Single hex-inverter IC makes four test gadgets

This Design Idea describes a simple way that you can use one hex-inverter package of an unbuffered HD14069UB CMOS (Reference 1) to make four test gadgets: a logic probe with well-defined logic-voltage windows and with an input impedance of approximately 1 MΩ; a continuity tester whose upper limiting resistance can be tens of ohms to tens of megohms; a single or train pulse injector or a modest signal generator; and a high-impedance audio probe. You can assemble these gadgets using the six inverter gates of a 4069, two or three transistors, and a few passive components.

In a two-gate CMOS/TTL-compatible probe, the resistor network comprising R₁ through R₄ biases the inverter’s inputs (Figure 1). Because of the gates’ high input impedance, R₁ through R₄ can have values of approximately 100 kΩ to 1 MΩ. The probe’s source/sink current is small at the probe tip because of the high resistances of R₁ through R₄; as a result, the probe tip essentially does not affect the logic-voltage level at the test point. Knowing the gates’ input threshold voltage, you can calculate the required values of resistors R₁ through R₄.

The upper gate detects logic zero, and the lower gate detects logic one. Set an upper limit to the logic-zero voltage and calculate the values of R₁ and R₂. Arbitrarily select R₁=1 MΩ and seek a value for R₂ such that the voltage at the input of the upper gate is just the threshold voltage. Thus, R₂=R₁(V_T−V_L)/ (V_S−V_T), where V_T is the threshold voltage, V_L is the logic-zero voltage, and V_S is the supply voltage. Similarly, set a lower limit on the logic-one voltage V_T and seek a value for R₄ in terms of R₃. By appropriately selecting R₃, keeping in mind the quiescent biasing of the gates to keep both the LEDs off in the probe’s suspended condition, you can obtain the value of R₄: R₄=R₃V_T/(V_H−V_T).

The following equation calculates the probe current: I_P=[−(V−V_I) (R₃+R₄)+V_I(R₁+R₂)]/(R₁+R₂)(R₃+R₄), where I_P is the probe current and V_I is the probe-tip voltage. It thus follows that the probe impedance for any voltage at the probe tip is greater than 1 MΩ. For packages of 4069 that exhibit a larger threshold voltage, such as 3V, you can help reduce it by including a diode followed by a 10-kΩ load resistor to ground in the positive supply rail to the chip.

Developers often use continuity testers (Figure 2 and Reference 2) as elementary test gadgets; such testers are indispensable on a work bench. One of the 4069’s gates, with its high input impedance and with a threshold voltage for the transition of the output of the gate, allows you to build the continuity tester with an upper limit on the resistance of the test circuit. The total of the resistance between the probes and the resistance in the switching arrangement forms a voltage-divider network, producing a voltage at the input of the gate. When the two resistances are equal, the voltage at the gate input is half the supply voltage. The transition threshold voltage of the gate is also nearly half the supply voltage; therefore, the selected resistor in the switching branch sets the approximate threshold-continuity resistance.

A useful alternative arrangement is to have a variable resistor in place of the switchable resistors. This approach allows you to set the threshold-continuity resistance arbitrarily by adjusting this resistor after including the desired resistance between the probe tips and observing the LED’s glow. The variable resistor should be set so that the LED just goes off. This method results in a compact arrangement, which a small package can accommodate. Another variable resistor (1 to 2 kΩ) is included in series with the negative probe to make it possible to have a threshold-continuity resistance of approximately 100Ω or less. You can also use a lower transition threshold voltage for the gate by including a couple of diodes followed by a 10-kΩ load resistor to ground in series with the positive supply rail. This arrangement can also be used to test for live ac-mains lines (Reference 3) with suitable modification, virtually making five gadgets.

Three gates still remain in the 4069 package; you can use two of them to make an astable oscillator/monostable single-pulse-generator circuit, which a complementary bipolar pair buffers to increase the drive current (Figure 3). You select between a single pulse or a pulse train with an SPDT (single-pole/double-throw) switch set to P (pulse) or A (astable). In pulse mode, pressing the switch produces a brief negative-going pulse at the input to the second gate as C₂ begins charging. The resulting high at the gate output causes a positive-going pulse at the junction of Q₁ and Q₂. It also is latched, and the switch is debounced by the positive feedback through capacitor C₁, which begins charging at a time constant that the selection of R₁, R₂, or R₃ determines. When the voltage across C₁ equals the threshold voltage, the second gate output returns low, again with positive feedback through C₁, driving the second gate input high and ending the pulse.

The diode in parallel with C₂ is always reverse-biased and serves as a very large-value resistor to discharge C₂. Assuming a typical diode leakage of 1 nA, the equivalent resistance at 2.5V is about 2.5 GΩ. The RC discharge time constant of approximately 125 msec is suitable for the human rate of pushing the button.

The values of R₁ through R₃ set the astable frequency or the one-shot’s pulse width. The 220-kΩ resistor at the input of the second gate is included to limit the leakage of current from the capacitor into the gate input when its voltage is below that of ground or higher than V_DD by 0.6V. The astable produces a frequency of approximately 1/(2.2RC), whereas the threshold voltage of the gate determines the pulse width of the one-shot, which is approximately 0.7 to 1.1RC.