The Quartz Watch Circuit

quartz watch circuit

Written By Alex Hamilton

Alex Hamilton is a watchmaker, collector of fine watches and writer of all topics in the study of Horology.

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In the previous article, The Quartz Crystal Oscillator, we saw how the electronic circuit in a quartz watch maintains the quartz crystal in oscillation. Most watch crystals vibrate at 32 768 Hz. A quartz watch mechanism with a second hand needs an electrical pulse once every second to operate the stepper motor which drives the train.
quartz watch circuit board

It turns out to be very simple to divide the frequency of an electrical signal by two, using just a few electronic components. The quartz watch circuit has a chain of such dividers, each halving the frequency, until we arrive at the required one pulse per second.

Quartz watches without second hands rarely operate their stepper motor once per second. By stepping less frequently once every 10,15, 20 or 30 seconds, they can reduce the drain on the battery. In turn, this allows a smaller battery to be used, leading to a reduction in overall size for the movement.

The dividers are all embodied in a tiny integrated circuit which is mounted on a circuit board, along with the quartz crystal and a number of other electrical connections.

You could treat the circuit like a “black box”: that is, it either works properly or it does not. The curcuit board cannot be serviced or repaired, but on most movements can easily be replaced. The picture above shows a couple of quartz watch circuits; the black arrows indicate the quartz crystals.

Inhibit or (grounding switch)

Almost all quartz analogue watches with second hands have a stop switch feature. When you pull the crown out to the time setting position, the train of wheels stops. This allows the user to stop the watch with the second hand at 12:00 o’clock, adjust the time and then start the watch at exactly the right moment. This allows the watch to be set to the correct time within one second.

The setting mechanism operates a switch, which causes the integrated circuit to stop sending pulses to the stepper motor. The switch consists of a contact on the circuit board and a lever connected to the main plate of the movement, (and thus the positive terminal of the battery). When you need to stop the watch, you pull the crown out, the lever comes into contact with this contact on the circuit board, connecting it to the main plate and signaling the integrated circuit to stop.

Setting the Rate

Quartz watch crystals are not perfectly accurate. Manufactures will typically specify the frequency as being accurate two within 20 parts per million (written as ± 20 ppm). That is, a 32,768 Hz crystal may operate anywhere in the range of 32,767.3 Hz to 32,768.7 Hz. Uncorrected this amounts to a time keeping error of approaching 2 seconds per day either gaining or losing time.
quartz watch block diagram

quartz trimmer

Older (20th century) quartz watches had an extra component on the circuit board called a trimmer. This would fine tune the frequency of the quartz crystal so it could be brought to a very accurate rate. The trimmer can be recognized by its characteristic slotted adjuster.

Trimmers have two disadvantages, the main one being that they add considerably to the cost of the circuit. The second disadvantage is that they add another source of unreliability to the watch.

Therefore modern watches no longer use a trimmer. Instead they use a system called inhibition.

Inhibition

Manufactures make the quartz crystal to operate slightly faster than 32,768 Hz, so that the watch will gain, even if the crystal is right at the lower end of its ± 20 PPM frequency tolerance.

The integrated circuit is then set to drop -or inhibit-one or more pulses from the quartz crystal at regular intervals. By measuring the actual rate during  manufacture and adjusting the number of pulses dropped, they factory can finely adjust the net rate. Time keeping within a second per week is not unusual for a good quartz movement that uses inhibition.

However, this approach has a strange effect that you must be aware of. The watch runs slightly fast for several seconds, and then pauses briefly before continuing. The irregularity in the motion of the second hand is far too small for a human to see, but it is obvious when the watch is on a timing machine.

Therefore, you must be aware of this and ensure you use the correct measurement interval. The movement manufacturers documentation will tell you the inhibition interval for that caliber, and quartz timing machines let you set the measurement interval accordingly.

We will talk more about adjusting the rate, if possible, when we service a quartz watch in our upcoming series.

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