When utilized to prescription drugs, the time period “off-label” suggests the (steadily found) sensible and useful makes use of for a drug which are totally different from the one it was initially developed for. This occurs for digital parts too, such because the venerable CD4013B dual-D CMOS flip-flop. Regardless of the 4013’s labeling as a conventional bi-stable logic component, it however has terrific off-label potential as an analog half.
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For instance, right here’s the way it works as a capacitance comparator as proven in Determine 1.
Determine 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 via the related R, C, and diode networks, turning it right into a ~100kHz RC oscillator. The length (T+) of the constructive oscillation half-cycle (pin 13 excessive) is managed by R2C2, whereas R1t (the trimmer’s high half) and Cx (humidity-sensor capacitance) management the length (T-) of the adverse (pin 13 low) half-cycle.
Growing humidity will increase Cx, thereby rising R1tCx and T- and vice-versa. In the meantime, as R1t is coupling the T- pulse to the timing ramp on Cx and S#2 pin 8, the underside half of the trimmer (R1b) is coupling it to a really comparable ramp on Cref and S#1 pin 6.
The relative distinction between time constants R1tCx versus R1bCref kinds the premise of the capacitance measurement. If R1tCx < R1bCref, indicating that humidity is beneath the setpoint on R1, then the ramp on pin 8 will cross the 0/1 threshold and finish T- earlier than the ramp on pin 6 does. It will enable FF#1 to be reset when clocked by the rising edge on pin 13 on the finish of T-, asserting Cref > Cx.
Contrarywise, if R1tCx > R1bCref, then pin 6 will cross threshold first, holding FF#1 set and asserting Cx > Cref.
As a result of each flip-flops inhabit the identical chip, their respective S enter thresholds will monitor one another carefully regardless of variations in temperature and provide voltage, enhancing switch-point precision and stability.
The identical circuit works equally properly in lots of different capacitance-sensing functions, corresponding to a non-contact place sensor/movement restrict change (Determine 2).
Determine 2 Movement restrict/place sensing proximity change utilizing the CD4013B.
Detection and management of the extent of liquid in a reservoir is one other appropriate utility, as Cx will increase as a result of rising liquid degree will increase capacitance between the liquid and an insulated probe.
Determine 3 Liquid degree sensor utilizing the CD4013B.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a methods. In all, a complete of 64 submissions have been accepted since his first contribution was printed in 1974.
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