Logic probe uses six transistors
Raju R Baddi, Raman Research Institute, Bangalore, India; Edited by Martin Rowe and Fran Granville - December 15, 2010
Transistors Q3 and Q4 form a comparator of their base voltages. The divider combination comprising R8 and R9 maintains the base of Q4 at a specific voltage, which is approximately 1.9V. Because the probe’s suspended voltage is greater, Q3 conducts, and no current flows through R6. Thus, Q6 and the red LED are both off. If the voltage at the probe tip goes below 1.9V, however, Q4 has a higher voltage at its base than Q3, and the common-emitter current through R7 diverts to R6 through Q4. This action produces sufficient voltage drop across R6 to turn on Q6 and, hence, the red LED. The following equation sets the low-voltage threshold: VLOW=[VR9/(R8+R9)].
The circuit in Figure 1 lets you
build a logic probe using three
NPN transistors and three PNP transistors.
Two transistors act as switches that
drive the LEDs; logic one is a green LED,
and logic zero is red. Q1 and Q2 test the
probe-tip condition for logic one, and Q3
and Q4 test it for logic zero. Q1 acts as a
zener diode in the emitter circuit of Q2.
The voltage divider comprising R12 and
R14 determines the diode’s value. That
value creates a lower limit for the breakdown
of the base-emitter junction of Q2
through VL. These values ensure that the
threshold value for logic one at the probe
tip is approximately 3.2V. Q1’s breakdown
voltage in the emitter circuit of Q2
is approximately 2.6V. The equation for
setting this threshold is VHIGH=1.2+(VR14/R12+R14),where V is the supply voltage.
Because VHIGH is a function of the supply
voltage, the probe is suitable for CMOS
transistors, as well. When the voltage at
the probe tip goes above this voltage, the
base-emitter junctions of both Q1 and Q2
are forward-biased, and they have a common
collector-emitter current that flows
through R4, producing enough voltage to
forward-bias Q5 and turning on the green
LED. Ideally, R1 and R2 maintain the
voltage at the probe tip at approximately
2.5V, which is less than 3.2V.
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The current through the probe tip is −50 to +80 µA for a logic voltage of 0 to 5V. This appendix details the derivation of these equations and the probe-tip current. Figure 2 shows the construction method for building a compact probe.
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