This logic detector probe circuit can be very useful device for those who may want to measure the logic levels of digital circuits frequently. Being an IC based circuit, it's implemented in CMOS technology, its aplication is more dedicated to test circuits using the same technology.
By: R.K. Singh
The power for the proposed logic tester is obtained from the circuit under test itself. However care must be taken not to put the power terminals in reverse, so when it is connected make sure to set the colors of each of the connecting wires.For example: Red Color, for the cable that connects with the positive voltage (CN2) and black color to the wire that goes to 0 volts. (CN3)
Operational details of logic tester probe with IC 4001
The operation is very simple. The 4001 CMOS integrated circuit has four two-input NOR gates, 3 LEDs and a few passive components used in the design.
Implementation also becomes crucial so that it is comfortable to apply while testing, therefore the printed circuit should be in the elongated in shape preferably.
Looking at the figure we see that the sensing signal is applied to CN1 terminal, which is connected to a NOR gate, whose inputs are in turn connected as a NOT gate or an inverter.
The inverted signal is applied to the 2 LEDs. The diode is switched depending on the voltage level (logic) at the output of the gate.
If the input is high logic level output of the first gate goes low activating the red LED.
Conversely if the detected is low, the signal is sensed is as a low level, the output of this gate is then rendered at high level illuminating the green LED.
In the event if the input signal is an AC or pulsing (varying voltage level constantly between high and low), both red and green LED light become on.
To acknowledge that a pulsed signal may be sensed, the yellow LED starts flashing here. This flashing is executed with the use of the second and third NOR gates, C1 and R4 which functions like an oscillator.
The oscillator output logic is applied to a 4th NOR gate connected as inverter gate which is directly responsible for activating the yellow LED via the given resistor. This oscillator can be seen continuously triggered by the output of the first NOR gate.
Parts List for the above explained logic tester probe circuit
- 1 Integrated circuit CD4001 (4 2-input NOR gate CMOS version)
- 3 LEDs (1 red, 1 green, 1 yellow
- 5 resistors: 3 1K (R1, R2, R3), 1 2.2M (R5), 1 4.7M (R4)
- 1no capacitor: 100 nF