Failsafe On/Off switch

[dmccormick001]

This past weekend one of the members of my club crashed a nice Extra 300. The plane and all the electronics were fairly new, and it had never been involved in any previous crashes. The pilot said that he suddenly lost all controls, absolutely no response. The damage wasn't too bad, just some damage to the landing gear and bottom of the fuse. When we took the plane over to the pit area to try to determine what happened, we noticed that even though the switch was still on and the battery still connected, the indicator lights on the switch were off. The switch is one of the kind that has a green, yellow, and red LED to indicate the battery's charge/condition. When we flipped the switch off and back on again, the lights and plane came back on. Evidentally the switch just stopped working for some reason. This isn't the first time a club member has lost a plane because of a bad switch, I'm sure a lot of guys have seen or had this happen.

Ok, here's my question/idea. How hard would it be to design a circuit that would make use of a momentary contact push-button switch, instead of a slide or toggle switch? The idea is to get away from a mechanical switch that has contacts that can corrode or fail in some way once the plane has been turned "on". The circuit could be toggled from an "off" state to an "on" state by holding the push-button down for, say, a couple of seconds. Once "on", it wouldn't depend on the switch to remain on, but could be turned "off" again by another two-second long depressing of the switch. Sound reasonable?

Part of the problem I've encountered in thinking about something like this before is the need to avoid a circuit that puts a lot of drain on the plane's battery when it's supposed to be "off". I know it's possible to reverse the action of a switch so that when it is "on", it holds the circuit in an "off" state, and when it is "on" the circuit is "off", so that if the switch fails the plane stays on. But everything I've tried like that has resulted in a lot of drain on the battery when the plane is supposed to be off. The momentary contact switch seemed like a good way to avoid that. I'm hoping it will be possible to use a MOSFET as a switch to handle large current demands even if the plane is a giant-scale.

My problem is, I don't have the design experience to be able to draw up a circuit like this from scratch. I can build just about anything from a schematic, and I've got some experience at programming a PIC, but I'm light on the theory/design side of things. If some of you guys will help me get started, I'll build it, troubleshoot it, work all the bugs out, and then make all the information available to everybody here so anyone who wants one can build it, complete with pictures, tips, etc. That's assuming, of course, that somebody agrees that this would be a worthwile project.

[rick.benjamin]

Take a switch apart.
The conducting parts are two tiny pieces of copper sheet.

What caused the switch to fail?
Maybe over-amp'd component?

The switch is rated for ?(20) amps.
De-rate by 125%
20 amps divided by 1.25 = 16 amps
Working load the switch can reasonably tolerate.

Sorry the plane crashed
Rick

[init4fun]

...

[pldaniels]

Init,

You might be talking about my BAC units.

I recently released the BAC-ESJ and the ESJ-CHG parts which basically deal with this very problem of mechanical faults causing momentary power supply disruption and subsequent reset/failure.

Paul.

[pldaniels]

Here's a link to the available devices;

Please note: these switches, unlike traditional designs, do not rely on mechanical contacts in the 3.5mm switch to turn on. These units turn on due to the absence of mechanical contact, hence they are 'failsafe'. Do not confuse these with the older "normally-closed" (NC) systems.


http://www.nqrc.com/?vp=PLD-ESJ-CHG-RX
http://www.nqrc.com/?vp=PLD-ESJ-CHG-08 (same as the RX but different wiring soldered on)
http://nqrc.com/images/s-PLD-ESJ-CHG-RX.jpg



http://www.nqrc.com/?vp=PLD-BAC-ESJ
http://nqrc.com/images/s-PLD-BAC-ESJ.jpg



http://www.nqrc.com/?vp=PLD-ESJ-CHG-08 (same as the RX but different wiring soldered on)

[Tomapowa]

I converted all my gassers and glows to A123 powered, regulated fail-safe systems using these:

http://www.fromeco.org/Products/04FRCKOD-440/Default.aspx

http://www.fromeco.org/images/fromeco/productpictures/Kodiak/PNG/KodiakwithFacePlatetext.png

For a DIY solution, depending on your max current draw... you could build this:
http://www.laureanno.com/RC/regulator.htm

http://www.laureanno.com/RC/Volt-Reg-Switch-2.gif

[init4fun]

...

[TomM]

Just replace the switch with any quality dpdt toggle switch available from Newark or Digikey.
The ones using a headphone jack...how many times have you had static/intermittent/connections on a NC headphone jack that you had to jiggle to get working... for a while. Add dirt and grime from the field to it and it will be worse. Plus- headphone jacks make a dead-short when plugging in and out. It's the nature of the design.
Honestly, looking around at all the electronic equipment I have at home, from consumer audio/video, to pro gear, all my ham radio gear, to test equipment I use at work, I haven't seen a slide switch on anything in 25 years. The only place I still see those cheap things is RC gear.

[DubbleD]

Deleted

[pldaniels]

There's nothing really hard about making a solid state switch.

The reason why my products use a 3.5mm plug is because of the multipole facility and relatively simple/compact installation compared to some other plugs I'd have preferred (especially since I'm aiming this at small gliders).

I should point out, the 3.5mm plug/socket configuration I use is not used for passing high currents during the operational/on process, it's purely signal level currents and that is only while plugged in to disable the system (at worst, these will turn on accidently, though you can add a parallel slide or toggle switch for a master "on/off" override).

Anyhow, making your own isn't difficult, all you have to do is configure a MOSFET in a low-side configuration with a default "pull-up" (eg, a resistor from V+ to the gate of the MOSFET) and then a 'hard pull down' when you insert/close the switch (eg, 1K resistor or a direct short).

The one characteristic though to watch when picking your "pull up" resistor is the quiescent (idle) current when you're turned off (as current will flow from V+ via the pull up resistor to GND via the "off" switch). You want something that's sufficiently strong to avoid accidental turn offs but also sufficiently large to keep your quiescent current low (else your battery will go flat quick).

[dmccormick001]

Well, obviously this has already been "done to death". :p

It figures. Tomapower, the Kodiak switch looks like a really nice solution, but I was hoping to avoid the cost of one like that with something I could build myself. I looked at the other link you posted, and it looks good, too, but it looks like you'd have to take the battery out of the plane every time to charge it, right? I'd like to have something that would allow you to leave the battery in the plane and charge it "normally".

The devices from pldaniels look like they are exactly what I was describing, and the ability to insert the mini-phone plug to charge the battery or check voltages is great. I just wonder about their current handling abilities, both seem rather small, with no obvious MOSFETs or anything to handle the current. How can they stand the 20amp rating?

Now I'd just like to be able to build one myself. :D

[pldaniels]

dmccormick,

I've got mine at 8A rating right now, most of the time it'll be the cabling that gives in before the MOSFETs as anything below 20AWG won't cope very well with constant currents above 10A.

See the little black brick on the top of the ESJ-CHG-08, that's the MOSFET. On the larger Kodiak there's a gang of 3 MOSFETs there doing the job as well.

Regards,
Paul.

[jbeanelliott]

A possible low cost solution to the problem of mechanical slide switch failure would be to wire two slide switches in parallel. You would need to remember to turn both switches to the ON position before flight. If either one of the switches failed you would still have power.

Well, just a thought for consideration.

-John Elliott

[pldaniels]

John,

That idea can work, so long as you're vigilant about checking the switches independently, else you'll get to the point where one switch has already failed... and the next is just waiting :eek:

[pldaniels]

Oh, just another thing to bring up, the switch contact-resistance can quite often be moderately high (0.02~0.05 ohms) which can cause a fair amount of heating and voltage drop at higher currents, eg,

3A draw, 0.05R resistance, V = 3 x 0.05 = 0.15V, heat/Power = IIR = 3 x 3 x 0.05 = 0.45W (that's quite a bit, certainly enough to make a switch noticably warm).

MOSFETs on the other hand are sub 0.01R resistance, the ones I'm using on the ESJs are 0.008R and I can swap that out for ones as low as 0.003R.

Paul.

[Hell-e-Guy]

pldaniels,

I've built and used a MOSFET based switch for my slope gliders. I would like to point out one thing regarding the PCB I see in your post. It's very important to make two none plated mechanical holes in the PCB for each wire for added mechanical strength.
In the PCB/Solder design you get the reliability and low resistance of a MOSFET but the wires can be pulled out over time from their pads.

Hell-e-Guy

[Acetronics]

For a DIY solution, depending on your max current draw... you could build this:
http://www.laureanno.com/RC/regulator.htm



Hi, Tomapowa

Having a closer look to the datasheet, it seems that the 5 cells batts or 2S Lipos are to avoid ... :o

7v5 as an absolute maximum input votage ... is VERY VERY VERY ;) close to 5 cells freshly charged voltage.

Alain

[pldaniels]

Hell-e-guy,

Although not shown in the photo of the item, the cables are additionally restrained by glue/blobs. I do understand what you're saying about the dual hole strain relief holes though I've found at times that the leads can still break internally over time (had this happen with receiver antennas). Of course, at least it won't take the PCB track with it :)

Paul.

[Tomapowa]

Hi, Tomapowa

Having a closer look to the datasheet, it seems that the 5 cells batts or 2S Lipos are to avoid ... :o

7v5 as an absolute maximum input votage ... is VERY VERY VERY ;) close to 5 cells freshly charged voltage.

Alain
Thanks for the concern Alain but this had been discussed previously... Myself and few others had that concern and most of us are using a 2 cell LiPo for input. Even with a full charge, the reg works perfectly and one in fact has been in use now for over 3 years. I also have another reg. in testing (1 yr) and the input is two cell A123 battery pack (6.6v). Again, this seems to be working just fine... and I probably will be converting to A123s for all my regs.

[rich smith]

A major cause of switch failure is not overcurrent but rather too little current. Heavy duty switches will stop working because the contacts are not "cleaned" by high current. DPST slide switch with contacts wired in parallel can be more reliable than "high quality" units costing many times more. You must be particularly careful of this when using it to control a solid state switch. Lowest possible rating is best.

[dmccormick001]

I really like the ideas demonstrated by the devices to which pldaniels gave links. If I understand them correctly, the sub-mini plug inserted into the PCB mounted jack is used to turn the unit "off". I could do the same thing with a servo jack mounted in the plane and a plug that had a jumper installed between two of the connections. The battery then could be charged via the same servo jack. I'd probably add a small LED to indicate power on and maybe one to indicate charging. That's exactly the kind of thing I had dreamed up, I'd just like to build it myself insted of buying one. No offence to pldaniels, his site looks like he's really got a great company going, and a great product to solve the problem.

But I'm just not good enough at the circuitry yet to be able to figure out what size resistors/capacitors I'd need with which MOSFET to build my own and make it work. I haven't had formal training in electronics, just what I've been able to pick up over the years by experimenting and reading and tinkering with stuff. Bad thing about learning that way is you often learn what NOT to do, but you don't always learn what TO do, if you know what I mean. But I have to say I've learned a heap about PIC programming by looking a source files off the 'net, and then changing them and hacking on them and seeing the results to the circuit.

Thanks anyway guys. I guess this one's already been solved, no need to re-visit it.

[pldaniels]

dmccormick,

Good luck with it. No offence at all taken, this is after all the "DIY" section and I'm a very big DIY'er, sometimes I just get carried away and turn it commercial. :)

Regards,
Paul.

[init4fun]

...

[dmccormick001]

If it didn't cost so much to ship to the US, I'd probably consider buying one of your units. You should try to get them into one of the larger online R/C retailers, like Tower Hobbies, Troybuilt, Atlanta Hobbies, somebody like that. I think you'd proboably sell the hound out of them.

[pldaniels]

dmccormick001,

Mountain Models will probably be picking up a few more of my electronics items including these ESJs. At the moment I'm working on sending out a few units here and there, checking feedback from people etc.

Paul.

[pldaniels]

Dave,

Package went out today.

Paul.

[dmccormick001]

Thanks, my friend.

Dave

[KraftyOne]

And I'm glad I still have few of the Noble 4PDT switches in my parts box... :D

Paul, hope you have a few brushed ESC in stock, gunna need a couple soon.

Cheers, EGB.

[pldaniels]

Hi there Krafty, been a while!

Yes, I've got a few in stock and you know I can always conjure up a few on demand as well :)

Paul.

[lazy-b]

Guys.....I almost Crashed my Helicopter due to Switch Failure.

I've been thinking making my own electronic switch using a simple SCR and POWER MOSFET......look like SCR will avoiding using a Pull-up resistor to turn-on the Power Mosfet.

You can purchase a Toggle Switch that alway return to its Center Position.

here is the detail discussion about my circuit:
http://www.rcgroups.com/forums/showthread.php?p=10304644&highlight=switch#post10304644

Ellion

[lazy-b]

look like there are already lots of solution for possible Switch Failure......Now, lets tackle the possibility of Battery failure.

Been thinking of connecting 2 Indentical battery in Parallel, now to prevent 2 Battery prevent from charging or dis-charging thru each other, will installed a Diode on each Battery Pack.

[pldaniels]

Yep, just a schottky will do fine. I've read with great amusement some of the writeups for that "amazing technology".... things like "Our finely tuned balance system ensures that your packs are constantly switched between the two packs to evenly use the power.", I'm a bit of a bitter man but some of that marketing talk makes me bitter.

[dmccormick001]

Yep, just a schottky will do fine. I've read with great amusement some of the writeups for that "amazing technology".... things like "Our finely tuned balance system ensures that your packs are constantly switched between the two packs to evenly use the power.", I'm a bit of a bitter man but some of that marketing talk makes me bitter.

LOL, I couldn't have said it better myself............

There are a couple of members of my R/C club who have had a string of unexplained crashes since converting to the Spektrum technology. These are guys who aren't rookies, and who don't make rookie mistakes. They've all had more than one occurance of a plane that just stopped responding, no warning, no previous problems, the only thing different being the new "amazing technology". I have no doubt that the underlying technology is indeed better and maybe even amazing, but I think the companies who have been racing each other to bring it to market have been guilty of making it available waaaay before they got the bugs worked out of it. As recently as this past month, at the SEFF gathering in Americus, GA, there were several unexplained crashes. The company reps claim they've looked into all possible scenarios and have made sure that the equipment can't possibly be the cause, yet the crashes continue. They claim all the problems have to do with low battery voltage, which causes the rx to re-boot for several seconds, but I can't help but wonder about their claims. How difficult should it be to keep the rx powered under any circumstance, regardless of the pack's voltage fluctuations? The rx shouldn't need but about 3 volts, and most Li-po packs are from 7.4 to 11.1 volts, with current ratings of 1300ma to 4000ma or more. And if that's the only problem, why don't they isolate the rx voltage from the motor/servo's voltage supply, and put a small photo-type battery in the rx to power it independently? Can't be that hard to design.

[pldaniels]

They -could- design these things to have separate power supplies but then it's more weight, more space and more complexity and probably another thing for the consumer to "break". Technically it's rather dead simple (just keep the grounds tied together).

Personally I'd be quite happy if they provided a separate 2 (or 3) pin interface strictly for RX power. In one move they'd have likely increased the survivability of a lot of planes.

Of course you can buy power-expanders but they're a lot more cumbersome/heavy/expensive because you're having to now isolate each and every servo channel, rather than just isolating the one power input.

If you're feeling experimental you can hack in your own 2-pin power lead to the receiver and solve the issues, of course you need to cut the track supplying power from the 3-pin headers to the RX regulator section. For some it's probably not so hard, for others it might be neigh impossible (eg, flood-fill pattern or lots of vias).

Paul.

Paul.

[dmccormick001]

I think some of the company techs are suggesting that you use a seperate 4.8 volt pack just for rx power, and some Spektrum users now swear by it, some are still just swearing at it. It hasn't eliminated the mysterious crashes, but who can tell if they are really due to a battery issue? The seperate track for the rx power seems the best and simplest solution. Oh well, for me it's gonna remain good ol' dependable channel 46.

[pldaniels]

I'm still holding onto my double-conversion 36MHz gear, very happy with it. One thing that is nice about the non-microcontroller/DSP solutions is that even if the power does 'brown out' for a moment it only takes about one or two frames before it's back in sync , less than a tenth of a second :)

[Tomapowa]

Spektrum does sell this "hold-up" capacitor for riding out power glitches..., never used it myself so don't really know its effectiveness.
http://www.spektrumrc.com/Products/Default.aspx?ProdID=SPM1600

Re: powering servos seperately from the receiver itself, I built a few simple adapters for people in the past (mainly 1/3+ scale). One even easily plugged directly into a Futaba 9ch PCM receiver... you simply attach your servos and connect a high current 5-6v BEC to the bus/adapter. It was originally designed with an onboard BEC (linear 5A) but most wanted a 10+amp config, hence an external switching BEC was used. No opto-isolation (built a few of those too), but at least it gave you the ability to power your servos from another external source without any ugly hacks. Some one actually sells such a similar, simple adapter... just can't remember who.

[pldaniels]

Tomapowa,

Was looking to start production of a similar unit, the big problem of course is that it's a whole lot more bulk and weight on top of something that could be solved by a simple receiver design change :(

As for the external capacitor that you often see being sold to 'solve' those glitch problems, it's really not going to provide a lot of protection since once again, it's on the common rails. A better solution would be to have that capacitor on the inside of the receiver's 3.3V or 2.7V rail, of course the inrush current to charge that could make for some interesting startup faults.

Wonder how long before someone in the 2.4GHz manufacturing realm does start offering the 'separate rx' power connection?

Paul.

[Dylwad]

I have been running dual redundant Nimh RX packs and switches for years, no diodes needed. Lost an important plane to a single switch failure once, once too many....

The packs have to be the same voltage, but can be different capacities.

Use quality switches (futaba digital) and quality batteries(sanyo) with quality connectors(futaba) and you wont have any problems. Have a close look at JR, Hitec, Futaba and cheap connectors and its clear Futaba wins.

Have a read at www.rcbatteryclinic.com about redundant packs.

Dylan

[lazy-b]

Guys, was able to test my Circuit this Morning using an SCR and Power Mosfet as posted in this thread. it so happen I do not have the SCR so I use 2 pcs PNP and NPN transistor part no. 9012 and 9013 wired as SCR.....works perfectly.....the only limitation is that it can work on a minimum of 3.8 Volts supply, I guess, its not a problem cause I plan to use 7.2 Volts and use a Regulator to drop it to 5.0 Volts.

I tested 2 different power mosfet, have install a Dummy load of 7 Ohms, and adjust the variable supply until I get a current of 1 amps.

IRLZ44 has a voltage drop of 0.018 Volts
IRFZ40 has a voltage drop of 0.040 Volts

look like its good enough 0.018 for 1 amps.....means about 0.180 for 10 amps.

I plan to use 2 pcs IRLZ44 in parallel this will further reduce the voltage drop in Half .

I just found all those compoent from my Junk Box, I eventually plan to use surface mount SOIC 8 pins.

This IRLZ44 Power Mosfet has the Lowest On-Resistance a few years ago.......any suggestion for a Much bettrer Power Mosfet ?

[pldaniels]

Hey there Lazy-b,

The IRLZ44 has a specced Rds(on) of ~28mR, so that means you should be seeing something more like 0.028V drop @ 1A, hrmmm.

For modern MOSFETs there's a lot of options down to about 2.5mR (1/10th what you're using now) but most that I know are surface mount.

Some cheap useful ones are as follows;

IRF8714 (~8mR)
FDS8874 (~7mR)
FDS8670 (~6mR)
PH2520U (~2.5mR)

They're all SO8 but they're still quite simple to handle.

If you want to stay with TO220, then

FDP8447L 8.7mR
IRF1324PBF 1.5mR (bit expensive ;) )

[pldaniels]

If you want to do 10A moderately often, then I'd probably suggest keeping the Rds down at or below 8mR to keep the heat dissipated down;

P = IIR
P = 10 x 10 x 0.008 = 0.8W that's still fairly reasonably hot though not out of spec.

Paul.

[lazy-b]

Paul, Wow..... 0.002 ohms , thats only .02 Voltage drop for a 10 Amps current.......I guess, its safe to assume a 10 amps peak, with 5 digital servo, 2 amps for each servo.

Thanks for your suggestion.

I was amaze by the simplicity of my Circuits, it has 000.0 microAmps in the OFF-States, No need to disconnect the Battery Connector .

Need to look for Schottky Diode, so that I can install 2 identical Battery......I guess, its better to use 2 pcs 5 amps Diode connected in Parallel, rather than using a single 10 amps Diode.

same with the Battery, I plan to use 2 seperate Battery of 1700 MAH per Battery pack.

look like this will be my Most Relieable Power System, its like using a Specktrum Dual Channel, Dual Antenna.

With Dual Battery, Dual Schootky Diode on each Battery, Dual Power Mosfet.......need also a Dual Power cable going to the Receiver........what other possibility can go wrong ?

Ellion

[pldaniels]

Just a warning regarding diodes, you shouldn't really put them in parallel to boost current capacity as otherwise they will 'fight' with each other. You really need to find a single 10A rated unit, or if most of the time the current is below 5A but only for short periods you go to 10A, you can use a 5A (just check the spec sheets, my 3A schottky's can cope with 10A peaks for short periods).

Paul.

[lazy-b]

Paul, Yes, I agree with you.......The only reason I use 2 Diode in Parallel is for Reduncy only, its most likely failure of Diode is Open, using 2 diode in Parallel will avoid open circuit.

As long as you do not exceed the current capacity of each diode, 2 diode will just share a Current Loading, you can not perfectly balance the Current loading for the 2 diode, But I guess, it will automatically Balanced, cause diode heat up its voltage drop will rise then some of the current will diverted to the other diode.

I've seen, a lots of Products uses Multi Diode connected in Parallel, I've seen other uses 5 Diodes connected in Parallel........other way to make it more evenly shared the current loading is to insert a very low resistance in series with the diode such as 0.1 ohms, have offen saw this techique in Parallel connection of Transistor.

anyway.......Thanks for your comments.

Guys, was able to get an Small SCR 2N5060 Yesterday. the circuits now performed better than using 2 pcs PNP / NPN transistor, I can now operate it in a much lower voltage.


Been thinking of using this electronic switch on a 4.8 volts Ni-Cad power supply.

here is the Supply Voltage, Voltage output on a 5 Ohms resistor load

Voltage input, Voltage Output
5.0 V , 4.92 V
4.5 V , 4.44 V
4.0 V , 3.83 V
3.9 V , 3.40 V
3.8 V , 2.90 V
3.7 V , 1.98 V

the circuits works perfect as long as your battery is always above 4.5 Volts.....look like the problem is that the Power Mosfet IRLZ44 can not be Fully TURN-ON a lower voltage......The Voltage Drop Created by the SCR circuit can not Fully Turn-ON the Power Mosfet, have check the IRLF44 Data sheet, you need at least 4.5 Volts to Full Turn-on the Power Mosfet.

one possible solution is:
1. to Increase the Gate Driving voltage thru the use of Voltage Double Circuits to drive the Gate of Power Transistor, dis-advantage of this it will make the circuit a little complicated

2. or use a Mechanical Relay that can operate a much lower voltage the Relay Coil connected to the Output, and the Relay Contact can be connected parallel to Power Mosfet.

3. or use a regular DPDT Switch, one side of the DPDT switch connect to ON/OFF SWITCH of the SCR Circuit , and the other side connected to the Power Mosfet.........This will create another Redundancy cicuit, its like Mechinical Switch connected parallel to Electronic Switch

In the Old days, I've seen a lots of Brushed Electronic Speed Controller, use this kinds of designed, Power Transistor and Relay, in slow speed, it use Transistor to Pulse wide modulate drive the Motor and Active the Relay on the High Speed to Further Reduce the Voltage drop across the Transistor.

Guys, you may have a better idea........

Ellion

[lazy-b]

I have been running dual redundant Nimh RX packs and switches for years, no diodes needed. Lost an important plane to a single switch failure once, once too many....

The packs have to be the same voltage, but can be different capacities.

Use quality switches (futaba digital) and quality batteries(sanyo) with quality connectors(futaba) and you wont have any problems. Have a close look at JR, Hitec, Futaba and cheap connectors and its clear Futaba wins.

Have a read at www.rcbatteryclinic.com about redundant packs.

Dylan

Thanks for the links......

Yes, you could do......its like using a Single Battery connected in Parallel, just like most Electric Helicopter connected in 2S2P......and you need to make sure that Both Battery must have almost the same Voltage Level before you can connect to each other.

look like thats another simply way of making a Redundancy Battery.....just be sure its should be the same Voltage level before connecting it together......cause a fully charge battery connected to a fully discharge battery, could produce a Huges amount of current and could Fries your Receiver Circuit board.

By just installing a Single Schottky Diode (0.2 Volts Drop) on each Battery pack would make it a lot safer.......You can use a 5 Cell Nicad and add a Regular DIode (0.7 Volts drop) to compensate for a Voltage drop created by the Diode.

With a Diode on each Battery pack, I can use different Kind of Battery.....my Main Battery could be Lipoly and my Back-up battery could be a smaller Nicad Battery.

I have encounter 2 switch failure using an Original Futaba Switch, I was lucky enough that I was able to detect it before I fly.

[lazy-b]

Guys, I was able to make my Electronics Switch using SCR and Power Mosfet to work as low as 3.5 Volts , its exactly using the same circuits, I just replaced the POWER MOSFET with exactly the same Part no. IRFZ44N (bought it a few years ago) But when I put back the Newly Purchase IRFZ44N, the operating voltage goes back to 4.0 volts.

Both Parts came from the same manufacturer, just different Manufacture date......if it can works as low as 3.5 Volts, I think its safe enough to use it on a 4.8 Volts Nicad system.

Guys, I've seen somewhere, that there is a Opto-Coupler design to drive a Power Mosfet, Inside the Opto-coupler is actually a Solar Cell the Light source from LED is converted to Electrical Signal to drive the Power Mosfet.....I plan to use this.

Ellion