5 State Digital IC and Circuit Tester

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 that does the job faster and easier by just touching this probe to different pins of various ICs and observing the logic levels.

Introduction to 5-State Digital IC and Circuit Tester

Most of the commercially 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 sometimes 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, besides the normal two logic levels, the above-mentioned three more logic levels are also required to be known in many instances. 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, thereby generating an 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 | 5-State Digital IC and Circuit Tester

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 T₁ does not conduct, allowing transistor T₂ to conduct heavily and go into saturation. This does not allow T₃ to conduct, and thereby voltage at the junction of R₄ and R₅, i.e. at an input NAND gate N₂, and that means one of the inputs of N₃, to be at logic ‘1’ level.

Input at point ‘B’ does not allow T₄ to conduct, so the second input of N₃ is at ‘zero’. Hence the output of N₃, which is an input to N₂, is at logic ‘1’ state.

The input of less than +50 mV at point ‘C’ makes diode D₁ conduct, which being germanium type allows a drop of 0.1V across it and one input of gate N₁ becomes maximum at +0.15V. The output of N₁, which is one of the inputs of N₄, will be thus at logic ‘1’ level.

Now, the fourth branch input at point ‘D’ makes transistor T₅, connected as a diode, to conduct; the potential at the base of T₆ does not exceed +0.5 volt, and so it does not conduct fully. Resistor R₉ and R₁₀ are adjusted such that T₇ conducts heavily, making one of the inputs of the gates N₄, and N₇ to be at logic ‘0’ level.

Thus, except the gate N₇ all the other N₄, N_5, and N₈ have both the inputs at logic ‘1’ level, and hence their outputs will be at the logic ‘0’ level. Only gate N₇ 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 the collector of transistor T₈ also does not glow. This is because outputs of N₅ and N₆ are low and the output of  N₇ is high. Resistor R₁₃, R_14, and R₁₅ being of the same value, the base potential of T₈ is more than +0.7 V, and as such it goes saturation and being down its collector potential.

Logic ‘0’ or ‘low state’ | 5-State Digital IC and Circuit Tester

Whenever the input voltage at the probe tip is 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 N₁, which is also one of the inputs to N₄, acquires logic ‘1’ state. Now, this time, the input voltage level at point D makes the voltage level at the base of transistor T₆ to be more than +0.5V, and it conducts sufficiently to bring down its collector potential below +0.8V. Resistor R₉, R_10, and R₁₁ are adjusted such that the collector voltage level of T₇ goes high to more than +2.5 V so that the second input of gate N₄ goes to logic ‘1’ state.

This brings down the output of N₄, which is also one of the inputs of gates N₅, N₆, N_7, and N₈, to the logic ‘0’ state. Thus, irrespective of conditions at the output inputs of gates N₅, N₆, N_7, and N₈, 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 (T₈ is deeper in saturation and collector potential of T₈ is low)

Tri-state level | 5-State Digital IC and Circuit Tester

When a voltage level at the output of an IC is more than +0.8V, but less than +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 the earlier two cases, and the output of gate N₂ is high.

At the input to branch B, although input is greater than +0.8V, resistor R₆ being 15 K, this voltage is not sufficient to make T₄ conduct sufficiently and so potential at the emitter of T₄ is still near zero. Therefore, the output of N₃ is high. Branch D also behaves similarly, as described in the condition of logic ‘0’ state, bringing collector of T₇ to a high state.

Only branch C behaves differently. Because the input is more than +0.8V, the drop across diode D₁ makes the input of gate N₁ to a low state and the output of N₄ goes high.

Thus, all the four gates N₅, N₆, N_7, and N₈ have both their inputs at a high state and, therefore, their outputs at a ‘low’ state blank all the elements a to f. This time, as the resistors R₁₃, R₁₄ and R₁₅ are connected to low potential, base voltage T₈ is pulled down to below +0.3 V, and hence T₈ is driven towards the cut-off condition. This allows elements ‘g’ to be illuminated.

Logic ‘1’ or ‘high’ state | 5-State Digital IC and Circuit Tester

A voltage level of more than +2.4 V is considered to be a logic ‘1’ or ‘high’ state. To distinguish this state from a 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 the earlier three conditions, except that transistor T₂ conducts lesser and T₃ starts conducting more. But still potential at the collector of T₃ is not sufficient to bring R₄ and R₅ junction to logic ‘1’ state, and so the output of N₂ is still high.

Branches C and D also behave as in the previous case of Tri-state condition, and hence outputs of N₄ and collector potential of T₇ are at a ‘high’ state. This time, the input at B is sufficient to make T₄ conduct, so that collector of T₄ is high enough to pull down the output of N₃ to a ‘low’ state.

Thus, outputs of N₅, N_7, and N₈ are low, whereas that of N₆ is high. Elements b and c only, therefore, light up.

Short-to-positive or +5V state | 5-State Digital IC and Circuit Tester

This state, as described earlier, is shown when input is greater than +4.95 V. it is important to check 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 the base potential of T₂ goes high to more than +4.95V, it conducts much less, making drop across R₂, R₃ less. This allows T₃ to go into saturation, and hence potential at the junction of R₄ and R₅ is more than +2V. Thus, the output of N₂ is at logic ‘low’. Outputs of N₂, N_3, and N₄ are therefore all low, whereas that of N₁ 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 the display. A 5.6V Zener diode (ZD₁) at the input point of the probe prevents the circuit from damaging, when touched accidentally to a voltage more than, +5.6V. since digital circuits have a 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, ZD₁ safeguards the logic probe in case of an eventuality.

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PARTS LIST OF 5-STATE DIGITAL IC AND CIRCUIT TESTER