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Introduction
History: how light learned to count people
Lesson 0. Gates from transistors: for those who start from scratch
0.1. The idea: a transistor is a switch
0.2. Building
0.3. Check
Lesson 0 paste (Cmd+I)
Lesson 1. The beam and CD4011
1.1. Idea: how a shop counts people
1.2. Idea: the CMOS 4000 series
1.3. Idea: NAND for everything
1.4. Build
1.5. Check
Lesson 1 paste (Cmd+I)
Lesson 2. The F/F flip-flop
2.1. Idea: a counter is flip-flops that flip
2.2. Idea: flipping through exclusive OR
2.3. Idea: a NAND with three inputs
2.4. Build
2.5. Check
Lesson 2 paste (Cmd+I)
Lesson 3. The CD4029B counter
3.1. Idea: which bit to flip
3.2. Idea: controlling the counter
3.3. Idea: the bubbles on the TI drawing
3.4. Build
3.5. Check
Lesson 3 paste (Cmd+I)
Lesson 4. The CD4511B decoder
4.1. Idea: seven strokes
4.2. Idea: latch, blanking, lamp test
4.3. Idea: a switch on two transistors
4.4. Build
4.5. Check
Lesson 4 paste (Cmd+I)
Lesson 5. Direction
5.1. Idea: two beams one after another
5.2. Idea: remember the first
5.3. Idea: the clock is when both are blocked
5.4. Build
5.5. Check
Lesson 5 paste (Cmd+I)
Lesson 6. The visitor counter
6.1. Idea: two displays on the same beams
6.2. Idea: two digits — two counters
6.3. Idea: Reset holds
6.4. Build
6.5. Check
Lesson 6 paste (Cmd+I)
Summary
All the parts
Instructions in pictures
Lesson 2 in pictures
← Back to the lesson: Lesson 2. The F/F flip-flop
2
The F/F flip-flop
load, flip, hold
kh
HT-04
F/F
Q
Q
Q
Q
IN
7×
OUT
3×
P
3×
N
3×
NOT
4×
0
NAND
2×
0
XOR
1×
0
0
3×
Hz
1×
3×
∞
Ctrl
+
G
NAND3
FF4029
© 2026 Daniil Khanin · khanin.info · HT-04
15
CANVAS
1
Delete
VCC
E1
R1
BA
Reset
Reset
RST
XA
SEEN
2
Delete
CD4011
A
BA
B
BA
C
RST
D
RST
E
BA
F
SEENN
G
nRST
H
SEEN
J
XA
K
nRST
L
SEEN
M
SEENN
© 2026 Daniil Khanin · khanin.info · HT-04
16
NAND3
3
IN
3×
OUT
1×
P
3×
9×
A
B
C
Y
Y
VCC
G
P
A
Y
VCC
G
P
B
Y
VCC
G
P
C
4
N
3×
9×
Y
m1
G
N
A
m1
m2
G
N
B
m2
GND
G
N
C
5
Ctrl
+
G
Library
…
Chip N
2×
Library
…
NAND3
Enter
NAND3
A
B
C
Y
© 2026 Daniil Khanin · khanin.info · HT-04
17
FF4029
6
IN
4×
OUT
2×
NOT
3×
0
6×
PE
J
TE
CL
Q
QN
NOT
I0
CL
O0
cp
NOT
I0
PE
O0
ld
NOT
I0
J
O0
jn
7
NAND
2×
0
XOR
1×
0
9×
NAND
I0
ld
I1
jn
O0
sdn
NAND
I0
ld
I1
J
O0
rdn
XOR
I0
Q
I1
TE
O0
x
© 2026 Daniil Khanin · khanin.info · HT-04
18
FF4029 …
8
NOT
1×
0
2×
NOT
I0
x
O0
d
9
NAND3
3×
12×
NAND3
A
sdn
B
n4
C
n2
Y
n1
NAND3
A
n1
B
rdn
C
cp
Y
n2
NAND3
A
n2
B
cp
C
n4
Y
n3
10
NAND3
3×
12×
NAND3
A
n3
B
rdn
C
d
Y
n4
NAND3
A
sdn
B
n2
C
QN
Y
Q
NAND3
A
Q
B
n3
C
rdn
Y
QN
© 2026 Daniil Khanin · khanin.info · HT-04
19
FF4029 …
CANVAS
11
Ctrl
+
G
Library
…
Chip N
2×
Library
…
FF4029
Enter
FF4029
PE
J
TE
CL
Q
QN
12
1
1Hz
2Hz
10Hz
60Hz
4kHz
…
MAX
13
0
3×
Hz
1×
4×
0
PE
0
J
0
TE
1Hz
CL
14
FF4029
1×
6×
FF4029
PE
PE
J
J
TE
TE
CL
CL
Q
Q
QN
QN
© 2026 Daniil Khanin · khanin.info · HT-04
20
CANVAS …
15
3×
3×
CL
Q
QN
1
PE
0
TE
Q
PE = 1, TE = 0: Q blinks half as often as CL
© 2026 Daniil Khanin · khanin.info · HT-04
21
← Back to the lesson
Lesson 2. The F/F flip-flop