1 Hertz Clock Timer

[ormo]

Clock Timer

Hi,

I'm looking to make a small clock - I have everything planned except the timer circuit. I need to find two circuits: one that will send its output high every second and one that will send its output high every 60 seconds (I haven't decided whether or not to include seconds yet!).

I was thinking something with the NE555 timer but so far I cant find the right resistor/capacitor values to make the time period exactly 1 or 60 seconds.

Thanks!

 

[Hero999]

A 555 timer isn't accurate enough, use a quartz crystal oscillator and a divider.

 

[ormo]

The smallest crystals I can found are about 3 MHz, I need one that's 1 Hz (I'm including seconds). Any ideas?

 

[Hero999]

32768Hz is a common crystal, just put a divider after the crystal oscillator circuit.

 

[Diver300]

You can also use a divider of 50 or 60 from the mains frequency.

 

[atferrari]

Cmos

You can also use a divider of 50 or 60 from the mains frequency.

I implemented a divider with two CD4017 Johnson counters giving a 1 sec pulse. The signal induced in a small wire acting as an antenna in the input was taking it from the 50 Hz in the wiring nearby.

Perhaps too bulky for you.

Have you dismantled a table clcok? Lots of them come with small xtals and the divider already giving 1 Hz output.

 

[ormo]

How's about this - would it work?

1 x 32.768 kHz crystal
2 x CMOS 4040

I use the 2 CMOS's to divide by 256 and then by 128 to convert the 32,768 Hz signal into a 1 Hz signal.

Would this work and how accurate might it be?

 

[Noggin]

Without knowing what a 4040 is (and without looking it up) if you properly divide by those two numbers it should be as accurate as the crystal. Thats the general way to get a 1Hz frequency. Use a 32768 and divide it down.

As mentioned before, you can also use a 0 crossing from mains power and divide that down.

 

[kchriste]

How's about this - would it work?
1 x 32.768 kHz crystal
2 x CMOS 4040

You'd need an oscillator for the crystal. Use the 4060 for the first stage as it has a built in oscillator.

I use the 2 CMOS's to divide by 256 and then by 128 to convert the 32,768 Hz signal into a 1 Hz signal.
Would this work and how accurate might it be?

You've got the right idea. It would be as accurate as the crystal specifications once you've tuned it. 32,768 Hz crystals are used all the time in battery powered clocks.

 

[MrAl]

Hello,

The 32k crystal idea is a good idea i think, but they do make single chip
solutions to divide this down to 1Hz. This chip has all the counters in
that one chip.
If someone wants to look this up that would be great, or else i'll have to
check to find the part number.

 

[on1aag]

Hi Ormo,

It can also be built with only one integrated thing.

on1aag.

 

[ericgibbs]

Hello,

The 32k crystal idea is a good idea i think, but they do make single chip
solutions to divide this down to 1Hz. This chip has all the counters in
that one chip.
If someone wants to look this up that would be great, or else i'll have to
check to find the part number.

hi Al,
Are you thinking of the HEF4521 24 stage divider with built osc driver, using a 32768HZ xtal, gating for 60secs.?

hi on1aag:
I must be slowing down, you beat me this time.!

 

[ormo]

Thanks everyone for the awesome advice!!

I'm going to go with the CMOS 4060 & 4040 with the 32.768 kHz crystal purely because of the range of part available at my local supplier of these parts.

Thanks again!

 

[frosty]

Hi, can I use the4521 and the 32,768 hz, to make the 15min timer, on for 15 and of for 15 with a relay, if so how would I hook it up??????Thanks

 

[MrAl]

hi Al,
Are you thinking of the HEF4521 24 stage divider with built osc driver, using a 32768HZ xtal, gating for 60secs.?

Hi Eric,

I think so, it's been so long now since i looked at these circuits.
For one reference i used a crystal osc in DIP package connected
to a divider (large divide ability) and so i didnt have to play with
the oscillator section at all. Two chips basically.

Frosty:
What kind of accuracy do you need? I ask this because you
may not need a crystal controlled oscillator just an RC type
which is a little easier to do.

 

[ericgibbs]

Hi Eric,

I think so, it's been so long now since i looked at these circuits.
For one reference i used a crystal osc in DIP package connected
to a divider (large divide ability) and so i didnt have to play with
the oscillator section at all. Two chips basically.

Morning Al,
A little memory jogger for the HEF4521 osc stage.

 

[Boncuk]

You'd need an oscillator for the crystal. Use the 4060 for the first stage as it has a built in oscillator.

You've got the right idea. It would be as accurate as the crystal specifications once you've tuned it. 32,768 Hz crystals are used all the time in battery powered clocks.

Hi kchriste,

do you have any experience how much the crystal frequency can be changed using the cap values given in your example?

We have a member in a German forum who wants 720 pulses within a 24 hour time period. This would mean a frequency of 0.008333Hz, almost impossible with a any crystal and divider.

Having the crystal with the nominal frequency the achieved closest result would be 0.0078125Hz using a 14 bit binary counter and another one with Q8 as output.

Thank you

Hans

 

[blueroomelectronics]

We have a member in a German forum who wants 720 pulses within a 24 hour time period. This would mean a frequency of 0.008333Hz, almost impossible with a any crystal and divider.

And not too difficult with any microcontroller, I was using 74HCT161s and noticed in the TI datasheet they were discontinued by TI. More and more 74 & 40 series ICs are becoming scarce.

 

[Diver300]

We have a member in a German forum who wants 720 pulses within a 24 hour time period. This would mean a frequency of 0.008333Hz, almost impossible with a any crystal and divider.

It's easy. It is one pulse every 2 minutes or 120 seconds
32.768 kHz
Divide by 2^15 (32,768)
Divide by 60
Divide by 2

That can be done with standard ICs, but it is less wiring with a PIC

 

[kchriste]

Hi kchriste,
do you have any experience how much the crystal frequency can be changed using the cap values given in your example?

It's been a long time since I've used that chip. It really depends on the crystal type and cut. Overtone SC cut crystals can't be tuned very far off. AT cut crystals can be pulled further off than SC cut crystals.
It looks like Diver300 has the correct solution for you anyway.