Circuit - Poor Man's Timer

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Last Updated on: 10/01/2011 08:13:09 AM

Poor Man's Timer Aug 24, 2008
designed by David A. Johnson, P.E.

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.

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.

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.

Watch Face

Clock Face

Watch Back

Clock Back, Battery Removed

Notch Filed in Watch Case

Wires Soldered to Battery Contacts

Wires Soldered to Battery Contacts

Wires Routed Through Notch

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.

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.