Dollar Watch: Timer - LCD Display

© Brooke Clarke 2009
Background
Operation
Mods
Related
Links
 Dollar Watch LCD Display Dollar Watch - Timer - LCD Display
Dollar Watch PCB & LCD glass

Background

It would be handy for many PIC micro controller circuits to have a small LCD display that would have a very low power requirement.  Trying to find these displays has been a very difficult search.  But I found these watchs at a local Dollar Store for, you guessed it, One Dollar each.

Operation

Documented

The display defaults to displaying the hour and minute with the colon blinking (by chance the camera took the photo above when it was off).

Pressing B1 (Mode) cycles the display through hh:mm -> Month Day -> :ss -> hh:mm.

Pressing B2 one time causes just the month to be displayed.  It can be incremented by pressing B1.
Pressing B2 a second time causes just the Day of the Month to be displayed. It can be incremented by pressing B1.
Pressing B2 a third time causes just the hour and either A or P to be displayed.  It can be incremented by pressing B1.
Pressing B2 a forth time causes just the minute to be displayed.  It can be incremented by pressing B1.
Pressing B2 a fifth time causes just the seconds to be displayed.  It can be incremented by pressing B1.

Undocumented

There is no provision on the watch case to operate Button 3.  When the display is hh:mm and B3 is pressed once the display alternates between:
hh:mm and mm  dd.  i.e. between the time and date.  Repeated presses of B3 then act like repeated pressing of B2 to allow setting.

The movement has notches that would support buttons B4, B5 and B6 but they have not been installed on this cheap movement.  The case has what appear to be buttons B5 and B6 but these are just for looks i.e. they are non functional.

Battery

The watch is powered by a single LR41 a 1.5 Volt button cell about 8 mm diameter.

Power Up & Down

If the battery is removed and then installed the watch starts running at 12:00 and Dec. first (12  1).
When the battery is removed the LCD goes blank.

Stopping Clock - Not

Pressing and holding the mode button changes the display from hh:mm to alternating hh:mm and mm  dd but does not stop the clock.

Mods

Timer

To use the watch movement as a timer the battery can be removed and the battery terminals connected to a pair of wires that will feed the watch 1.5 volts to start the timer.  The problem is how to stop the timer.  For example if the crystal is shorted out the LCD goes blank.  When the crystal is shorted the LCD goes blank, BUT when the short is removed the LCD resumes the prior count.  This means the cheap watch CAN be used as a timer good for up to one year.

For a timer that will be good for up to 24 hours, many days if you manually keep track of the days, you can use an analog quartz watch.  I found one for $8 at the local Big Lots store.  That's eight times more than the price of the digital watch that included time and date functions.  If that was built into a product there's a large impact on the selling price.

LCD Display

The LCD might be removed from the rest of the movement.  More needs to be done to understand how to drive the LCD.

There are 13 contacts on the PCB which connect to the glass using a zebra strip.
Contacts No. 1 and No. 13 both have a signal that has three levels: +150 mv, 0, -150 mv.  The time between transitions is about 100 uS.
All the other contacts (No. 2 through No. 12) have signals that switch between +150 mv and -150 mv.

The LCD glass is specifically made to be a watch 12 hour time display.  This means the tens of hours digit can only be "1" or blank.   Notice that the "1" in the time display above is right aginst the left edge of the glass, i.e. there is not room for more segments like would be needed to display a "2". In a similar way the tens of minutes digit can only be blank, 1, 2, 3, 4 or 5.  This reduces the number of segments that need to be controlled to 22 which matches the use of two control and 11 segment contacts.

Related

Quartz Clock
Real Chronograph Pocket Watch
Time & Frequency

Links

Back to Brooke's Products for SaleTime & Frequency, Personal Home, PRC68 Home personal home, page
hits sinceNov 10 2009