Re: Simple 60 Khz Transmitter Design?
- From: chrisgj198@xxxxxxxxxxxxxx
- Date: Mon, 4 Feb 2008 16:28:29 -0800 (PST)
On 4 Feb, 12:01, mike <spam...@xxxxxxxxx> wrote:
John G wrote:
<pol...@xxxxxxxxxxxxxx> wrote in message
news:95e4a0eb-b87b-49a9-9019-cd3fa794b036@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I'd like to be able to update my watches with the NIST atomic time
broadcast, which unfortunately we don't have here in Australia. I've
looked at the format of the broadcast and think it should all be quite
achievable except the actual RF, which I have no experience in. I
would like to know if anyone experienced with transmitter design is
out there and can confirm for me if this simple idea might work.
To my way of thinking 60 Khz is extremely low frequency, so I was
thinking of just using a 555 timer (wired for 50% duty cycle), with
the output hooked through a capacitor (to make it AC) and a coil (to
make it more sine-waveish) to a short length of wire for an antenna.
Would that constitute a transmitter?
Sean
I hope before you (and a lot of members of this NG) waste any more
"time" on this problem you realise 2 things.
You're overthinking this. If I understand the project, it's to set a
watch...not recreate WWVB.
Some of us think it might be fun to do the
experiment with someone else actually building it.
1. a 555 timer is less acurate than a klepsydra and certainly less
accurate than a 1925 long case pendulum clock I have in my lounge room.
And probably more accurate than the spray can of cheese whiz in my
fridge...and about as relevant.
The 555 is the carrier. All it has to do is stay in the bandpass of
the receiver. So, the real question is, "what are the receiver
requirements for carrier accuracy and stability?" I'm betting that the
$19 WWVB atomic clock hanging on my wall has a pretty sloppy receiver.
While I'm at it, let's ask one more relevant question...
WHEN does the watch update itself. I'm just guessing that the power
consumption of the WWVB receiver might be orders of magnitude greater
than the power consumption of the part that goes tick-tock.
That suggests that the receiver might be OFF most of the time.
Won't help to have a synchronization setup that's not near
when the watch is listening. And what if it can't find a signal?
Does it retry? How often? Does it ever give up and quit trying
altogether? Battery drain may go through the roof. Or it may just
stop trying to synchronize.
Knowledge of the update algorithm might be critical here.
2. There is a whole lot of data embedded in the 60khz signal that you
cannot generate without first owning an atomic clock to establish the
time.
Most of us DO have access to an atomic clock. It's called GPS. For the
rest of you, the time clock in your computer, synchronized to a time server,
is plenty accurate to set a watch...my watch anyway...
The whole project is silly. If you've got a wrist watch that cost over
$9.99 and needs to be set more than once a year, it's broke. Take it
back. If you need action on a schedule that accurate, you need
something more reliable than a human with an accurate watch
to orchestrate it.
But who among us has never delighted in a project that others
thought a waste of time?
The MSF (UK) radio-controlled clock that I disassembled contained a
crystal used as the filter in the receiver, so it probably has a
receiver bandwidth of only a couple of Hz. You wil need a crystal-
controlled transmitter. Probably the easiest way is to use a
microcontroller with an appropriate crystal (e.g. 6MHz ? 24MHz etc.)
and then configure one of the counters inside the microcontroller to
act as a divider. The micro could also generate the time code
modulation. A coil of wire would be the most effective antenna,
ideally with a capacitor across it so that it resonates at the right
frequency.
The clock that I disassembled also contained a 32768Hz watch crystal
which was used for the time-keeping most of the time. At the end of
each hour it would turn on the radio receiver and wait until it
received a time code with correct parity and then it would set the
time and turn off the receiver. Unfortunately, when receiving pure
interference it would have a 50% chance of receiving correct parity,
at which point it would set the time to some nonsensical time like
34:81, because they did not think to check whether the hours were less
than 24 or the minutes less than 60.
Whether or not building the proposed trnasmitter would be legal where
you are, it is unlikely to be noticed amongst all of the BPL hash, and
so as long as you don't go telling people about it, probably noone
will care. If you were to build the whole arrangement inside a metal
box with your watch, then that would quite likely be legal, and so for
the purpose of this discussion that is what I will assume you did.
I met an engineer who (whilst a student) built a time code transmitter
so configured that it would make all of the clocks at his university
run roughly backwards, to the amusement of all.
Chris
.
- References:
- Simple 60 Khz Transmitter Design?
- From: pollen
- Re: Simple 60 Khz Transmitter Design?
- From: John G
- Re: Simple 60 Khz Transmitter Design?
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- Simple 60 Khz Transmitter Design?
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