- Jan 1, 1970
Greg said:I've decided to build a BCD clock and after searching for a circuit
I've decided on Bill Bowden's circuit.
After examining the circuit, though, I'm curious about the circuit
giving a pulse every second. There are two NAND gates attached to the
4040 binary counter which then feed into an OR gate. Am I correct in
assuming that they should be feeding into AND gate to get a pulse
every second? As the circuit stands, I would think it's going to give
a pulse on 0.8 sec and on 0.2 sec, or two pulses per second.
I'm fairly weak on logic circuits so forgive the stupid question.
I'll take a shot.
The CD4040 is a binary counter that resets to all zeros out
with Q1 being the 1 bit, Q2 being the 2 bit, Q3 being the 4
The bottom NAND outputs a low when both inputs are high, and
these inputs are the 4 and 8 bits.
The upper NAND gate outputs a low when its two inputs are
high, and these bits are the 16 and 32 bits.
So starting at a reset, the first count that produces a low
from both gates at the same time happens at count
32+16+8+4=60. This low state is slightly delayed by an RC
filter and inverted to a high by the inverter. This rise
from low to high clocks the second counter every 60th line
cycle and also resets the counter back to zero before the
61st cycle occurs, so when it does, it is counted as 1 of
the next 60.
The duration of the second pulse is set by the time delay
between when the reset pulse is sent to the counter and how
long it takes for the disappearance of the 60 decode passing
through the RC filter. That is roughly a millisecond.