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
Description:
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