|
Precision
1Hz Clock Generator using Chip-on-Board
|
Usually
the circuits for generation of 1Hz clock for applications
in digital clock and counter circuits make use of ICs
in conjunction with a crystal and trimmer capacitors,
etc. However, the same or better accuracy can be achieved
using a chip-on-board (COB) device found inside a digital
clock which is readily available in the market for Rs
15-20. This COB consists of IC, capacitors and quartz
crystal, etc which are mounted on its surface. It works
on 1.4 volt DC source. This COB can be used to derive
1Hz clock. 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. The op-amp CA3140 is connected in a non-inverting
mode, and its gain is set by resistors R4 and R3. Capacitor
C2 reduces the AC gain and unwanted stray pick-up and
thus improves stability of the circuit. The input impedance
of IC CA3140 is very high and thus there is no drop at
the input when 1Hz clock signal of low level is connected
across its input terminals from the COB. Amplified 1Hz
clock pulse is available at its output pin 6, which is
further amplified by transistors T1 and T2 to drive the
digital clocks and timers. 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 us-ed
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. |
|
|
Click on the Image for its larger version
|
