Yeah well, for AC mains 50/60Hz is just a guideline. So I bet this clock will differ in time within a matter of a few day, possibly few hours. If the feq is off by 1/2Hz, it would become off by 12min a day.

@David Millions of synchronous motor driven mechanical clocks disagree with you.

Building a clock using logic chips and line frequency was a standard procedure in computer lab in 1986.

@TRB,
An RC oscillator will be far less accurate than line frequency. It would be sensitive to temperature changes (as well as the zodiac).

About 18 years ago, there was a group of people on Usenet that were tweaking their digital watches for accuracy. A digital watch typically has one adjustment, a capacitor which can slightly influence the crystal time base. Some were wrapping wire around the watch to read the Xtal frequency with a frequency counter and atomic time base and would adjust the capacitor for a “better” second. Some even left their watches on their wrists while they slept to stabilize the watch temperature, i.e. using their bodies as an Xtal “oven”.

Actually, the grid frequency is maintained long-term so that clocks are accurate.

http://en.wikipedia.org/wiki/Utility_frequency#Long-term_stability_and_clock_synchronization

@Leigh Hahah thanks, amazing what happens when you go out and look for the answer instead of assuming you must already know it…

I am working on a project using ac line frequency as a timebase. It is corrected over the long term so it is useful for clocks and suck. The short term error can be quite high however.

Here is a beginner project to get at the 50/60Hz signal (and later they even use the same 4017/4022 chips for a one-second reference):

http://www.play-hookey.com/digital/experiments/line_clock.html

I’m certain that I’m much older than most readers of this blog and it’s amusing to me that those who have grown up with microprocessors or micro-controllers have difficulty with what once was first year digital logic in school. On the other hand I all too frequently get stuck on Arduino issues. It’s no surprise that when people collaborate to find a solution, it’s generally very effective.

With regard to the accuracy of using the AC line as a time base, I can’t speak for other countries, but in the US all utility companies count the number of cycles they output and they tweak generator speed up or down as required to keep the cycle count exactly correct over a nominal amount of time.

@David

Short-term average power line freq stability is OK. Long-term average power line freq stability is excellent.

From wiki:

“Regulation of power system frequency for timekeeping accuracy was not commonplace until after 1926 and the invention of the electric clock driven by a synchronous motor. Network operators will regulate the daily average frequency so that clocks stay within a few seconds of correct time. In practice the nominal frequency is raised or lowered by a specific percentage to maintain synchronization. Over the course of a day, the average frequency is maintained at the nominal value within a few hundred parts per million.[17] In the synchronous grid of Continental Europe, the deviation between network phase time and UTC is calculated at 08:00 each day in a control center in Switzerland, and the target frequency is then adjusted by up to ±0.02% from 50 Hz as needed, to ensure a long-term frequency average of exactly 24×3600×50 cycles per day is maintained.[18] In North America, whenever the error exceeds 10 seconds for the east, 3 seconds for Texas, or 2 seconds for the west, a correction of ±0.02 Hz (0.033%) is applied. Time error corrections start and end either on the hour or on the half hour.[19][20] A dynamicdemand.co.uk/grid – Real-time frequency meter for power generation in the United Kingdom is available online. Smaller power systems may not maintain frequency with the same degree of accuracy.”

The only problem with the mains time is that if you don’t filter the signal correctly you might count additional spikes that come from switching appliances and thus make your clock fast.

Of course, your clock is stuck (or worse, reset) when you loose power.

In some designs, there is a battery and an R/C onboard oscillator (it doesn’t need to be very accurate) to keep the time (not the display) during power outages.

It would be fun to see that implemented in pure logic.

My favourite is still the neon clock: http://www.vf.utwente.nl/~ptdeboer/ham/neonclock/

@everyone – Wow, I was wrong (spoke before doing research). After some reading, the frequency does fluctuate but is averaged over one minute to be pretty well on the money. Very surprised to hear that much goes into the phasing of mains power. I tended to think mains are a crude/rough “good enough” power source, saying anything sensitive tends to go through power conditioning. Thanks for the correction/info!

-and knowing is half the battle.

So, the short-term variance of the electric grid might not be that great, but over the longer term (farther along the X axis), it is managed, so it would have a smaller variance.

@S2H, Simulations? Why would you use a simulator for such basic digital logic? As fopr the analog section a diode, transistor and a cap is all I have needed to make a 5 volt squarewave out of a 12V sine wave.

You young folk sure do need your hands held tight when it comes to this stuff.

Seriously guys.. You make not think you can hire an EE for crap pay, but have you looked into hiring EE students?

In other words, please send Mike Szc… back to the world where windows solitaire is the most complex item one needs to understand!

IsotopeJ:
> When the breaker was thrown, the grid would actually PULL the 1200 megawatt generator into phase.

Wow, that’s not something you’d think could be done, but I guess it’s obvious when you think about it. Do you have to “idle” the turbines and generators to do that, or do you just end up with them delivering a bit less electricity to the grid before they’re in sync?

@magetoo
>Do you have to “idle” the turbines and generators to do that, or do you just end up with them delivering a bit less electricity to the grid before they’re in sync?

There is a single generator driven by a massive turbine. The turbine/generator would be spun up first. IIRC: The generator is spun slightly faster than the grid frequency. There is an analog dial right on the outside of the generator housing that spins in relation to the difference in phase between the generator and grid. When closing the breaker, the operator had to look at this dial and flip the switch as the needle approached a mark that indicated phase-sync. Failing to do so would result in the generator and turbine immediately moving in phase with the entire grid – meaning goodbye turbine building. If you think that’s crude, you’d be surprised – a lot of things that happen at nuclear power plants are purposefully low tech.

@S2H: that’s multisim 10, it is simulating perfectly, proteus is also a good software.
@Adventure in: D3 is there along with C1 making a half wave rectifier. the cathode of D3 is not connected to GND.

the line frequency is very stable, i did some research before building this, and maybe it shifted 200mS during the night, and -200mS during the day. it was plugged all day for the last year, and it didn’t shifted at all, at least not notoriously.

@Lucassiglo21: Gracias amigo. I am using multisim 10 and it tends to give me a lot of convergence errors even when simulating some very basic circuits, so it has hindered me from designing anything large-scale. Any advice? I will also take a look at Proteus.

@croftj: While I agree that the components used in this circuit are by themselves quite basic, would you go ahead and design a PCB for this device without drawing up a schematic and simulating/breadboarding? Let me know if you need me to hold your hand…

most of times multisim works just fine, and when the convergence thing happens, there is some kind of convergence problem solver that shows and solves the problem automagicly(included in version 10).

in my experience, proteus is much better for microcontroled systems and digital/mixed designs. and the pcb layout software (proteus ares) is very nice, i use it a lot.

is you want full analog analysis with complex things, multisim works better.

that’s just my opinion.

@S2H

I’ve used Multisim for years and haven’t hit a convergence error yet. What kind of simulation are you doing? DC operating point? Transient analysis? Fourier analysis? AC analysis? Don’t bother with simulating HP function generators and scopes. Simulated equipment sucks. Also, make sure to have all your sources setup correctly.

Here’s an example of a simple amplifier circuit which should have about 20dB of gain with a maximum output power of 2 watts from roughly 20-20000Hz. If you enter this exactly how it is, and you can’t get a simulation working, well, I suppose there could be something wrong with your copy of Multisim.
http://dl.dropbox.com/u/449518/amplifier2.png

@natrix: Thanks for the suggestions. I was getting errors when doing a dead-simple simulation of a comparator circuit (none of the fancy analysis stuff, just pressing the “play” button and sticking probes in). I was able to repeatably cause an error by just sticking a probe at one of my op-amps inputs.

What do you mean by having my sources set up correctly? In the case of the comparator, the only source I had was a battery going through a voltage divider (if a first year electrical student could do the math in their head, then surely the software could too?!).

Anyone doing such a design try to be sure to internationalise it if they’re putting it only and build in a small DIP switch for 50/60Hz selection.

@magetoo

Following up what Isotope* wrote. My former supervisor worked with a transportable weather radar. When a storm approached, they switched to standby power. He started the standby generator and watched a panel of three lights. Each light was connected between one leg of the mains and standby generator (3 phase). When all three lights dimmed out, (indicating the phase difference between the mains and standby was not enough the light the bulbs) he threw the transfer switch.

i know it’s not a big deal, but the clock is nice. Today is not a standard thing to do something like this without a uC.

so it’s accurate until the fourth digit in a couple of days. so the error is aprox 0.0009%, a crystal oscillator is 30 PPM i think, wich is an error of 0.003%. Sooo, the line is more accurate in a couple of days.

if i did the math wrong please correct me

@magetoo
>Do you have to “idle” the turbines and generators to do that, or do you just end up with them delivering a bit less electricity to the grid before they’re in sync?

Inre: “idle”, Even when a power plant is shutdown for maintenance/repairs they like to keep the generator/turbine rotating (maybe this only applies to a horizontal shaft) to keep the shaft from “settling” or distorting, which can happen if the plant is idle for an appreciable length of time (weeks?)

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