A 1Hz clock generator circuit with COB

This is a 1Hz clock generator circuit with a chip on board (COB) . Typically, circuits that generate 1Hz clocks for digital clock and counter circuit applications use an IC in conjunction with a crystal and trimmer capacitors , etc. Even so, equal or better accuracy can be obtained using a chip on board (COB) device found within digital clocks that are readily available in the market. This COB includes ICs, capacitors , and quartz crystals, etc. mounted on its surface . It is powered by a 1.4 volt DC power supply . This COB can be used to obtain a 1Hz clock.

The resistor R1, capacitor C3, diodes D1 and D2 shown in the circuit change 5V DC to 1.4VDC. A Hz clock is accessible at terminals A and B with a phase change of 90o. The two outputs are put together using capacitors C1 and C2 to get a full 1Hz clock. This 1Hz clock pulse has a very low amplitude of the order of a few millivolts and cannot be used to drive digital circuits immediately . This low-level voltage is amplified several times by the operational amplifier (op-amp) ICCA3140.

The operational amplifier CA3140 is connected in non-inverting mode with its gain set by resistors R4 and R3. Capacitor C2 minimizes AC gain and unwanted spurious pickup, thus improving the stability of the circuit.

The input impedance of ICCA3140 is very high, so when a 1Hz low level clock signal is connected from the COB to its input, there is absolutely no drop at the input. The amplified 1Hz clock pulse is accessible at its output pin 6, which is further amplified by transistors T1 and T2 to drive the digital clock and timer .

Preset VR 1 is the offset zero control and is used to adjust the precise 1Hz pulse at the output terminal "E". Connect an LED with a 220 ohm resistor between terminal "E" and ground and change preset VR1 until the LED flashes once per second. When using a COB, stick it on a general purpose PCB using a rubber based adhesive and neatly solder the terminals using thin single strand wire.

Note: The settings of the COB used by different watches may vary. But through trial and error, we can always find the correct points corresponding to points A, B, C and D.