Resistor R1, capacitor C3, diodes D1 and D2 shown in the circuit convert 5V DC into 1.4V DC. A ½Hz clock is available at terminals A and B with a phase difference of 90o. The two outputs, are combined using capacitors C1 and C2 to obtain a complete 1Hz clock. This 1Hz clock pulse has a very low amplitude of the order of a few milli-volts which cannot be used to drive the digital circuits directly. This low-level voltage is amplified several times by op-amp IC CA3140.
Preset VR1 is offset null control used to adjust proper 1Hz pulse at the output terminal ‘E’. Connect one LED in series with 220-ohm resistor between the terminal ‘E’ and ground and adjust preset VR1 till the LED blinks once every second. When using the COB, affix the same on a general-purpose PCB using rubber based adhesive and solder the terminals neatly using thin single-strand wire.
Lab Note: The COBs used in different watches may differ some what in their configuration. But by trial-and error one can always find out the appropriate points corresponding to points A, B, C and D. Figure of a second COB used by EFY Lab is shown alongside. The points A and B (on the COB used by us) were observed to have complementary 1Hz outputs and hence anyone (only) could be used as input to opamp CA3140.