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Project Notes

#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.


Here’s a quick demo..



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


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 D C B A LE !BLK !LT 7-Seg Char
(float) 0 0 x 0 0 0 1 (blank)
low 0 1 0 0 0 1 1 4
high 0 1 1 0 0 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 a 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 a 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 floatinf around, perhaps even difference PCB designs from different vendors
  • the red LED1 is indended to be a power indicator, but it never turns on. There is a simple fix however - see the next section.


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

Input “LOW”:


Input “HIGH”:


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 anough power to enable the circuit, LED1 will light.



Input not connected/floating:


Input “LOW”:


Input “HIGH”:


Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.