CMOS Logic Probe

Parts List:
R1 = 2M2             C1,C2 = 0.1µF (100 nanoFarad)
R2 = 6M8             IC = MC4001 (CMOS)
R3 = 1K8             Leds = D1-Red, D2-Green, D3-Yellow, 3mm.
R4 = 1K5             (R2: 6.8M or two 3.3M)
R5 = 1K8
      A logic probe is a device which is used when testing digital circuits, and it shows the logic state at the selected test point. In common with most designs this one can indicate four input states, as follows:
1. Input high (logic 1)
2. Input low (logic 0)
3. Input pulsing (pulse)
4. Input floating

      This circuit uses the four 2 input NOR gates contained within the 4001 CMOS IC, and is primarily intended for testing cmos circuits. The probe derives its power from the supply of the circuit being tested. The first gate, N1, has its inputs tied together so that it operates as an inverter, and it is biased by R1 so that roughly half the supply potential appears at its output. A similar voltage appears at the juntion of R4 and R5, and so no significant voltage will be developed across D1 and D2 which are connected between this junction and gate 1's output pin 3. Thus under quiescent conditions, or if the probe is connected to a floating test point, neither D1 or D2 will light up. If the input is taken to a high logic point, gate 1 output will go low and switch on D1(red), giving a logic 'high' indication. If the input is taken to a low test point, gate 1's output pin 3 will go high and light D2(green) to indicate a logic 'low'.

      A pulsed input will contains both logic states, causing both Led's D1 and D2 to switch on alternately. However, if the duty cycle of the input signal is very high this may result in one indicator lighting up very brightly while the other does not visibly glow at all . In order to give a more reliable indication of a pulsed input, gates N2 to N4 are connected as a buffered output monostable multivibrator. The purpose of this circuit is to produce an output pulse of predetermined length (about 1/2 a second in this case) whenever it receives a positive going input pulse.
The length of the input pulse has no significant effect on the output pulse. Led D3 is connected at the ouput of the monostable, and is switched on for about 1/2 a second whenever the monostable is triggered, regardless of how brief the triggering input pulse happens to be. Therefore, a pulsing input will be clearly visible by the yellow Led D3 switching on.
The various outputs will be: Floating input -- all Leds off. Logic 0 input -- D2 (green) switched on (D3 briefly flashes on). Logic 1 input -- D1 switched on. Pulsing input -- D3(yellow) switched on or pulsing in the case of a low frequency input signal (one or both of the other indicators will switch on, showing if one input state pre-dominates.
The finished project can easily be housed in a magic marker felt pen or something. The probe-tip is made of a piece of piano wire, but a bronze finishing nail is a good choice too. Solder the probe wire to the nail or pianowire and you are all set. Have fun building it and make it part of your trouble-shooting equipment.

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Copyright © 1987 Tony van Roon