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DiscoverCircuits.com -- Hobby Corner
"Hobby Circuits for your
Hobby Projects"
Last Updated on:
06/19/2009 03:17:17 PM
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Poor Man's Timer
Aug 24, 2008
designed by David A. Johnson, P.E. |
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Often during testing of certain
equipment and components, you would like to keep track of the elapsed time in hours,
minutes and seconds. There are some nice commercial digital timers and counters
available for just this sort of application. However, although it may not be as
accurate, there is a cheaper way to go. I’m sure many of you have some old wrist
watches or battery powered dial faced alarm clocks collecting dust in some drawer.
With just a bit of effort you can turn these into value pieces of test equipment.
You must use the analog clocks and watches with dial faces and not digital types, since
the analog devices retain the elapsed time information, even after the power is
interrupted. With a wrist watch that has a day/date feature, you can time things
up to 31 days, to an accuracy of a few seconds. |
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Most wrist watches are powered by 1.5v
button cells while many dial face alarm clocks are powered by 1.5v AA or AAA cells.
These timing devices draw very little current and can easily be wired into an
electronic circuit. The circuit might be connected to a microprocessor or some
sensing device. When power is applied to the clock, the elapsed time is recorded.
When power is turned off, the elapsed time information is retained. |
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To convert these devices into a timer,
first open the back of the watch or the battery compartment of the clock. Remove
the battery. File a 1/8 inch wide by 1/16 inch deep notch into the metal of the
watch case or the plastic of the battery compartment lid. Next, solder two wires
onto the battery contacts. Use a thin red wire, about 12 inches long, for the
positive side and a darker color, such as black, for the negative terminal. If you
have difficulty applying solder the clock’s battery contacts, you can first press some
adhesive copper tape onto the contacts, then solder the wires onto the tape. Route
the two wires through the notch. Then close up the back plate or compartment. That
is it. You are now are ready to time something. |
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Watch Face |
Clock Face |
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Watch Back |
Clock Back, Battery Removed |
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Notch Filed in Watch Case |
Wires Soldered to Battery Contacts |
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Wires Soldered to Battery Contacts
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Wires Routed Through Notch |
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A simple voltage
divider circuit can be used to convert the +5v logic signal from a computer or a logic
device into a 1.5v signal, to power the clock. I suggest a capacitor be included,
to provide the needed peak current pulse needed by the clock’s solenoid. |
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What might you use
this thing for? One application might be to time how long a door remains open
during the day or week. Maybe you could use it to time how long a battery can
power a device or maybe how many hours a computer is being used per month. I will
later show how this simple device can be used to measure the milliamp-hour capacity of a
button or coin cell battery. |
