Mesh/02 February 2016

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sudomesh meeting 9 Feb 2015


Daniel, MaxB, Alex, Marc, Matt

Gear from Yahel

He is not budging on prices.

We're going to buy from him:

  • The two AF5 units
  • The two Mimosas
  • Two of the toughswitches

Finding bandwidth

Alex is going to talk to Monkeybrains (he has a contact there).

Discussion about voltages and why we want toughswitches

  • Ubiquiti Airfiber gear takes 48 VDC
  • Normal Ubiquiti gear takes 10.5 VDC to 26 VDC
  • Old Ubiquiti PoE supplies are 15 VDC and new supplies are 24 VDC
  • Toughswitches allow us to do remote reboot
  • We could use a passive multi-port PoE and hefty power supply instead but no remote reboot

Our routers give us access to peoples homes

  • How do we communicate this to people in a way that doesn't sound terrible?
  • Should we investigate a standard piece of hardware that we can sell to people who are concerned?
  • We should have a policy that is written up somewhere public on how we handle root ssh key access to the mesh

How much interference do microwaves cause on 5 ghz

Marc will test tonight (didn't do it. got distracted by ordering the 12v cable supplies).

Make cables for people to power on their nodes from 12v / car power

Rationale: We have had this idea that the mesh could be more disaster resilient than other connectivity but in reality installing solar and battery systems is expensive and simply isn't going to happen for most nodes. The ony power source that most people will have immediately available following a disaster is their car and car battery. It would be cheap and simple to make kits containing a long cable with cigarette plug and battery clip connectors on one end and a PoE on the other. This would allow people to power on their nodes after a power outage. It would also make it simple to hook up directly to a 12v solar panel.

Marc ordered the following to make 10x test units:

  • The schottky diode protects from inverted polarity (in case someone attaches the battery clips the wrong way around).
  • The capacitor protects from voltage spikes (in case someone attaches battery clips while node is plugged in)
  • The zener diode protects againts over-voltage (in case someone attaches it to a voltage source higher than 22vdc)

Why not cut off at 25 volts with the zener diode so people can use 24 volt batteries as well? Because 24 volt batteries are often actually outputting voltages above 26 volts (especially when being charged) and Ubiquiti gear dies at above 26 volts. Better to have a higher margin of error and just be explicit that it doesn't support 24 volts.

Total cost for enough to make 10 units (with some extra components): ~$75 (that's without fuses, bags, cost of CAT5 and instructions)

If we sell the kits for $10 we're more than breaking even.

We should put the electronics in some common tin/plastic container (dollar store or candy container (Alex's idea)). Maybe we just hot glue that box under the passive PoE.

Apart from the PoE there should be two additional output wires with 5.5 x 2.5 mm male barrel plugs and we should include an additional passive PoE for people who have two extender nodes. This way there will be enough plugs to power both home and extender nodes.

We should include a few extra fuses in the kit and a guide on how to replace them.


We probably want less than 1 volt voltage drop over the cable since there will be additional voltage drop over the ethernet cable. We probably want at least 50 feet of cable so people can realistically reach their car from their home (or maybe we just offer different lengths).

This calculator says that 50 feet with 16 awg cable at 12 volts and 1.5 amps will have a 0.6v drop. That's probably acceptable, that means we can support up to 82 feet of cable while staying under a 1 volt drop:

You can get 1000 feet of 16 awg cable for about $50. That means a 50 foot cable costs $2.5

Compare this to using all pairs in 26 awg CAT5 cable which gives a drop of 1.53 volts over 50 feet:

This calculator says that the drop over the nice 24 awg outdoor cable (using two pairs for PoE) with a 12v input over 50 feet and assuming about 7 watts of draw for a nanobridge/nanostation results in a 0.77 volt drop:

So total drop from car to extender node assuming 50 feet from car to home node and 50 feet from home node to extender node will be about 1.4 volts. If the extender nodes stop working at 10.5 volts that means that 11.9 volts will be the minimum allowed output from the car battery. Many car batteries will probably drop below that point at some point. Some say that Ubiquiti gear will work down to 9-10 volts though.

Low voltage cutoff

  • We know that completely discharging a car battery is bad for the battery (also then your car won't start)
  • What happens when home and extender nodes drop to below their minimum voltage. Do they still draw power?
  • What is the minimum voltage for home nodes?

Either way we shouldn't discharge car batteries below 11 volts. How do we prevent that? What is the simples/cheapest low voltage cut-off circuit?

Higher voltage extended version

This version allows voltages up to 36 VDC as input. It has the normal circuit and in addition it has a double throw switch that disengages the 22V zener diode and instead engages a 36v zener and a LM2596S buck converter board (less than $1 from china, but we should add a heat sink). The reason why the switch is important is that the buck converter needs 1.25 volts more input than the output, so if the output is set to 12v then the input would have to be 13.25 minimum. If powering from at 12v battery then any voltage drop is bad, since ubiquiti absolute minimum is 10.5 volts and long ethernet cable runs introduce an additional voltage drop. We might want to use the same kinds of switches that switch between 110v and 220v input on the back of computer PSUs to avoid accidentally switching (which could fry the fuse).

  • Ordered 36V 5W zeners
  • Already have some LM2596S buck converters (we should add tiny heat-sinks to them)
  • Ordered a few double-throw switches (but the "easy to accidentally switch" kind)

Solar panels

A home node + extender node probably draws about 12 watts total (need to test this) so a 20 watt panel might be enough but a 50 watt panel would be better.

  • 20 watt panels are about $50
  • 50 watt panels are about $75
  • 20 watt panels with a charge controller are about $70
  • A 12v 18Ah deep cycle battery is about $35

Total cost is probably between $100 for something minimal to use in an emergency and probably closer to $300 (or maybe more) for something that is reliable enough to be in operation year round. Not sure what local laws are regarding having something like this permanently mounted on/at your house.

Of course actual minimal setup is just the 20 watt solar panel and no battery and that might actually work for several hours a day.