Let's familiarize with a COB - chip on board device by building a precision 1Hz clock generator. Circuits that usually generate 1Hz clock makes use of ICs with variable capacitors but they are not very precise. ( micro controller based one should be precise but it costs large ). better accuracy can be obtained by a COB found inside a digital clock, which is cheaply available in the market.
|1Hz clock generator circuit diagram : Click on the image to enlarge|
The COB is a small pcb consist of an IC chip capacitors and quartz crystal, etc. Here's the some important points to note before you assemble the clock generator.
- R1, C3, diodes D1 and D2 shown in the circuit convert 5V DC into 1.4V DC which is the supply voltage to the COB.
- A ½Hz clock is available at terminals A and B with a phase difference of 90o, these pins are coupled using capacitors C1 and C2 to obtain a complete 1Hz clock.
- Op-amp IC CA3140 used to amplify the output of the COB to a considerable level.
- R4 and R3 is used to set the gain of the op-amp.
- T1 and T2 transistors further amplifies the clock output to drive the digital clocks and timers.
- Preset VR1 is offset null control used to adjust proper 1Hz pulse at the output terminal ‘E’.
- LED blinks every one second interval.