Quite often electronics engineers and technicians come across the problem of fault finding or testing electronic circuit boards full of digital integrated circuits. A handy and quick look-tool ‘5-State Digital IC and Circuit Tester’ for such an analysis is a logic probe which does the job faster and easier by just touching this probe to different pins of various ICs and observing the logic levels.

Most of the commercial available logic probes show only two distinct levels of logic ‘0’ and logic ‘1’ corresponding to voltage levels of 0V to +0.8V and +2.4V to +5V respectively, for TTL circuits. But sometime it becomes necessary to know the voltage levels at the pins of the IC designated as ‘+Vcc’ and ‘GND’ pins-to state correctly, ‘short-to-positive or +5v’ and short-to-ground or 0V’ respectively. Similarly, in circuits using tri-state logic IC in microcomputer boards, it is very much essential to know whether a particular pin of an IC is at ‘tri-state’ level or not-tri-state level voltage being that in between +0.8V and +2.4V/.

Thus, beside the normal two logic levels, the above mentioned three more logic levels are also required to be known in many instance. Hence, a 5-state logic probe is described here.

### Principle of Operation of 5-State Digital IC and Circuit Tester

The voltage to be sensed by the tip of the probe is made to pass through four different paths. Depending upon the sensed voltage level, one or more path allows the signal to flow and affect the logic levels in that channel further, and thereby generate appropriate logic state at the input of the 7-segment display to show the correct logic state.

The voltage levels at various points in the circuit change as per the input voltage level at the tip for different logic states.

### Short-to-ground State

A short-to-ground state at the tip of the probe is detected whenever the input voltage level is between 0V and +50mV. Since the voltage level is below +50mV at the input point ‘A’, transistor T_{1} does not conduct, allowing transistor T_{2} to conduct heavily and go into saturation. This does not allow T_{3} to conduct, and thereby voltage at the junction of R_{4} and R_{5}, i.e. at an input NAND gate N_{2}, and that means one of the inputs of N_{3}, to be at logic ‘1’ level.

Input at point ‘B’ does not allow T_{4} to conduct, so the second input of N_{3} is at ‘zero’. Hence output of N_{3}, which is an input to N_{2}, is at logic ‘1’ state.

Input of less than +50 mV at point ‘C’ makes diode D_{1} to conduct, which being germanium type allows a drop of 0.1V across it and one input of gate N_{1} becomes maximum at +0.15V. Output of N_{1}, which is one if the input of N_{4}, will be thus at logic ‘1’ level.

Now, the fourth branch input at point ‘D’ makes transistor T_{5}, connected as a diode, to conduct; the potential at the base of T_{6} does not exceeds +0.5 volt, and so it does not conduct fully. Resistor R_{9} and R_{10} are adjusted such that T_{7} conducts heavily, making one of the inputs of the gates N_{4}, and N_{7} to be at logic ‘0’ level.

Thus, except the gate N_{7} all the other N_{4}, N_{5} and N_{8} have both the inputs at logic ‘1’ level, and hence their outputs will be at logic ‘0’ level. Only gate N_{7} has one of the inputs at logic ‘0’, and so its output will be logic ‘1’, illuminating element ‘d’ of the 7-segment display. Element ‘g’ of the 7-segment display connected to collector of transistor T_{8} also does not glow. This is because outputs of N_{5} and N_{6} are low and output of N_{7} is high. Resistor R_{13}, R_{14} and R_{15} being of the same value, the base potential of T_{8} is more than +0.7 V, and as such it goes saturation and being down its collector potential.

### Logic ‘0’ of ‘low state’

Whenever the input voltage at the probe tip between +50 mV and +0.8V, a logic ‘0’ state is said to be existing. This voltage level has the same effect on branch C as before. Thus, the output of N_{1}, which is also one of the inputs to N_{4}, acquires logic ‘1’ state. Now, this time, the input voltage level at point D makes voltage level at the base of transistor T_{6} to be more then +0.5V, and it conducts sufficiently to bring down its collector potential below +0.8V. Resistor R_{9}, R_{10} and R_{11} are adjusted such that collector voltage level of T_{7} goes high to more than +2.5 V so that the second input of gate N_{4} goes to logic ‘1’ state.

This brings down the output of N_{4}, which is also one of the inputs of gates N_{5}, N_{6}, N_{7} and N_{8}, to logic ‘0’ state. Thus, irrespective of conditions at the output inputs of gates N_{5}, N_{6}, N_{7} and N_{8}, their outputs are pulled up to logic ‘1’ level and hence six elements a, b, c, d, e and f of the 7-segment display are illuminated. Only elements g is not illuminated because of similar logic as in the first condition (T_{8} is deeper in saturation and collector potential of T_{8} is low)

### Tri-state level

When a voltage level at output of an IC is more than +0.8V, but less then +2.4V, a Tri-state or high-impedance state is said to be existing. When the probe comes in contact with such a pin, branch A behaves similarly as in earlier two cases, and output of gate N_{2} is high.

At the input to branch B, although input is greater than +0.8V, resistor R_{6} being 15 K, this voltage is not sufficient to make T_{4} conduct sufficiently and so potential at emitter of T_{4} is still near zero. Therefore, output of N_{3} is high. Branch D also behaves in a similar fashion, as descried in the condition of logic ‘0’ state, bringing collector of T_{7} to high state.

Only branch C behaves in a different manner. Because input is more than +0.8V, the drop across diode D_{1} makes input of gate N_{1} to low state and the output of N_{4} goes high.

Thus, all the four gates N_{5}, N_{6}, N_{7} and N_{8} have both their inputs at high state and, therefore, there outputs at ‘low’ state blank all the elements a to f. This time, as the resistor R_{13}, R_{14} and R_{15} are connected to low potential, base voltage T_{8} is pulled down to below +0.3 V, and hence T_{8} is driven towards cut-off condition. This allows elements ‘g’ to be illuminated.

### Logic ‘1’ or ‘high’ state

A voltage level of more than +2.4 V is considered to be logic ‘1’ or ‘high’ state. To distinguish this state from supply voltage of +5V, state ‘1’ is considered to have a voltage level of more than +2.4V but less than +4.95V whenever this condition is present at the input, branch A still behaves as in earlier three conditions, except that transistor T_{2} conducts lesser and T_{3} starts conducting more. But still potential at collector of T_{3} is not sufficient to bring R_{4} and R_{5} junction to logic ‘1’ state, and so output of N_{2} is still high.

Branches C and D also behaves as in the previous case of Tri-state condition, and hence outputs of N_{4} and collector potential of T_{7} are at ‘high’ state. This time, the input at B is sufficient to make T_{4} conduct, so that collector of T_{4} is high enough to pull down output of N_{3} to ‘low’ state.

Thus, outputs of N_{5}, N_{7} and N_{8} are low, whereas that of N_{6} is high. Elements b and c only therefore light up.

### Short-to-positive or +5V state

This state, as described earlier, is shown when input is greater than +4.95 V. it is something important to heck that the supply voltage pins of the ICs are correctly at +5V. For this input voltage, branches, B, C and D behave exactly as in the previous logic ‘1’ state condition. But this time, in branch A, since base potential of T_{2} goes high to more than +4.95V, it conducts much less, making drop across R_{2}, R_{3} less. This allows T_{3} to go into saturation, and hence potential at junction of R_{4} and R_{5} is more than +2V. Thus, output of N_{2} is at logic ‘low’. Outputs of N_{2}, N_{3} and N_{4} are therefore all low, whereas that of N_{1} is high. Thus, only element ‘a’ is illuminated.

A common-cathode 7-segment display as the one used here or any other equivalent available would do the job of display. A 5.6V Zener diode (ZD_{1}) at the input point of the probe prevents the circuit from damaging, when touched accidently to voltage more than, +5.6V. since digital circuits have maximum recommended operating voltage of +5.5V, one is not supposed to come across any potential higher than this in a digital circuit board. Nevertheless, ZD_{1} safeguards the logic probe in case of eventuality.

### PARTS LIST OF 5-STATE DIGITAL IC AND CIRCUIT TESTER

Resistors (all ¼-watt, ± 5% Carbon) |

R_{1}, R_{3}, R_{8}, R_{9}, R_{13}, R_{14}, R_{15} = 1 KΩ
R R R R R R |

Semiconductors |

IC_{1} (N_{1} – N_{4}), IC_{2} (N_{5} – N_{8}) = SN7400
T T D ZD DIS |