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Fig. 4. Display and drivers.

and IC 11 (Fig. 4), which also drive seven-segment displays DIS1, DIS2, and DIS3. Current limiting for each display is performed by resistors R2: 1, R1:2, R3:3, P5A R4:6, R7:5, and P6:7. (This method ot idenffying the resistors is discussed in the Construction section of the article.)
Now we'll examine the capacitance meter's autorange circuitry (Fig. 5). Overflow pulses from the last BCD decade counter (IC17) are applied to 4-bit binary counter IC16. This IC has four weighted binary outputs, A, B, C, and D, which are inverted by IC15. Lines A, A, B, and b are decoded by the NAND gates in IC8 to provide control signals for the Tri-State logic switches in IC5 and selection of the proper display decimal point. OutlSuts C and C either sink or block current from overrange indicators LED 1 and LED2.
Assume that counters IC17 through IC19 have counted 999 pulses and the display reads ".999." Upon receipt ot the next pulse, the decimal point is shifted one position to the right and the display reads "0.00." Tri-State switch IC5 then passesthe +10referenceoutputof/C3 to decade counters /C17through IC19. One-shot IC21 and IC22 then produce a pulse which advances the most significant counter and (leftmost) display by one so that the displays now read "1.00." If necessary, this process is repeated once or twice, resulting in an autorange function of 1000:1. Alter the third counting sequence, the overflow pulses cycle the two overrange LED's to indicate a count of l 000 pulses.
The 7400 series IC's require + 5 volts, which is provided by the projects's power supply (Fig. 6). Transformer T1 re

 

duces the line voltage to a convenient value. The low-voltage ac is rectified by DI and D2 into pulsating dc and smoothed by C1. A regulated dc output at +5 volts is provided by IC23. Although the regulator IC can provide a 1 ampere output, the capacitance meter circuitry requires only about 700 mA. Construction. For the most part, the circuit is not critical and any assembly technique can be used to reproduce it. However, the measuring circuit comprising IC20 and its associated components is critical, and should be properly shielded and decoupled trom the other stages. Etching and drilling and parts placement guides

 

The 42 feedthrough points are identified by circles on the component placement guide (Fig. 8). Sockets or Molex Solercons are mandatory for the LED displays and decoder/drivers. By cutting a socket lengthwise or using Molex Soldercons on the outside pin rows, as shown in Fig. 9, a trough is provided under the displays and decoder/drivers into which the current-limiting resistors are placed. Numbering the holes from the center both up and down will allow quick resistor placement. For example, the leads of R2:1 occupy the second hole up and the first hole down. (See Fig. 9B.) Use small, 1/4

Fig. 5. Schematic of meter's autorange circuit.

for a suitable printed circuit board are shown in Figs. 7 and 8.
The pc board holds all components of the capacitance meter, less those in the power supply. It is a double-sided board on which many connections must be made between the top and bottom foil patterns. If you cannot make plated through holes, you must use wire feedthroughs to make the necessary connections. Component leads must be soldered on both sides of the board when pads are available.
Sockets or Molex Soldercons should be used to hold the integrated circuit and display packages. However, it is impossible to solder leads to pads on the component side of the board when they are under an IC socket. Because of this, all feed-through pads are accessible to the sides of the sockets. Molex Soldercons present no problem, as they can be soldered on both sides of the board.

 

watt resistors and, where necessary, insulate leads with sleeving. The critical components on the board are L l, C18 through C21, which determine the frequency of the reference oscil lator, and R1 through R4 which with IC20 form the basic capacitance measuring circuit.
High-quality polystyrene capaci tors and metal-film fixed resistors with temperature coefficients of less than 50 ppm/'C should be used. These components, to-gether with IC20, will determine the long-term accuracy of the meter and measurement error as a function of temperature. If high-quality components are used and the meter is properly calibrated, its accuracy will be at least 1 % at room temperature.
Checkout and Callibration. A properly functioning unit will respond as follows, and should then be calibrated. Rotate RI, R3, and R10 fully counterclock

Fig.6. Power supply circuit
has a voltage regulator IC

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