GPS-Disciplined Oscillator

GPSDO Drives Electro-Mechanical (Analog) Clock

Circuit converts unipolar GPSDO output to bipolar clock motor drive signal.

Max Carter

A GPS-disciplined oscillator (GPSDO) provides precisely timed 100 mS unipolar (3.3V) pulses at one-second intervals (1 pps), synchronised to Universal Time (UT). A GPSDO can be had for around $100 from eBay.

The pulses can be used to synchronize an ordinary $10 battery-powered (minus the battery) clock motor (1 pps, non-continuous sweep). This requires that the pulses be converted to a bipolar drive signal. The drive circuit is shown in Figure 2.

Figure 1

Figure 2

The input pulses from the GPSDO drive a CMOS CD4027 dual flip-flop. One section (1/2) is wired as a one-shot multivibrator, the other section (2/2) as a 'polarity alternator'. The output from the one-shot drives a PNP pass transistor. The outputs from the polarity alternator drive a conventional H-bridge circuit made of complementary (PNP/NPN) transistor pairs. Series resister R limits the voltage across the motor winding to 1.5V. The value of R is determined by multiplying the winding resistance by 7/3 (2.33). The 1k resistor and 1 µF capacitor dampen high-frequency ringing in the motor winding.

The circuit requires positive DC power in the range of 7-35 volts. It can be powered from the same source (wall wart) that powers the GPSDO.

As Built

Clock Modification - Connecting to the Motor Winding

  • Disassemble the clock to gain access to the solder pads on the circuit board where the motor winding is connected. Be sure to note the order in which the wheels (gears) are removed.
  • Using a utility knife or small hand grinder, cut one of the traces on the circuit board so as to isolate the winding from the on-board circuitry.
  • Tack solder a length of AWG 28 (.08mm2) two-conductor wire to the pads.
  • Route the wire to the outside of the motor case.
  • Reassemble the clock. (It's not necessary to reinstall the battery.)
  • Reinstall all hands pointing at 12. Verify the hands are parallel and not touching. Do not reinstall the plastic face cover (if there is one).


Initialization is done manually. Go to Official U.S. Time or other UT source. With the clock running, move the minute and/or second hand as necessary until the clock displays the correct time. Do not force the hour hand. The clock will continue to tell the correct time unless power is interrupted.


The GPSDO is on the left. The converter/driver is in the white plastic box on the right.


Other Available GPSDO Outputs

A precise 10 MHz frequency reference output is also provided, useful for frequency calibration, along with a serial RS232 9600 BPS NMEA signal for use with GPS geolocation software. Neither of these outputs are used in the above clock application but are always available.

Notes regarding RS232 output
  • The serial interface on this GPSDO is a 3.5mm tip/ring/sleeve audio jack on the rear panel.

    Use an adapter like the one shown below (from Amazon):

    DE9 9 Pin Female to 3.5mm Male Plug

    Or roll your own:

    Figure 3

  • Serial (RS232) COM ports have disappeared from desktop and laptop/notebook computers, but a serial COM port is easily provided using a serial adapter. The cable from the adapter plugs into a USB port on the computer. The adapter shows up in Device Manager as a COM port, typically COM5. This one is from Amazon.

    USB to Serial Adapter

    Figure 4

  • Some GPSDOs are equipped with a USB interface, eliminating the need for the serial adapter. These will also show up as a COM port in Device Manager, probably COM5.
  • Data can be checked with a terminal program such as Termite. Settings: COM5 (or whatever), 9600 BPS, 8N2, no handshake. Disconnect the terminal program before starting your geolocation program.

Related Links

A GPSDO can be had for around $100 from eBay

Official U.S. Time

Geolocation/Time Sync Software

DE9 9 Pin Female to 3.5mm Male Plug

USB to serial adapter


Schematics produced with DCCAD.

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