[Freedombox-discuss] Time

Paul Gardner-Stephen paul at servalproject.org
Fri Dec 13 03:56:10 UTC 2013

Hello Adrian,

A fine question, and one that is probably as easily answered by trying it
as anything else.
Many phones have an internal temperature sensor, so some correction for
temperature could be attempted.
Similarly, the temperature sensor could be used with the phone's screen
and/or GPS receiver as heaters to maintain the temperature as constant as


On Fri, Dec 13, 2013 at 1:32 PM, Adrian Gropper <agropper at healthurl.com>wrote:

> How close could we get with just software on an Android phone? The phone
> would be disabled to whatever point it took to maintain a counter, charge
> the battery, occasionally listen for GPS or wifi. How stable could that be?
> Adrian
> On Thursday, December 12, 2013, Doug wrote:
>> It seems that time is important.  So how do we know what time it really
>> is, without leaving a trace?
>> Many years ago I knew someone who did some experiments with time.  He
>> wanted a really accurate timekeeper, but did not have a lot of money to
>> spend.  This was long before GPS existed, and before NTP servers became
>> easily accessible to nearly everyone.  You could tune a radio to WWV, or
>> call up the time lady at the phone company, but that was about it.
>> So he built his own OCXO.  He built a little circuit board, with a
>> crystal and an oscillator circuit.  It also had another circuit,
>> electrically isolated from the first:  a heater circuit, consisting of a
>> transistor, and a temperature-sensing element, in a feedback arrangement.
>>  (I think this was right around the time when fairly precise and
>> inexpensive temperature-sensing ICs first became available.)  He set this
>> circuit to maintain the temperature on the board at a temperature higher
>> than any likely to be encountered in his room, I think about 60C.
>> He had a small Thermos brand vacuum jug, the kind used to keep a bowl's
>> worth of soup warm.  Of course this has a double-wall glass envelope,
>> aluminized on the interior walls of its vacuum chamber, and is an excellent
>> thermal insulator.  He cut a small hole through the screw-on cap and ran
>> wires through it, and suspended his circuit board in the middle of the jug.
>> Outside the jug he set up a power supply for the crystal circuit,
>> designed to be as stable as he could afford, and another power supply for
>> the heater circuit.  He powered it all up and waited for hours for the
>> temperature to equilibrate.  He then used this module as a clock source for
>> his other projects that needed really stable time.
>> (He had to wait for hours because he designed the heater circuit to max
>> out at a fairly low power level, knowing that very little power would be
>> needed to maintain 60C in that insulated jug.  He wanted to keep overall
>> power consumption as low as possible, because he wanted to operate time
>> bases over long periods of time and therefore needed to run his clock
>> source, and his counters, on trickle-charged battery power, and didn't want
>> to invest a lot in batteries.)
>> There were a number of problems he had to solve.  The insulating jug was
>> so good that he had to worry about the temperature overshooting the set
>> point, as the temp feedback circuit was not the only thing in there that
>> was dissipating power, and it itself dissipated power even when the heater
>> transistor had no current.  He had to choose the right kind of crystal, as
>> different kinds have different degrees of stability.  He had to choose the
>> right frequency range.  He baked the crystal for a long time to age it.  He
>> had to put small faraday cages around both the crystal circuit and the
>> heater circuit to keep noise out of the oscillator.
>> IIRC he estimated a stability of something like one second per year, at a
>> very low cost (most of the parts coming out of his spare parts box, or the
>> kitchen).
>> Of course these techniques have been in use in commercially available
>> clock sources for decades, and I see you can now buy OCXO modules for as
>> low as $50 at places like Digikey.  But I am not aware of any cheap
>> mass-produced OCXO-based appliances suitable for use in a personal NTP
>> server you can keep in your home.
>> Imagine such a module, with a small low-power computer attached (open
>> hardware, perhaps an Arduino would be enough, or perhaps this one?
>> http://www.kickstarter.com/projects/214379695/micro-python-python-for-
>> microcontrollers ), with a lithium battery so you can carry it outside,
>> and with a USB and/or ethernet connector.  You have to connect it to an
>> accurate clock occasionally, so it knows what time it is.  Then after that,
>> it just sits there and counts.  Connect it to your local network, or plug
>> it directly into your Freedombox, and there you are.
>> With this kind of frequency stability, you don't need to calibrate it
>> against external time bases very often.  Perhaps carry it outside
>> occasionally so it can see GPS signals, or carry it somewhere where it can
>> see someone else's wifi.  Perhaps just design it so that you can sync it up
>> by plugging it into a smartphone.  Or all of the above.
>> With the growing importance of accurate timekeeping, and of leaving
>> minimal traces behind while doing it, perhaps the time has come for an open
>> hardware project for such a timekeeper.  (Kickstarter, anyone?)
>> Seems to me such an appliance could be produced for less than what a lot
>> of us spend on a good gateway router.
>> Would something like that be accurate enough for our purposes?
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> --
> Adrian Gropper MD
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