What types of circuits are used in digital watches?

[Chrisau]

Hi,

I would like to recreate the circuitry of a standard digital watch. One which simply shows the time, and also allows the user to set an alarm (single-instance or repeating). The alarm output would energize a vibration motor to make the watch vibrate.

I am currently researching trying to learn what circuits are involved in such a device and so far I have pieced together information about;

• Crystal oscillators to provide the pulse signal
• Truncated ripple counters to take the pulse signal and produce a binary output
• The binary output is used to feed the seven-segment displays
• A set-reset-latch circuit is required in connection with the ripple counters to achieve a programmable (timed) output to drive the vibration motor

This is all new to me, but I feel I can learn quickly with the right information. What I wanted to ask, before I go too far in a possibly wrong direction, is am I on the right track? Can anybody point me in the right direction (websites/literature/videos/kits/diagrams/ideas etc.) in finding information that will help me understand and reproduce the circuits in such a device?

Any input is much appreciated. Thanks in advance.

 

[Nigel Goodwin]

You do it with a micro-controller - one chip, one 32KHz crystal - plus what ever display and buttons you want.

 

[KeepItSimpleStupid]

Some people like to build a clock from scratch - nothing wrong with that. A watch is quite different. You could also try to simulate the design instead of building it.

I'll comment on clocks in general.

Crystal oscillators to provide the pulse signal

Resonators, crystals and the line frequency are options with options to switch to a crystal time base when power goes away. 50/60 Hz selection if applicable. One other option for a clock is to use WWV, a radio signal. Others have use the Internet NTP (Network time protocal) to get the time. 32768 kHz is popular because it's easily divisible to 1 pps and low power.

Truncated ripple counters to take the pulse signal and produce a binary output

Actually they are modulo BCD counters, not binary. Two modulo 60 counters for minutes and seconds. A modulo 24 counter for hour if you choose that display method.. The 12/01 and AM/PM indicators are a bit harder to do.

BCD is Binary Coded Decimal, Each digit is binary encoded in 4 bits.

The binary output is used to feed the seven-segment displays

The output is BCD and is sometimes multiplexed. Nixie, vacuum florescent (VFD), LED and LCD displays are possible. Brightness control is often employed.

A set-reset-latch circuit is required in connection with the ripple counters to achieve a programmable (timed) output to drive the vibration motor

Or a comparator and timer.

There was a digital clock chip made by National, I think the 53xx (11-14) that basically used a 50/60 cycle select, 12/24 hour select, multiplexed displays, fast/slow, and hold/run. I built one of these as part of an explorer's post at Hewlett-Packard. I think our groups was the second complete and the first to work. We did the PCB routing at 4X using tape. The components were already placed for us. I still have that clock, No alarm. No brightness control.

I have a vacuum florescent timer for a clock now, The 120 VAC output runs a piezo buzzer that's loud enough to wake me. I have a dip in my hearing at 2600 Hz due to antibiotic use.

My watch uses 4 buttons that run the watch and backlight. You can set the individual digits. It includes a calendar and timer.

I had another watch that used a "stem" which could be pulled out, but it also acted like an encoder, It could rotate left, right and be pushed as well as pulled out. Setting were made pulled out. There was also one small pushbutton.

Nearly all clocks allow setting to the second and that was the purpose of hold/run. Others set the seconds to zero when you exit setup mode.

Dallas/Maxim make a lot of clock calendar parts, thus your programming skills could be put to use on other parts of the project.

For those that don;t want to use chips, there is this https://makezine.com/2008/06/18/transistor-clock-kit-uses/ kit.

I kinda wanted an alarm that was set with BCD switches way back when.

Setting of the digits got better because they didn't cause the counters to overflow. My car clock uses two buttons, hrs and minutes. When pushing minutes, hours won't advance.

I've been thinking about a UPS for my room for a small lamp, alarm. laptop and light just as my elderly mom has.

EDIT: BCD, NTP added.

 

[dr pepper]

Watches tend to be a custome chip with all the functions on board, however you could make your own circuit, getting it to watch size isnt going to be easy, nor getting power consumption tha low, but if you want to learn the technology start with something a little bigger.

Takea look at this instructable, for a watch circuit you'd use a lcd instead of a led, that would mean even more circuitry:

https://www.instructables.com/id/24-Hr-Digital-Clock-only-with-basic-CMOS-Chips/

A microcontroller would be a much better way and maybe possible for the home constructor.

P.S. welcome to the forum.

 

[Chrisau]

Thanks guys.

I have done some further research from your suggestions and it seems that what I am chasing is a Programmable Interval Controller, with an LCD display. I have been looking at the PIC range of microcontrollers from MicroChip.

Can anyone tell me if it is possible to set up one of these series chips in repeating alarm mode, instead of just one shot? I am having troubles understanding the terminology in the datasheets, still all new to me.

Here is a datasheet example from a chip in the series; **broken link removed**

 

[Nigel Goodwin]

Thanks guys.

I have done some further research from your suggestions and it seems that what I am chasing is a Programmable Interval Controller, with an LCD display. I have been looking at the PIC range of microcontrollers from MicroChip.

Can anyone tell me if it is possible to set up one of these series chips in repeating alarm mode, instead of just one shot? I am having troubles understanding the terminology in the datasheets, still all new to me.

Here is a datasheet example from a chip in the series; **broken link removed**

You can program it to do whatever you want, it's essentially a small fully integrated computer.

 

[Chrisau]

Thanks Nigel! Will get a hold of some parts and get started

 

[dr pepper]

The pic16f628A has some timkeeping code in the datasheet.
There are various projects online, I suggest you replicate one of these.
Hardware is easy, software demands more knowledge.

 

[Pommie]

The pic16f628A has some timkeeping code in the datasheet.

The clock example code is in the 16F88 datasheet.

Mike.

 

[Chrisau]

Ah thanks guys. What page is it on, I will download the datasheet soon and have a look at it.

Cheers

 

[dr pepper]

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