DIY Logic Probe

Simple Logic Probe.

CMOS 4001 Quad 2-Input NOR gate is used in this circuit for detecting HIGH, LOW, PULSE and HIGH IMPEDANCE state in the digital circuit.

Components Used:

IC 4001 --- 1

1k Ohm Resistor --- 3

2.2M Ohm Resistor --- 1

4.7M Ohm Resistor --- 1

100nF Capacitor --- 2

LED's GREEN, RED, BLUE --- each 1

Circuit Operation

Oscillator: The logic probe uses a 4001 quad NOR gate IC, resistors and a capacitor. The frst NOR gate(IC1a) is used as a basic oscillator. With the inputs connected together the frst NOR gate is transformed into an inverter. 2.2M ohm resistor is used to feed the output back into the input which results in oscillation if the input is unconnected (floatng).

HIGH LOW Indication:

Hi(RED) LED is connected between power through 1K ohm resistor and the output of the NOR gate while Lo(GREEN) LED is connected to ground through 1K ohm resistor and the output of the NOR gate. If the input to the NOR gate is a logical 1 then the NOR gate will output 0V and this will result in Hi(RED) LED turning on (current flows through 1K ohm resistor, through Hi(RED) LED and then into the output which is at 0V). If the input is OFF then the output of the NOR gate will be VDD and this results in Lo(GREEN) LED turning on instead of Hi(RED) LED (current flows from the output of the NOR gate through Lo(GREEN) LED and then to ground through 1K ohm resistor).

PULSE Indication:

The next two NOR gates(IC1b & c) form a basic mono-stable circuit which is used to detect an oscillatng signal. If for example the input signal was oscillatng at a high rate (more than a MHz), then any LED that is used to show this signal would not be very bright and thus make it hard to tell if the input is oscillatng. To solve this the mono-stable formed by the second two NOR gates, capacitor (100nF) and resistor (4.7M ohm) will be constantly triggered and re-triggered by the oscillatng input. This results in a constant output which will make an LED bright so long as the oscillaton is present. The last NOR gate(IC1d) is used as an LED driver for the mono-stable.

FLOAT Indication:

If no input is connected (floatng), the NOR gate will weakly oscillate around VCC / 2. The voltage seen by both Hi(RED) and Lo(GREEN) LED will be approximately half VCC and thus results in neither LED turning on and therefore indicates a floatng input. This weak oscillaton, however, is able to set and reset the monostable which keeps the Pulse (BLUE) LED on.

LED ----------------- Condition

RED (Hi) ----------------- HIGH

GREEN (Lo) ----------------- LOW

BLUE (Pulse) ----------------- Floating

ALL LED's ----------------- Oscillating

Now connect the positive and negative clip to the circuit under test power supply and probe the signal traces to check the status.