Logic Probe
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Notes:
This logic probe uses a single CMOS IC and shows three logic
conditions, High, Low and Pulsing. In addition if the probe input is neither hi
or low (the high impedance state of tri-output logic ic's) then no LED's will light. Power from the
logic probe is taken from the logic circuit under test; using a CMOS IC enables
logic circuits to be tested using voltages from 3 to 15 volts. IC1a is
arranged as a buffer with a difference. Under no input, i.e. probe not
connected to circuit the gate will oscillate due to feedback from the 2M2 resistor.
Output voltage at IC1a is approximately half supply voltage. The Hi and Lo
logic indicator LED's are also connected to a potential divider consisting of
the two 1k resistors. Voltage at the junction is half supply voltage
hence with no input , or high impedance no LED's will
light. A Hi or Lo logic condition will cause IC1a to rest in a permanent state
indicated by either the Hi or Lo LED illuminating. With a fast oscillator
or clock signal both Hi and Lo LED's will light but will be quite dim. This is
the reason for IC1b and IC1c. These two gates form a monostable
oscillator, time constant determined by the 100n capacitor and 4M7 resistor.
With a clock signal this is effectively slowed as the monostable
is continually triggered and retriggered. IC1d acts as a buffer to drive the
pulsing LED.
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