Clock Tester

Clock TesterWanting a device that can measure the swing of pendulum clocks accurately to quickly adjust a clock and to record the swinging of clocks over longer periods of time in order to produce a graphical representation of the clocks' accuracy, I've developed my own clock tester.

The tester is based on an arduino board, for which I have written the code. An LCD display shows the accuracy of the clock, and a phototransistor + LED serve as a capturing sensor.

The device has a compartment for 4 x AA batteries, and alternatively runs from any USB power source (laptop, phone charger, etc.)

The Clock Tester measures each double-swing in milliseconds (with microsecond accuracy) and shows the last swing duration, the average swing duration, the number of double-swings measured, and the accuracy factor.

The accuracy factor gives you an idea about the quality of your clock by showing you the maximum duration - the minimum duration divided by the average duration multiplied by 10000.

A good clock (e.g. my local church clock) has an accuracy factor of 2. My Junghans ATO clocks have about 7, and my rather average 1930's striking pendulum clock a factor > 100. Within 20 swings or so, the accuracy factor gives you a very good idea about the quality of the clock you are measuring. This is very useful when buying a clock.

The green button is the reset button (to start measuring), the switch is for switching the battery power on/off (if not connected via USB), and the 3.5mm plug socket is for the light sensor. There is also a USB connector to connect the device to either a laptop, desktop, or charger for power supply.


The first video shows the clock tester in action with our local church clock…

The second video shows you the clock tester hooked up to a 1950's Junghans Ato pendulum clock and a laptop.

You can see the sensor, which fits into any standard-size microphone holder, so you can use goosenecks, clamps, etc. which are readily available on eBay.

Verge & Foliot ClockYou can also see how the Clock Tester records the data as standard csv to the computer, so you can use the data in your favourite spreadsheet (Excel, LibreOffice, OpenOffice, etc.) to produce great graphics, such as the one shown here, which shows the accuracy of a Verge & Foliot Clock over a 7 hour interval.


Each clock tester needs to be calibrated individually as each quartz on the arduino boards is slightly different.

The calibration is done over a one-hour period by measuring the internal quartz of the arduino board and comparing it to a computer that is synced every 10 minutes via ntp to an atomic clock. With that, a correction factor is calculated and uploaded to the tester, which will then use the correction factor for accurate measuring.

Data Capture

The Clock Tester sends the measured data through the USB connection to any computer. You can either use a perl script that I supply, which creates a CSV file, or your own software to capture the data.

Light Sensor

Light SensorThe light sensor consists of an LED and a phototransistor. It comes with a cable and a 3.5mm plug that fits into the main unit. The light sensor fits into any standard microphone holder so that you can use standard microphone clamps, goosenecks, etc.


DisplayThe two-line, 16 x 2 character LED display shows the following:

  • top left: duration of last double-swing
  • bottom left: average duration of double-swings
  • top right: number of double-swings captured
  • bottom right: quality factor (maximum - minimum) / average * 10000

Sorry for the quality of the photo, but it appears to be hard to capture these back-lit LED displays on photo. The display in real life is bright and easily readable.

tester.txt · Last modified: 2011/05/10 15:19 by Christian Dannemann
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