Four test pieces made with a six-inverter IC

The diode in parallel with C2 is always reverse biased, and it acts as a very large resistor to discharge C2. Assuming a typical leakage of 1nA for the diode, the equivalent resistance at 2.5V is about 2.5GΩ. The RC discharge time constant of about 125ms is suitable for the speed at which a human hand presses the key.

The values ​​of R1 to R3 set the astable frequency, or one-shot pulse width. The 220kΩ resistor at the second gate input is used to limit the leakage current from the capacitor into the gate input when the gate voltage is below ground, or 0.6V above VDD. The one-shot pulse generates a frequency of approximately 1/(2.2RC), while the threshold voltage of the gate determines the pulse width of the one-shot, which is approximately 0.7RC to 1.1RC.

Sometimes, it is necessary to listen to an audio signal at a certain point in the test circuit. The 4069 has a high input impedance and sufficient output drive current of about 6.8mA to drive a small PCB-mounted speaker. This method can build a simple audio probe (Figure 4). The resistors at the gate input in Figure 4a are used to protect the gate when the voltage of the unit under test is higher than the gate supply voltage.

Figure 4 shows two ways to drive the sound transducer, depending on the loudness required. Figure 4a is a direct connection to a piezoelectric transducer. If the sound is louder, a small resistor can be connected in series with the speaker in Figure 4b to control the volume and prevent possible damage to the speaker or transistor. Figure 4c shows an alternative method to increase sensitivity by biasing the gate input. [page]

The bias voltage is given by the voltage divider network as follows: VB = R2VS/(R1+R2)-R2VL(1-ξ)/(R1+R2)-R2VHξ/(R1+R2), where VS is the supply voltage, ξ is the input signal duty cycle (TH/(TH+TL), assuming a rectangular wave), VH is the logic high voltage, and VL is the logic low voltage. The recommended value for R1 is 1MΩ; the user can select R2 (1MΩ is recommended) based on the bias voltage formula. Many R2 values ​​are possible, as shown in Figure 4c. Capacitor CC (0.1μF is recommended) is connected in series with the signal to be measured to provide a bias voltage in series with the signal. The minimum signal strength is limited by the input threshold window of the gate and has different values ​​for different logic gates. For example, for a rectangular wave signal that switches between zero and the signal voltage, the bias voltage should be below the threshold window, while the sum of the bias voltage and the signal voltage should be above the threshold window.

There is a strict case where both values ​​are at the edge of the window. Therefore, for rectangular signals or attenuated digital signals, VSIG=ΔVT is the minimum required signal strength. Usually, ΔVT is different for different gates; some are wide (CD4069), some are narrow (CD4011). However, when an AC signal (such as a sine wave) is loaded, the negative phase of the signal will reduce the bias strength to VB-VSIG. Therefore, one phase is enough to produce a change equivalent to half the window width. Therefore, for AC signals, the minimum signal strength standard is VSIG=ΔVT/2.

Finally, for an inverting gate, ΔVT = VT2 – VT1, where VT2 is the input voltage at which the gate's output is fully settled at logic 0, and VT1 is the input voltage at which the gate's output is fully settled at logic 1. R1 and R2 help select a signal strength above the set minimum gate threshold window. If R2 is about 1 MΩ, the R2CC time constant is 0.1 seconds, corresponding to 10 Hz, which seems adequate.

For normal digital signals, it is sufficient to ignore R2 and CC. In other words, R2 and CC are equal to zero. It should be noted that this coupler does not compensate for severe signal conditions that would prevent a constant drain current at the output. Its purpose is to provide a typical transistor amplifier bias for the audio probe's gate. The concern for severe signal conditions is accomplished by the 10μF capacitor in series with the gate output in Figure 4b.

Users who prefer to use a traditional AC-coupled RC circuit (with the left terminal of R2 connected to ground rather than the probe tip) can calculate V\'B as a function of several parameters using the following equation: V\'B = VSR2/(R1+R2)-ξ(VH–VL)–VL, where all terms have their usual meanings.

Compare this to the previous equation. In the V\'B equation, the bias voltage is dependent on VH or VL through a single product factor of the duty cycle, while in the bias voltage equation, it depends on another product factor R2/(R1+ R2)<1, thus reducing the dependency and producing a flatter data curve with duty cycle, as shown in Figure 5, with a rectangular wave of VL/VH=0/4V ​​and a varying duty cycle.

All of these small devices can be put in a small container, such as a hose, and probed to perform various tests (Reference 4). The probe power supply can be two 3V CR2032 lithium batteries; the CMOS4069 is a low-power device. However, it should be noted that the thresholds of 4069 devices vary greatly from manufacturer to manufacturer, so the values ​​should be checked when selecting the device to make the test equipment, especially for the first three devices.

The key to these test devices is the high input impedance of the CMOS gates. Other packages (such as CD4011/4001) can also make multiple devices, because the key is to use inverting gates.

Editor's Note: In all the circuits discussed here, the probe ground should be connected directly to the ground of the device under test. Although the design example does not discuss this, some readers may want to add CD4011/4001NAND/NOR logic to combine the open circuit tester, monostable oscillator and audio probe to provide an audible open circuit tester.