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PARTS LIST
C l -4000-~&, 16-V electrolytic capacitor
C2,C4,C8 through C16, C23--0.01-IiF disc ceramic capacitor
0-0.0033-~&, l 0% Mylar capacitor
C5--O. l ~& disc cerannic capacitor
C6,C 17-4.7- ~LF, 16-volt tantalum capacitor
C7-220-ILF, 16-volt electrolytic capacitor
C 18-0.0 1 -~&, 5% polystyrene capacitor
C19-820-pF, 5% polystyrene capacitor
C20--470-pF, 5% polystyrene capacitor
C21-220-pF, 5% polystyrene capacitor
(722_0.005-~&, 10% Mylar capacitor,
D1,D2-IN4002 silicon diode
D3 through D5-IN4154 or HEP R0600 sili- con fast-recovery diode
DISI through DIS3-DL707 common-anode, seven-segment LED display
Fl, F2-1/4-ampere fast-blow fuse
ICI,IC2,IC3,ICl7,ICl8,ICl9-7490 decade counter
1C4,IC 15-7404 hex inverter
IC5-741 25 Tri-State quad buffer
IC6,IC20-555 timer
IC7,IC8,IC22-7400 quad Two-input NAND- gate
IC9, IC l 0, IC 11-7447 BCD to seven-segment decoder/driver
IC l 2,IC 13-7474 dual D edge-triggered flip- flop
IC 14, IC21-74121 monostable multivibrator
IC 16-7493 4-bit binary counter
IC23-LM309K 5-volt regulator
L l - l 3-ILH inductor
LED 1, LED2-20-mA light emilting diode
R l - l 00,000-ohm pc mount trimmer potenti- onneter
R2-1-megohm, 1% tolerance, 50 ppm/'C metal film resistor
  R3-100-ohm pc mount trimmer potentiometer
R4-1000-ohm, 1% tolerance, 50 ppm/'C metai film resistor
RIO-25,000-ohm, panel mount linear taper potentionneter
The following are 1/4-watt, 5% tolerance carbon composition resistors.
R5-1000 ohms
R6, R7- l 00,000 ohms
R8, R9-1500 ohms
R l I,R l 2,RI3-100 ohms
R 14, R 15-3300 ohms
R 16 through R20--470 ohms
R2:1, RI:2, R3:3, R5:4, R4:6, R7:5, R6:7 (one set for each of three decades)-330 ohms
S 1-3-pole, 3-position rotary switch
T l - l 6-volt center-tapped transformer
Misc.-Suitable enclosure, banana jacks or binding posts for Cx terminals, printed circuit board, fuseholders, knobs, hook-up wire, IC sockets or Molex Soldercons, hardware, solder, etc.
Note-The following items are available from Dage Scientific Instruments, Box 1054, Livermore, CA 94550: CM-6 complete kit of parts, including tested IC's, cabinet, bardware, miscellaneous items, calibration capacitor, arid assembly manual, $69.95 in U.S. arid Canada. CM-68 partial kit iincludes etched arid drilled double-sided pc board, l 3-~i.H inductor, polystyrene capacitors (C18 through C21), calibration capacitor, arid assembly manual for $20 in U.S. arid Canada. U.S. residents add $1 postage arid handling, Canadians add $2. Californians add sales tax.

or twice, resulting in four automatically selected ranges. Additional overflow pulses are displayed by two LED's located to the left of the display.

Circuilt Detalls. Refer to the ap propriate schematic (Figs. 2 through 6) for the following detailed circuit description. Free-running 555 timer IC20 (Fig. 2) is the basic capacitance mea suring circuit, comprising the comparators, reference voltages, flip-flop, and discharge transistor described previously. The timer's discharge period is used to measure the component under test. When MODE switch Sl is in the nF position, the discharge period is determined by Rl, R2, and CX. In the ~LF position, the interval is determined by R3, R4, and CX.

 


Fig. 2 Input stage has
free-running 555 timer


A second free-running 555 timer, IC6 Fig. 3), is employed in an auto cycling circuit which autornatically updates the capacitance mea surement. The reference frequency (about 1.4 MHz) is sup

plied by a Colpitts oscillator made up of IC4, Ll, and C18 through C21. Signals from the reference oscillator and timers IC6 and IC20 are combined by dual-D flip-flops IC12 and IC13. One half of IC12 synchronizes the output of IC20 with the 1.4-MHz reference frequency, providing dual-phase (0 and 0) outputs. The other half of IC12 and IC13 select one discharge pulse from IC20 after the output of autocycle timer IC6 goes high. The flip-flops disable IC6 until the discharge pulse is completed. The reference oscillator output is gated by IC7 so that it passes to the counting stages during one discharge pericid of CX per measuring interval. Moncistable multivibrator IC14, when triggered by

Fig.3. Oscillator, sync.
and reset circuitts

the leading edge of the synchronized discharge pulse, resets decade counters IC 16 through IC 19 and dividers IC 1 through IC3. When S l is in the nF POSition, the width of the reset pulse generated by IC14 is controlled by the setting Of ZERO trimmer potentiometer RIO. This allows the user to keep stray capacitance out of the measure ment. The gated reference signal is divided by decade counters IC1, IC2 and IC3. Output signals from these counters, at 1 / 1 000th, 1 / 1 00th, and one-tenth the input frequency, are applied to Tri-State logic switch IC5 (Fig. 5), which passes the appropriate pulse train to decade counter IC19. Overflow pulses from this BCD decade counter are applied to counter IC18, whose overflow pulses in turn are counted by ICI 7. Binary coded. decimal outputs from these three decade counters are decoded by IC9, IC10


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