Simple capacitance meter bins parts

-March 24, 2014

This Design Idea describes a simple two-chip CMOS circuit that can sort capacitors into 20 bins over a wide range (100pF to 1μF), using 10 LEDs to display the value range. The circuit is power efficient and can be run using two CR2032 cells. As such, it can be built into a handheld probe.

The heart of the circuit is the RS flip-flop using 4093 NAND gates along with transistors Q1 and Q2, which discharge the reference and test capacitors respectively. The reference capacitor (CREF) and the test capacitor (CX) are charged from zero volts, followed by a reset pulse applied to the bases of Q1 and Q2. Depending on the values of the capacitors and their respective series resistors, either CX or CREF reaches the gates’ VT before the other; accordingly the outputs of the RS flip-flop are set/reset. The outputs are so wired such that when CREF reaches VT before CX, a clock pulse is applied to the CD4017 counter, whose outputs (Q0-Q9) charge CREF through one of 10 different series resistor values (R0-R9).

Figure 1  Capacitance meter schematic. A 4011 NAND can exhibit some hysteresis, but a 4093 is recommended.

The master clock that runs the circuit is derived using a single 4093 Schmitt NAND gate. This steps the 4017 decoded decade counter, the flip-flop “comparing” the voltage ramps across CX and CREF while charging CREF through the different values. When CX reaches VT before CREF, then 4017 is not clocked, and the appropriate LED starts flashing indefinitely. Depress the reset switch to take another reading, which resets the 4017 and selects the minimum resistor value, R0. There are two ranges, each divided into 10 sections, which cover 100pF-10nF and 10nF-1μF. The range is selected by an SPDT switch which connects a CREF of either 100pF or 10nF.

A particular flashing LED indicates that the value of the test capacitor lies in the range between it and the next lower LED. Q3 takes care of lighting the LED in the appropriate phase of the master clock. The anodes of the 10 LEDs are connected to the outputs (Q0-Q9) of the CD4017. It seems that lower values of charging resistors can result in more reliable readings due to smaller errors in the charging current (contributed mainly by reverse conduction in all the diodes). R0-R9 can be scaled appropriately, together with an inverse scaling of the master clock frequency. Figure 2 gives a handheld probe construction example using a glue stick tube.


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