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Simple Logic Probe

This circuit effectively functions as a logic-level indicator or logic probe for TTL

circuits. It gives an indication of any of the following four conditions that might befound on a typical digital circuit and thus provides a versatile troubleshooting aid:

1. Line high (logic 1)2. Line low (logic 0)

3. Line tri-state (high impedance)

4. Line changing between high and low state alternately (pulsating).The logic probe is powered from the power supply of the digital equipment under

test. The logic state of the line under test is indicated by two LEDs (LED1 andLED2). If the line being probed is at logic 1 (high) state, LED2 would glow

brightly. If the line is at logic 0 (low) state, LED2 would be off. If the line is at tri-

state or open then LED2 would be dimly lit (about half of its full brightness). Ifthe line has a pulsating signal or a clock signal, LED1 would flash on and off at a

rate of 1 Hz. Thus LED2 gives an indication of static signal states, while LED1indicates dynamic signal conditions on the line being probed. The various states

of LED1 nd LED2 are summarised in Table I. In order to obtain a visible indication

of a clock signal presence on LED1, a divider, which comprises two monostablemultivibrators, is made use of. The monoshots are realised using IC1 (dual timer

556). The output of the first monoshot section (pin 5) is applied to the triggerinput (pin 8) of the second timer. Assume a clock frequency of 10 kHz is applied

to the probe tip. The falling edge of the clock at pin 6 of IC1 triggers the first

monoshot section. The output of the first monoshot stays high for around 0.5

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seconds and then goes low. This low going edge triggers the second monoshotsection, making its output (pin 9) high for around 0.5 seconds. While pin 9 is

high, the first monoshot section is prevented from being retriggered with the help

of transistor T1. When pin 9 reverts back to the low state, the first monoshot canbe retriggered. This arrangement ensures that LED1 flashes on and off at a

visible rate of around 1 Hz, thus indicating a clock presence on the line under

test. The circuit will detect clock frequencies between 1 Hz and 100 kHz,approximately. For any clock frequency within this range, LED1 will flash at a

constant rate of 1 Hz. When the probe tip is connected to a line at logic 1,currents flowing through resistors R9 and R10 add up in LED2 and make it glow

brightly. When probe tip is connected to a line at logic 0, the voltage at thejunction of resistors R9 and R10 is not sufficient to turn on LED2 (hence it

remains off). When the probe is not connected any- where or is connected to a

tri-stated line, LED2 remains dim because of current flowing through R9 only. Thewhole circuit can be enclosed in a small plastic tube with LED1 and LED2 exposed.

A pin can be used as the probe tip. The usefulness of this logic probe is limited tolow frequency digital circuits.