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 be found 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 and LED2). 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). If the 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 LED1 indicates 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 monostable multivibr- ators, 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 trigger input (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 seconds and then goes low. This low going edge triggers the second monoshot section, 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 can be 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 the junction 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. The whole 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 to low frequency digital circuits.