Touch switch with two outputs - only driving LEDs

iimagine

Joined Dec 20, 2010
479
Ah, yes, iimagine. It seems the PMV28 was proposed in the end. Would that do the job in your circuit?
I dont see why not. Its time to get used to surface mount components anyway. Although those 2N7002 are WAY cheaper and should probably do the job since you are only lighting up leds, not much currents.
I’ll have to source fresh LEDs, too, since I can’t remember the spec on the ones I’ve got
You might not have to, if you have some resistors laying around, start with 200ohm connect it in series with the led and connect to the 3V battery. If it does not lights up or dim, change the resistor to lower value. do this till you find that lowering the resistor value does not brighten it any more, that is the resistor to use. This is much easier if you have a pot resistor and a multi-meter.
 

iimagine

Joined Dec 20, 2010
479
Revising my circuit I realized that we do not need the 3rd mosfet, this should be better:
DeleteMe2.PNG
Note: I'm just using random leds resistors, they need to be calculated or measure
 

ci139

Joined Jul 11, 2016
1,359
those 2N7002 are WAY cheaper and should probably do
?! probably . . . (note the upper rightmost graph should have VgtN and Vss-VgtP swapped vertically to match the appropriate MOSFET . . . but since thei're equal it doesnot much matter here . . .)

dummy~SWc_x04c.png
but it probably will do if no heavy loads are to dragg the terminal voltage more down than on the following graph (3.0 to 0.5mA)
source
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the Energizer's d/s allows us to make a simplified CR2032 Spice model
. . .
it's 2.85V 15Ω with no load @ half/empty (for primary simulation it's quite practical to use constant such ...)

Re low power low voltage comparator control (the use of LTC6702 is coz i got no any lo power 1.8V supply open drain one Spice models ... but that cuts just 2 Diodes off the design...) ::
dummy~SWc_x05c.gif + opting for lower skin conductance → dummy~SWc_x05d.gif/!\ but it has to be verified in practice -- the revese diode dynamic resistance behaviour /!\
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Because of the potentially high component count (and no real life confirmation of the circuit) also the compact layout required . . . i'd suggest you consider the MCU approach as an option (either way it's gonna be a hell to fit the design in to such tight space - you need to address the possible noise issues also - takes a lot of patience and ultimate will >.,< iik)
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one more option : dummy~SWc_x11d.gif
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(Update) Nothing to do -- a theoretical revision ::
dummy~SWc_x11e.gif dummy~SWc_x11e2.png
 
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danadak

Joined Mar 10, 2018
4,057
If you ever think of this as a commercial product can be done as follows.
However don't consider this for a one off design. The package almost impossible
to solder to by hand.

I posted this just to show you what is possible.

This can be done one part, 3 touch inputs, drive all LEDs if a few mA each.

Package looks like -

upload_2019-8-13_7-15-56.png

upload_2019-8-13_7-18-1.png


Buttons are capsense, basically a pad on a PCB or a film. Something conductive.

There are consultants that can do the design. The coding for this pretty trivial. the
"tuning" of the capsense buttons takes some testing to get it right.

Basically a one chip design, except you would need external Rs to limit LED current,
and a cap to bypass the chip power pin .


Regards, Dana.
 
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Thread Starter

HappyPixie

Joined Aug 4, 2019
13
Thanks to iimagine for all the input. I think I'll order the parts and give your circuit a try. Thanks also to djsfantasi for the extra Arduino info and guidelines. I'll keep it in mind in case I decide to make more of this project. Thanks to ci139 and Danadak for the detailed info. That may be too hairy for a newbie like me, but perhaps one day...
I may well post an image of the finished project when it's done, but it's take three years to get this far so don't hold your breath!
Kind regards to y'all.
Steve
 

Thread Starter

HappyPixie

Joined Aug 4, 2019
13
Well, here we are seven months later and I've finally got around to finding a suitable bits of circuit board and a few components. I assembled it all and the three LEDs stay on constantly with the touch switch not making any difference.
When using the surface mounted 2 × 2N7002 I practically melted them trying to get them to solder to the board. The finished circuit is shown attached but is practically unrecognisable in comparison to iimagine's design. You'll see my touch switch which is a brass cog from a watch surrounded by resin inside a copper tube. Touching it makes the circuit between the copper and the brass.
I've checked it over and I'm sure it's a match to the circuit design. Does anyone have any suggestions of what to try out or should I just build another attempt?
Thanks20200313_225029_edit.jpg
 

danadak

Joined Mar 10, 2018
4,057
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AnalogKid

Joined Aug 1, 2013
8,377
In surface mount electronics, anything that is "practically melted" is in fact completely melted. The 2N7000 is the same transistor in a much more robust package.

ak
 

dl324

Joined Mar 30, 2015
10,520
I practically melted them trying to get them to solder to the board.
I'm surprised, and disappointed, that datasheets are no longer providing guidance regarding maximum soldering temperatures and durations. They used to do that to avoid damaging components and boards due to thermal stress.
 

Thread Starter

HappyPixie

Joined Aug 4, 2019
13
Thanks for the swift responses and suggestions. I've just ordered a pack of the 2N7000 because they look much more forgiving of my clumsy soldering. I'm intrigued by the MePads. If I had more space, they might have been appropriate.
Hopefully I'll be up and running soon.
 

Thread Starter

HappyPixie

Joined Aug 4, 2019
13
In surface mount electronics, anything that is "practically melted" is in fact completely melted. The 2N7000 is the same transistor in a much more robust package.

ak
True to the suggestion, the 2N7000 was much more robust. I created the circuit and tried it out. The touch switch made no difference to the two LED part of the circuit staying on and just flickering. I had hoped that perhaps the battery is low but the voltmeter shows it is delivering 2.9v.
Just can't figure out what's wrong.
In the images below, you'll see the new circuit and the cavity in the bottom of my wood sphere that the battery and circuit have to fit in. You can also see the hole on the side which the touch switch fits in.
Anyone have a suggestion of what to try next?
Thanks in advance.
Steve

20200321_163021_small.jpg20200321_165156_small.jpg
 

dl324

Joined Mar 30, 2015
10,520
the 2N7000 was much more robust. I created the circuit and tried it out. The touch switch made no difference to the two LED part of the circuit staying on and just flickering. I had hoped that perhaps the battery is low but the voltmeter shows it is delivering 2.9v.
Just can't figure out what's wrong.
2N7000 isn't an appropriate device for 2.9V. Some devices won't even start to turn on with Vgs that low.
1584811006026.png
1584811026301.png

You need a logic level MOSFET.

This device has a Vgs of 1.45V max.
1584811212011.png
 

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Thread Starter

HappyPixie

Joined Aug 4, 2019
13
2N7000 isn't an appropriate device for 2.9V. Some devices won't even start to turn on with Vgs that low.

You need a logic level MOSFET.

This device has a Vgs of 1.45V max.

Thanks, Dennis.

Considering I melted the last surface-mounted MOSFETs, I kinda need something a bit bigger that can cope with my ham-fisted soldering.

The other problem I've discovered is the dullness of the LEDs I'm using – even when I hook them directly to the battery – I may have to reconsider my plans. Someone suggested I try out different resistors to use with each LED until it didn't change brightness. If I then use these LED/resistor pairs in the circuit, doubled or tripled in parallel, could that be why they're so dull?

Very confusing.
 

iimagine

Joined Dec 20, 2010
479
Hard to tell if you had everything hooked up correctly from that mess, you should get a perf board. Try hooking it up to a 5V supply and see if it works. I think 3.3V is the bare minimum though. If all else fail and you must work with a 3V there are alternatives but requires more components
 

Thread Starter

HappyPixie

Joined Aug 4, 2019
13
Thanks, iimagine. At the risk of testing your patience, how many more components would be required to suit a 3v disk battery as in my photo? You can see the space is severely limited, so that's why I ignored the perf board and went freestyle. If this style of battery varies from 3v down to, say, 2.?v what kinda circuit would make the touch switching work and still light the LEDs?
To be honest, when I attach the battery straight to the three LEDs, they're not that bright. I'm wondering what else I could do to get brightness. Excuse the stupidity, but should I have bought 3×1v LEDs (run in parallel) to work with the 3v battery?
I know which end of a soldering iron to hold but I barely know one end of a diode from another.
Thanks for your help.
 

iimagine

Joined Dec 20, 2010
479
Here's the BJT version of the above circuit. As you can see, the bias current consumption is quite large. Your battery will not last long, but can run on 0.7V if you can find LED that lights up at that voltage. I think it is best that if you can somehow stack 2 batteries on top of each other to get 6V and use the mosfet circuit. But if you want to, you can try this on a bread board and test out before soldering anything. I tested it on both simulators but have not built it.
 

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ci139

Joined Jul 11, 2016
1,359
Here's the BJT version
i appreciate your effort . . . but even the Spice (being a lot more accurate than the Falstad ) is not very good for estimating the actual power draw . . .

. . . that unless you backward match your simulation to match your actual circuit . . . and even then depending on the "mode of operation" the power consumptions can differ up to some 10% to 15% or occasionally perhaps even more depending on the particular circuit

likely - a still no use for now : "In 2014 researchers identified a liquid sodium–cesium alloy that operates at 50 °C (122 °F) and produced 420 milliampere-hours per gram. " ◄ ? versus ? ► "Their prototype battery demonstrated a power capacity of 420 Ah/kg at 150° C (300° F), which matched the performance of the traditional design, though at much lower temperatures. In addition, their battery was also able to retain 97 percent of its storage capacity after 100 cycles, "
 
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Thread Starter

HappyPixie

Joined Aug 4, 2019
13
Here's the BJT version of the above circuit. As you can see, the bias current consumption is quite large. Your battery will not last long, but can run on 0.7V if you can find LED that lights up at that voltage. I think it is best that if you can somehow stack 2 batteries on top of each other to get 6V and use the mosfet circuit. But if you want to, you can try this on a bread board and test out before soldering anything. I tested it on both simulators but have not built it.
Interesting, thanks. If I double up the battery will the 2×2N7000s and the values of resistors I used for the LEDs in the original "mess" still be OK? There is just about enough space for two batteries and a few components tucked around the edges. I was planning in embedding them in opaque black resin anyway, so nobody will see the ugly mess, just so long as the LEDs light up!
When you say the batteries won't last long, presumably that's only when the LEDs are on? Will the circuit draw voltage even when the LEDs are off?
I've ordered a breadboard off EvilBay. Should be leaving China any time now. :)
Thanks again.
 
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