#612 Hi-Lo Logic Tester Kit

Building a neat little hi/low logic indicator kit using a CD4511 and a few passives; also fixing a circuit bug that prevents the power indicator from working.

Build

Here’s a quick demo..

Notes

This is cheap little kit that one might want to pick up for a few reasons:

good for some through-hole soldering practice
learn about the interesting CD4511 BCD-to-7 Segment Latch/Decoder/Driver
study the neat trick used to get a “H” or “L” indicator with just a few passive components

Kit and Parts

kit_parts

Ref	Item	Description
R1	2kΩ
R2	1MΩ
R3	1MΩ
R4	120kΩ
R5	1kΩ
R6	10kΩ
R7	51Ω
C1	221 - 220pF
C2	4.7µF
C3	473 - 47nF
VD1	1N4148
LED1	5mm Red LED
LED2	7-segment	5161AH
U1	CD4511
VT1	9014 NPN
VT2	9014 NPN
	4-pin header	for probe input
	2 wires	for power - unused, I replaced with XH2.54 connector

Principle of Operation

The input level varies the B, C and !BLK values according to the following table:

INPUT	C	B	!BLK	!LT	7-Seg Char
(float)	0	x	0	1	(blank)
low	1	0	1	1	4
high	1	1	1	1	6

When input floating:

VT1 base pulled high, which pulls C and !BLK low
since !BLK low, no 7-segment output regardless of other input

When input high:

VT1 base pulled high, but also VT1 collector via VD1, hence C and !BLK pulled high
B pulled high via R2
sets DCBA = 0110 i.e. 4511 output “6”,
the 4511 Output maps to the 7-segment elements a-g as follows. i.e. displays “H”:

4511 Output	b	c	d	e	f	g
a : 0 (n/c)
b : 0			0
c : 1 (n/c)
d : 1	1	1
e : 1						1
f : 1					1
g : 1				1

When input low:

VT1 base pulled low (off), hence C and !BLK pulled high
B pulled low via C3
sets DCBA = 0100 i.e. 4511 output “4”,
the 4511 Output maps to the 7-segment elements a-g as follows. i.e. displays “L”:

4511 Output	b	c	d	e	f	g
a : 0 (n/c)
b : 1			1
c : 1 (n/c)
d : 0	0	0
e : 0						0
f : 1					1
g : 1				1

Original Schematic

Here’s the schematic for the board I received. Note however:

there are a few variations of the schematic floating around, perhaps even difference PCB designs from different vendors
the red LED1 is intended to be a power indicator, but it never turns on. There is a simple fix however - see the next section.

original_schematic

As build per the kit and original schematic. It works, just without the power indicator.

Input “LOW”:

test_low_v1

Input “HIGH”:

test_high_v1

Modified Schematic

Here’s a redrawing and modification to circuit to allow the LED1 power indicator to work correctly. Essentially:

PCB LED1 footprint not populated (open circuit)
LED1 and R5 connected in series in the PCB footprint provided for R5
end result - LED is in the pull-up connection for VT2 base. So as long as there is enough power to enable the circuit, LED1 will light.