touch switch

[mrbump82]

Ok so I used the schematic for a touch switch and it didn't work .so I did like breaktru said in a previous post and added another 10mohm resistor in series and it will fire with touching just one wire and slightly harder it I touch both but it seems it's not opening fully. I'm getting like 2.8 volts from a 5 volt power source

[CraigHB]

The MOSFET is not seeing enough voltage at the gate terminal to turn on fully.  You could try increasing your 10 Meg resistor to 20 Meg or even higher, but you can run into problems with leakage currents if that resistor it too high.  It can drive up the gate voltage so it never fully turns off.

Try another transistor.  The gate terminal is extremely delicate on a MOSFET in terms of exposure to ESD (static discharge).  Damage can cause the MOSFET to fail to turn on when it's supposed to.  Static basically blows a hole through the gate junction which is very thin and delicate.

Touch switches configured this way are vulnerable to ESD damage.  You can use an ESD protected MOSFET or add a TVS diode from the gate to source terminals to protect a standard MOSFET.  Select a TVS diode with a breakdown voltage an amount lower than the gate-source breakdown voltage which is 16V for this transistor.  A 10V TVS diode would be good.  However, leakage from the TVS diode can interfere with the operation of the touch switch (it will reduce gate voltage) so you need one with an especially low leakage current, could be a problem.

The 1k resistor between the pad and gate in the position shown does nothing.  It should be on the positive terminal of the battery to limit current in the event the positive touch pad gets grounded somehow.  As shown, you could short the battery directly through the positive touch pad if contact to ground occurs.

If you still can't get it to work, try a transistor with a lower gate-source threshold voltage.  It will require less voltage at the gate to turn on fully.  That's actually a "5V specified" MOSFET which means it's design to be driven by a 5V source.  You would want a 2.5V specified MOSFET ideally, but you can't find those in a leaded package like that one. 

[Breaktru]

Yeah Craig pointed out to me that the resistor on gate is not necessary on an N-Channel mosfet a long time ago.

I've use the IRLU3114 N-Channel which is similar to the 3103 but a lower RDS(on) resistance. I had to go to 100 Meg ohm between Source and Gate to full turn on and is fine for shutting off.
Also the 3114 is smaller (I-Pak & D-Pak). The 3103 is a T0-220 footprint


Note:
          Be aware that humidity and skin/finger resistance is a factor on turning on the Mosfet.
          The resistor between the Gate and Source may have to be tweaked for the Mosfet to turn on fully and fully off. Put a volt meter on Drain to ground.
          And.... make sure you also test it out w/ a load attached. Further tweaking maybe required.

[Visus]

I redrew the switch the way Craig says all good

I hover the button too much, had to abandon my touch switch modding with constant firing while surfs.. 

Break uses a 14ga wire  or 12ga center core, i dun remember  and uses the wires sheath as isolator with his full metal jacket 44 switches, its really cool way to it..

I must use that led idea thanks for this load..  I just soldered some itty bittys 0402  mofos bright as heck..
takes up almost 0 space lol..

I made a circular flat lense out of a water bottle cap looks great/pro... ya see why i like your diy with fet picture load  woot..
doing that on next mod ima makin

They have cool things
http://www.lumex.com/products/detail/0805_package_bi-color

[mrbump82]

It seems to work good but if it's got potential to fail I'll just use the tact switches that I ordered as backup. And my leds didn't work out they had dual positive leads instead of negative. Just though it would be a cool switch for mynew hooka box

[Visus]

The led I linked has separate leads its small but bright as heck.

The touch switch you gotta play around with it,  like turning on a faucet for water flow, the exact same principle but the resistor is the lever to the  valve.
That sounds awesome for an e hooka mod.. 

Heres a thru hole
http://www.mouser.com/ds/0/Bivar/3BC-3-CA-F-6545.pdf

[CraigHB]

You'd think it would not be the case, but multi-color LEDs can be headache to set up.

Through-hole bi-color LEDs with 3 leads tend to use a common cathode which means they share ground.  I actually use a one with a common anode (shared positive) and it's the ~only~ one I've found;

http://www.mouser.com/ds/2/244/SSL-LX3059IGW-CA-94876.pdf

I do have some complaints with this part, it does not have the best diffusion or brightness, but I can't find anything better.

I have a common cathode one on-hand that is much nicer, way better diffusion, much better brightness, but it's harder to work into my circuit because of the common negative;

http://www.mouser.com/ds/2/239/_100_LTL1BEKVJNN-53867.pdf

The SMD ones make the issue moot.  You can mount them next to each other in any configuration you want.  You can line up as many as three of the 0603 ones under 1/8" light pipe no problem.  It's actually pretty easy to make a light pipe, an acrylic rod will do nicely, the stuff is cheap and widely available.  However, you still have to set it up which can be problematic in terms of the enclosure.

You can find lots of two lead bi-color through-hole LEDs, they're actually more common, but they're a pain to wire because they have the LEDs set up inversely.  Takes extra parts to wire those up.  You need to switch positive and negative to switch colors.

[mrbump82]

Yeah I'm having issues trying to wire this stuff up as it is. I'm trying to fit it all into a tube cause I want them to look like hooka wands.

[Visus]

The one I loaded is  also common anode + but only 6mcd yours are mcd 30 and you say they are dim,  the one i loaded must be very dim then..

 I just did an order earlier today before reading your post they also have light pipes, I  didn't even think about ordering some 
Lumex smd bi colors are nice and bright but they dont have bicolors  with wings which makes soldering them so easy..
But yeah adding smd side by side duh' easy enough and no solder bridging on a single dbl-chip.. 

Meh' my technique is sound now,  for even the 0402's but it took 3 mess-ups to get it right without melting the led lol..
0603's were easy but I also melted those before success..   I parts hacked a cable modem for spare part smd things.  Having fun with that modem its loaded with every resistor value and lots of smd leds 0402's.  I use a razor and take up the whole pad/trace area then solder tinned 32ga silver wire to em.  My eyes cross soldering them lol.  They have two 0402 leds for each light pipe should have taken hint there.

It funny I just watched Evolv's warehouse tour and they made sure to show advanced modders how to put on screens its so easy when you see how it's done, I had a giggle.  All in technique, its uber simple.

Break gotta see that vid he was one of the 1st to have screen problems.  Laoding it in videos..

[CraigHB]

One other advantage with LEDs having a higher MCD rating is they require less current for the same brightness.  That can be be a consideration.  For example, if I want to power an LED directly off an MCU port, the lower the current demand the better.  I don't like to power things directly off MCU port pins, but to use the much nicer common cathode part that's what I have to do.  So the fact I get the same brightness at 3mA as I do at 10mA using the common anode part is a big deal.

[mrbump82]

So I'm going to do it the way break was talking about with the p channel. I noticed today that is was very leaky and would auto fire and then I noticed that when I put my lips to it it shut off

[slopes]

You can use an ESD protected MOSFET or add a TVS diode from the gate to source terminals to protect a standard MOSFET.   

Anyone? Double checking if this is correct. I've seen similar basic circuit layouts for MOSFETed switches (ie, not ecigs) which all seem to place the TVS diode between the source and DRAIN (with resistor between source and gate).

[CraigHB]

Are you sure you're not confusing that with the body diode.  That's often shown as a standard diode symbol between source and drain.  All three wire MOSFETs have a body diode (there are 4 wire MOSFETs that do not have one).  Sometimes it's shown in the symbol diagram and sometimes it's taken for granted. 

A protected MOSFET will show a diagram with a TVS diode between gate and source like the one in this data sheet; https://www.fairchildsemi.com/datasheets/FD/FDS6679AZ.pdf

If you want to protect an unprotected MOSFET, you can do the same thing with an external TVS diode.  In most cases, it only needs to be unidirectional rather than bidirectional as shown in that data sheet.  Also, a 1/2 Watt Zener can handle the job sufficiently as well, doesn't specifically have to be a TVS diode.

[slopes]

CraigHB - This guy's circuit for a triggered airgun switch is laid out similarly to the various e-cig touch switches. He puts the TVS diode between D and S - so I wondered what the difference is?

http://unconventional-airsoft.com/2009/08/26/how-to-make-a-basic-mosfet-switch/

I've bought a couple IRLU3114 N-Channel's after reading Breakthru's post above and want to do all I can to protect them. This is after blowing out several IRL3103's using Raidy's circuit in a very tight space (including with various resistor values between G and S up to 100Mohm).

If I do place the TVS diode between G and S (as in your post), I take it it sits in tandem with the (100Mohm) bridging resistor? 

[CraigHB]

That's something different.  It's still a protection diode, but not for ESD.  Motors are inductive loads that can create large voltage spikes when a source is disconnected.  That's something called a flyback diode that allows the energy stored in the magnetic field of the motor's coils to dissipate when the power is shut off.  Just about all motors and transformers require a flyback diode unless they are not switched or use low power.  Resistive loads with no appreciable inductance to not require a flyback diode.

Yes an ESD protection diode connects gate to source in parallel with any other components.  Keep in mind for high speed switching there are other considerations, but none present for a main power switch.

[slopes]

Thanks for this info, Craig.  I'm getting a 500mw 10V zener diode (BZX79C) which my local store stocks and will string it between the G and S pins on the IRLU3114. Hopefully that'll provide a more robust switch.

[CraigHB]

You want to run the Zener somewhat lower than the maximal gate-source voltage so it clamps at some point below that.  If the max Vgs for that transistor is 20V then 10V would be a good choice for one cell or two cells in series.  It will protect the MOSFET nicely, actually better than a MOSFET with a built-in TVS diode.  Zeners are usually good for about 15kV HBM ESD.  The heaviest human body static discharge is still below that.  A strong one is about 10kV.  The protected MOSFETs are only good for about 6kV HBM ESD.