to fully on or
fully off with no chance of an inbetween gate state.

Im glad you mentioned it though, because im sure most
people will want to use a standard SPST switch to V+
with maybe a 10 megohm resistor to ground on the gate.
This allows a wider selection of switches to be used.

Also, while re-thinking the various solutions, i'd now
want to check the maximum gate current rating for
when the switch is first closed. The inrush current into
the equivalent gate capacitance could be quite high.
Perhaps a SM size 10 ohm resistor in series with the
switches' arm to limit the current spike. This wouldnt
add to the size much at all.

As for the latching mechanism, perhaps two small momentary
switches -- one to ground and one to V+ with a series
current limiting resistor, and a large cap connected to
the gate. One switch, when pressed, would charge the cap
turning the light on. When released, the cap would remain
charged for quite some time, untill the other switch is
closed, discharging it. This would take some testing
though to see if the cap ever reaches a very power consuming
inbetween state which could burn up the transistor.

Maybe i'll look into posting a schematic at some point.
Any other ideas or suggestions let me know.

Good luck with your LED circuits,
Al

 

[JSWrightOC]

Mr. Al,

SPDT, never thought of that. /ubbthreads/images/graemlins/smile.gif

Gate resistors are good, though interestingly I have seen pulse-shaping networks for driving FETs that have a small value capacitor wired in parallel with the gate resistor, to sharpen up the pulse...go figure.

When selecting the capacitor and designing it into your circuit, use a very low leakage unit, such as a Tantalum or similar. Regular electrolytic caps have far too much leakage for this to work reliably.

Another option to the capacitor (although somewhat larger) would use a pair of CMOS inverters wired back-to-back through resistors in a latch circuit. Pressing one switch forces one of the gate's inputs high and the other low, latching the circuit. Press the other switch and the gates flip states.

This is something completley unrelated save for the use of MOSFETs, but a flashlight could be designed so that it had two contact surfaces that you would invarably bridge with your hand when you grabbed it, turning the light on. Let go (or move your finger, for instance), and it would turn off (maybe even with a delay?). I know for some applications this wouldn't be practical, but it would make a great momentary switch that would *never* wear out! Just make sure to install shunting diodes on the gate circuit so ESD charges won't damage the FET.

 

[MrAl]

Hello again JS,

I like the idea of a touch switch too, for apps that
wont get wet or anything.

I'd like to try using two sets of contacts to charge
and discharge the capacitor (mentioned in previous posts)
which would make a very cheap latching circuit.
Im just not sure yet if an in between gate state will
damage the transistor. Im sure it would work for driving
the smaller LED's like the 20ma type, but i'd have to look
at the 350ma type closely before i did anything like this for
that higher rated LED.

Take care for now,
Al