#636 CD4001 Logic Probe
A simple logic probe using NOR gates.
Notes
This logic probe design can detect and indicate:
- logic high (red LED on)
- logic low (green LED on)
- oscillating signal (yellow LED on)
About the CD4001
The CD4001BC is a Quad 2-Input NOR Buffered B Series Gate. Key features:
- operating range (VDD): 3-15V
- monolithic CMOS
- low power TTL compatibility
- all inputs protected by standard CMOS protection circuit; voltage at any pin −0.5V to VDD+0.5V
See the CD4001 datasheet for more.
Construction
The first NOR gate is used as an inverting buffer of the input signal.
When LOW input:
- first NOR gate output will be high
- with high output, D2 (green) on, D1 (red) off
- a fixed high signal will stall the oscillator on the 2nd and 3rd NOR gates
- R4 pulls the 3rd NOR gate input high, thus final NOT gate output will also be high, thus D3 (yellow) off
When HIGH input:
- first NOR gate output will be low
- with low output, D2 (green) off, D1 (red) on
- a fixed low signal will stall the oscillator on the 2nd and 3rd NOR gates
- R4 pulls the 3rd NOR gate input high, thus final NOT gate output will also be high, thus D3 (yellow) off
When input is oscillating:
- D2 (green), D1 (red) will flicker based on instantaneous input state
- oscillating input to the 2nd NOR gate will enable the oscillator on 2nd and 3rd NOR gates
- final NOR gate output will oscillate, causing D3 (yellow) to light up
Note:
- R4, R5 values are not critical. High values such as 1MΩ are satisfactory
- R5 is optional - it prevents oscillation when input is high Z/not connected.
- the link/short between the anode of D1 and cathode of D2 is used to inhibit D1/D2 when there is no strong input signal high or low
- this is optional; the circuit works without it.
- removing it will reduce power consumption of the circuit but D1/D2 will tend to be on when there is no strong input signal
Breadboard test, with logic high input:
Breadboard test, with logic low input:
Breadboard test, with oscillating input (10kHz square wave from FY3200S signal generator):
Protoboard Build
I transferred the design to a piece of protoboard to capture the circuit for future use. A quick sketch of the layout:
Protoboard test, with logic high input:
Protoboard test, with logic low input:
Protoboard test, with oscillating input (10kHz square wave from FY3200S signal generator):
