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. |
