Wanted an external BEC for 2s lipo
anyones got one?

Is this for the xt? If yes you should be looking for a lipo-cutoff if no please ignore this post .

If you want DIY,
An LM7805 do the trick. TO220 Package without heatsink can run 1.5A and can consume 1.9 watt. The device will looks like below, a 3 pin device like transistor cost about Rs 5-10/-.

     _______
     |   O   |
     --------
     |\______\
     | | 7805 |
     | |        |
      \|_____|
       █  █  █
       |   |   |
       1   2   3

You can use series regulator
So, a for a 2S Battery, Input Voltage will be 2x3.7 = 7.4 volt and output voltage = 5 Volt.
and taking 30% duty cycle, power will be ((7.4-5) x 1.5) x .3 watt = 1.08 watt

means need no heatsink.

Now connect Pin 1 to Batt+ Positive and 2 is common negative for batt as well as output.
and PIN 3 is +5Volt +ve output.

But the most simplest circuit is Add 3 series and 3 parallel 1N 4007 diode, connect in series from bat+ to +ve terminal. 1.2-1.8 volt drop with 3A capacity will be available that will serve the perpose.
================================Added========================on Request
an zener regulator may be like this.


and if transistor is like Darlington pair with S8050 and C2233 transistor,
(Darling pair used to avoid continuously current drain in base resistance of Normal Power Transistor.)

Vbe = 1.2-1.4 Volt(Typ), 2.5 Volt (Sat)
hfe = 20 x 100 = 2000. hfe of c2233 = 20, hfe of d8050 = 100
Ic = 4A or less. So, Ib1 = 4/20 = 200mA (for c2233) and ib2 = 200mA/100 = 2mA.

Zener Calculation
therefore, this base current will pass through Rb resistance in excess of zener or at Iz=0.
Zener voltage = 5+1.4 = 6.4 Volt -Take 6.8 Volt.
And at zero load current, ib1=ib2=0, Zener current = 2mA.
.: Zener watt = .002 x 6.8 = 0.0124 watt = 12mW (any watt as min is .125 watt available)

Resistance Calculation
.: R = (7.4-6.4)V / 2mA = 500 Ohm.
Now as the current of BEC not go upto 4A, take next higher level resistance = 560 Ohm.
And Power = 1Vx1V/560 = 2mW, i. e. 1/4 watt or 1/8 watt will be enough (Crosscheck).
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And if u use only C2233, Ib = 4A/20 = 200mA.

Zener Spec
Vzener = Vout + Vbe drop = 5+1.2 = 6.2 Volt
IZener = 200mA, Pzener = 6.2 x 0.2 = 1.24 watt.

Resistance Spec
Resistance R = (Vin - Vzener)/Ib = (7.4 - 6.2)/ .2 = 6 Ohm
Rwatt  = I x I x R = 0.2 x 0.2 x 6 = .24 watt
- Use 0.5 watt 6.2 Ohm or two 12Ohm 0.25 watt in parallel.

Advantage
You can get good regulation.
Disadvantage
You have constant current drain of 200mA.
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For switching regulator, google with "roman black SMPS"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you need Readymade BEC,
search
www.RCElectro.com
www.rcforall.co.in
www.rcbazaar.com
www.rcdhamaka.com

http://www.rcbazaar.com/products/807-avionic-sport-bec.aspx

Thanks

Thanks, Kalyan sir. it will hwlp me too.

Could you also give a DIY, circuit with Zener Diods.

Added to original post.

Thanks da..
But now I guess IN 4007 diode method is the best.

Abhay,

please don't mind, its 1N4007 Not IN4007.
1 Stands for No of junction = 1, i. e. 2 layer device N & P
Similarly 2N stands for transistor (3layer device, PNP or NPN)
3N Stands for 3 junction i.e. thyristor(SCR,UJT,SCS,GTO) with 4 layer.
4n Stands for 5 layer device with 4 junction like MOSFET(N-Ch,P Ch, Enhance/Deplation mode).

RCforall has them in stock

@ Kalyan Sir, i never mind it..
We can also learn frm mistakes. 
I have used these diodes several times but I never went to buy these diodes, as i can get in most of the elctronic PCBs i have. And always read it IN4007 instead if 1N4007. So had a misconception of what seemed to be written on them. now i got it correct..