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3 <title>ClunkPi: driving pulse clocks with a Raspberry Pi</title>
7 <p>ClunkPi is an add-on board to interface a Raspberry Pi to a
8 pulse clock. It is designed to fit all models of Raspberry Pi,
9 from the Model A onwards. It can operate any clock that's
10 compatible with the British Post Office class A system,
11 including movements made by Gents, Synchronome, and English
12 Clock Systems. ClunkPi can be driven by the
13 <a href="/clunk/">Clunk</a>
14 software to provide an NTP synchronised pulse clock.</p>
15 <h2>Specifications</h2>
16 <p>ClunkPi fits the 26-pin I/O header on the original Raspberry
17 Pi. It should also work on the first 26 pins of the 40-bit
18 header used from the Model B+ onwards. The board is smaller
19 than either a HAT or a uHAT, so it should fit many cases.</p>
20 <p>ClunkPi is essentially a switchable current source. It
21 provides a current of 250 mA ±10% into a resistance of up
22 to 12 Ω. It is switched by GPIO line 17 from the Pi. It
23 is powered by the Pi's 5 V line, from which it needs
25 <p>In theory, it should be possible to run ClunkPi off a higher
26 voltage to drive multiple clocks, up to the 50 V limit of
27 the drive transistor.</p>
29 <p>I designed ClunkPi using
30 <a href="http://kicad-pcb.org/">KiCad</a>. The KiCad source
31 files, along with the Gerber files for PCB manufacture, are
32 available for download.</p>
33 <h2>Circuit description</h2>
34 <p>The circuit is a simple current source controlled by a shunt
35 voltage regulator. U1 and R2 control the base current in Q1 to
36 achieve 1.24 V across R1. D1 suppresses any voltage spike
37 caused by the inductance of the clock coil.</p>
39 <p>Plug the board into a Raspberry Pi with the square solder pad
40 connected to pin 1 of the GPIO connector. Connect the clock
41 (either way around) to the pads labelled "J2".</p>