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Pretty Accurate Digital Wall Clock

.000202943 % slow or fast max.

Pretty Accurate Digital Wall Clock
The crucial component in this project is a Maxim IC type DS3231, qualified by its manufacturer as an “extremely accurate I2C real time clock (RTC) with integrated temperature compensated crystal oscillator (TCXO) and crystal.” Maxim also says that the integration of the crystal resonator enhances the long-term accuracy of the device, guaranteeing a maximum error of less than 64 seconds over a year, and over a temperature range 0 to 40 ºC (32 to 104 ºF). The device incorporates a battery input which maintains running of the device in the absence of external power.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor Members can exclusively download these files for free as part of their Membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service. Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

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The BOM (Bill of Materials) is the technically exhaustive listing of parts and other hardware items used to produce the working and tested prototype of any Elektor Labs project. The BOM file contains deeper information than the Component List published for the same project in Elektor Magazine. If required the BOM gets updated directly by our lab engineers. As a reader, you can download the list here.
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Combined for microcontroller board, display board
(5%, 0.25W)
R1 = 82Ohm
R2-R7,R15 = 27Ohm
R8-R14,R16,R22,R25,R28,R31,R34,R37 = 680Ohm
R17,R18,R19,R39,R40,R41 = 10kOhm
R20,R23,R26,R29,R32,R35 = 100kOhm
R21,R24,R27,R30,R33,R36 = 150Ohm
R38 = 47Ohm
R42,R44 = 100Ohm (optional, see text)
R42-R45 R43 = 1kOhm (optional, see text)
C1,C2 = 22pF ceramic
C3-C7,C10,C11 = 100nF
C8,C9 = 470µF 16V radial
D1 = LED, red, 5mm IR LED
D2 = LED, bicolor, e.g. Kingbright L-93WEGW
IC1 = LM7805CT
IC2 = TSOP31238 (Vishay Semiconductor)
IC3 = DS3231SN (Maxim Integrated Products)
IC4 = PIC16F876A-I/SP, programmed, Elektor Store # 110167-41
LD1-LD6 = SA15-11SRWA 7-segment display, CA (Kingbright)
T1-T15 = BC548
T16-T21 = BC327
BT1 = 3V Lithium button cell with holder.
K1 = 2-pin PCB screw terminal block, 0.2’’ pitch.
K2/K4, K3/K5 = 14 pin SIL connector pair (pinheader and receptacle).
S1,S2 = pushbutton, PCB mount, tactile feedback, e.g. Alps SKHHALA010.
X1 = 20MHz quartz crystal.
PCB # 110167-1 (microcontroller board).
PCB # 110167-2 (display board).

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