UltraGuard Backup – Fully automatic

I was approached by a competition sailplane-flyer who is using UltraGuard as  backup-power. He said they are having some issues due difficult placement of the UG unit, it’s hard to reach and as we know when the day is over it’s advisable to disconnect the battery on UG so it would not deep discharge.

I thought about it a while and here is the solution.

Fully automatic UltraGuard – Setup and forget

The idea is as we know simple, UG senses when there is power coming to the main leads, it then adjusts itself to 0.5V lower voltage and sits there. If there is need to charge the 2S LiPo it does that, if not, it’s just idle.

Now, if main power is cut off due for example BEC-failure UG is giving voltage to RX and servos, giving the possibility to rescue the model. So clever and yet so simple.

There’s two issues here. First is that due difficult placement of UG in some models it’s hard to turn off, it’s done by pressing a switch on the UG. This has been resolved earlier with LogicSwitch if using Jeti, the schematic and guide is here.

Using only the traditional UltraGuard + LogicSwitch approach you still need to disconnect the 2S LiPo on UG. So, lets make that automatic too.

Schematic and comments

UltraGuard uses a 2S LiPo via balance-connector. That means 3 leads. In order to make the automatic disconnection of LiPo we need to cut two wires of those three. Easiest way is to use a two-circuit relay:


Function is very simple.

Powering up the system

  • UG Main leads get’s voltage from RX-circuit
  • Relay connects two out of three LiPo-wires, connecting LiPo to UG
  • UG senses voltage on main leads and starts normally

In case of BEC-failure

  • UG gives 0.5V lower voltage to main-leads
  • Since there’s still voltage on main leads relay is still connecting LiPo to UG

Normal shutdown of model

  • Disconnect main power (BEC, RX-battery or whatever)
  • UG keeps power on RX-circuit
  • Shutdown UG from Jeti transmitter
  • LogicSwitch shuts down UG
  • Since there is no more voltage in UG main leads relay disconnects UG LiPo
  • LiPo is disconnected, no need to physically disconnect anymore

What do you need?

A suitable dual-circuit relay. These can be found for your RX-voltage, there’s plenty of for example 8-9VDC relays, just pick one and test.

A LogicSwitch, of course.

I would personally put a capacitor to relays coil also. Just to make sure that in case of hard servo-usage on backup-power the relay would not disconnect momentarily resetting UG. I do suggest a very thorough testing, as always.


Yes, benchtested. I did not find any strange behavior whatsoever. Of course as usual, I’m not taking any responsibility on what you do to your model.

6 thoughts on “UltraGuard Backup – Fully automatic

  1. I would suggest replacing the relay with 2 small P channel FETs and a few resistors.
    The FETs eliminate the power loss in the relay coil and are much more reliable than a set of electromechanical contacts, and you eliminate the coil dropout voltage problem. While there would be some very small leakage current when the FETs are off, it would be on the order of the LiPo internal self discharge rate, so. I problem there with respect to storage between flights.
    Turn off the Rx or unplug the Rx and the FETs are off. Switch on your Rx channel and the FETs are on.
    The whole circuit wouldn't be much bigger than your LogicSwitch and shouldn't cost more than $0.25 or so to build. In fact you could put both circuits on the same PCB for a neat and compact solution.
    Easy, a few hours to design, simulate and build.

    1. You're correct in that relay is not the most refined approach. I would however consider this more as proof-of-concept. Like I wrote, I do not take any responsibility :)

      The idea on integrating everything to one PCB is very good. However the demand is so low that I do not see myself in making those at least right now. There are FET-driven relay-circuits widely available for a buck or two already.

  2. Lewis : The problem with both is that not every one is an electronics guy.
    Can you come up with a part number, or diagram with parts to make your version.
    Will the logic switch work with a Taranis or Spectrum systems?
    Thanks guys!

    1. LogicSwitch needs digital output on receiver, to my knowledge Spektrum cannot do that, I do not know what receivers in Taranis does.

  3. Wait a second, if the BEC fails, doesn't the Relay disconnect the UG's battery, and everything crashes?

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