When applied to pharmaceuticals, the term “off-label” suggests the (frequently discovered) practical and beneficial uses for a drug that are different from the one it was originally developed for. This happens for electronic components too, such as the venerable CD4013B dual-D CMOS flip-flop. Despite the 4013’s labeling as a traditional bi-stable logic element, it nevertheless has terrific off-label potential as an analog part.
For example, here’s how it works as a capacitance comparator as shown in Figure 1.
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| Figure 1. | Circuit diagram with the CD4013B used as a capacitive humidity sensor. |
The #2 flip-flop has its Q outputs (pins 12 and 13) and S/R inputs (8 and 10) cross-connected through the associated R, C, and diode networks, turning it into a ~100 kHz RC oscillator. The duration (T+) of the positive oscillation half-cycle (pin 13 high) is controlled by R2C2, while R1T (the trimmer’s top half) and CX (humidity-sensor capacitance) control the duration (T–) of the negative (pin 13 low) half-cycle.
Increasing humidity increases CX, thereby increasing R1TCX and T– and vice-versa. Meanwhile, as R1T is coupling the T– pulse to the timing ramp on CX and S#2 pin 8, the bottom half of the trimmer (R1B) is coupling it to a very similar ramp on CREF and S#1 pin 6.
The relative difference between time constants R1TCX versus R1BCREF forms the basis of the capacitance measurement. If R1TCX < R1BCREF, indicating that humidity is below the setpoint on R1, then the ramp on pin 8 will cross the 0/1 threshold and end T– before the ramp on pin 6 does. This will allow FF#1 to be reset when clocked by the rising edge on pin 13 at the end of T–, asserting CREF > CX.
Contrariwise, if R1TCX > R1BCREF, then pin 6 will cross threshold first, holding FF#1 set and asserting CX > CREF.
Because both flip-flops inhabit the same chip, their respective S input thresholds will track each other closely despite variations in temperature and supply voltage, enhancing switch-point precision and stability.
The same circuit works equally well in many other capacitance-sensing applications, such as a non-contact position sensor/motion limit switch (Figure 2).
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| Figure 2. | Motion limit/position sensing proximity switch using the CD4013B. |
Detection and control of the level of liquid in a reservoir is another suitable application, as CX increases because rising liquid level increases capacitance between the liquid and an insulated probe (Figure 3).
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| Figure 3. | Liquid level sensor using the CD4013B. |
