Part XVII Overview: Build the Pure Silver Timer

An enlarger has no shutter. The lamp is the shutter, and the exposure lasts as long as somebody leaves it on. That sets this part four jobs a kitchen timer does none of: repeat an interval closely enough that two prints from one negative match; generate a series of intervals in equal visible steps; change an exposure by a stated amount and have that instruction still mean the same thing at a different enlargement; and do all of it with both hands over the easel in a nearly dark room.

The rule of the part

The timer has one output and it is at extra-low voltage. Part XVI put every lamp this course builds — enlarger head, safelight, contact printer, UVA unit — on the low-voltage side, so the timer switches a transistor and nothing else. HSE’s first control is the course’s: limit the supply voltage to the lowest that will do the job, because 230 V AC can kill and the risk is greatest in wet surroundings, which is what a darkroom is. No mains conductor enters this instrument, and no page in this part opens a mains enclosure.

What the survey took out of the design, and why you are being told

Until 5 September 2026 this part promised two outputs. The second was an isolated dry contact, whose only purpose was to close the trigger of a bought, certified, sealed mains appliance switch so that a legacy tungsten enlarger could be timed. Whether such a product could be bought at hobby cost was open question 1.4 in the course’s own register, and the rule was fixed before the search began: if no acceptable product exists, the build changes, not the safety rule.

The survey ran against five criteria written in advance and found none. The pattern is not an accident: the devices that take a low-voltage trigger arrive on screw terminals for an installer, and the sealed plug-and-socket appliances take no trigger at all. RH Designs say it about their own timer in one sentence — its foot switch exactly replicates the operation of the Start/Stop key, which starts the StopClock’s own programmed exposure and can carry nobody else’s.

So the mains half is removed rather than deferred, and a legacy tungsten enlarger gets three routes and no fourth: convert the head to low voltage as Part XVI describes; leave its own switch in manual use and count the exposure by ear on the metronome; or buy a certified enlarger timer with its own switched socket — £112.20 to £429.00 on 5 September 2026 — and let it own the mains.

The pages, and what each must deliver

Page	Must deliver	Proved by
F-stop timing	Why a series is geometric, and the arithmetic of one	A worked strip checked against the paper’s own ISO(R)
Switching lamps safely	The boundary, stated once for the whole course	The survey, criterion by criterion
The timer hardware	An instrument workable by touch that does not fog paper	The fog test against a control sheet
The timer firmware	Intervals that can be trusted and a log of every one	Loopback measurement and a soak test
The switched output	One channel, sized, fused and thermally checked	Command-to-light delay and an interlock test
Calibrating the timer	The shortest exposure the whole system can honestly repeat	Three measurements and a certificate
The timer that lies	Recognising an exposure fault from a chemistry one	Discriminating tests on one session’s evidence

The order, and why it is that order

Arithmetic first, because a timer resolving a tenth of a second is useless to somebody who does not know why a test strip must be geometric. The boundary next, because it decides what the instrument may be before any of it is soldered. Then hardware, firmware and output, each the thing the next one tests — and measurement last, because you cannot calibrate an instrument that cannot yet log.

Safety framing

Level A for the hardware and the firmware: extra-low voltage, a soldering iron and print chemistry under Part XIV’s rules. Level B for the switched output, the calibration and the break/fix page — earned by the UVA array and the high-output LED head rather than by mains work, because there is none.

What you need first

Part XIV’s electronics primer, assumed in full and never re-taught: the Pico, the low-side MOSFET, the busy-wait, the datasheet discipline. Part XIII for what a stop means on paper. Part XVI for what is being switched and for the fog test this display must survive. Part XV for the calibration.

Cost, and what the planner cannot price

The builds sit in the planner’s stage 4, and the planner carries no electronics line at all: the parts for Parts XIV to XVII are among the gaps it names. Every page here therefore quotes a specification rather than a price and reads its subtotal as a floor. The one dated figure is the bought alternative.

Alternative route

Three of these seven pages need a dark room. Without one the part still works: the arithmetic and the safety lesson need nothing, the hardware and firmware are bench work, and a bought timer replaces the build with the calibration page applying unchanged — a bought instrument’s offset and repeatability are as unmeasured as a built one’s. What no route substitutes is the fog test on your own display. If you cannot run it, the emission is unknown and the instrument goes outside the room.

!One limit to hold from the first page

A tenth of a second is a specification, not an achievement. Whether this timer delivers one depends on the lamp it switches, and the answer differs by an order of magnitude between an LED head and a tungsten one. The calibration page measures it; until then nothing here claims a resolution it has proved.

The pages of this part

Part17 of 28Level3 — AdvancedPages7Estimated time17.0 hoursHighest safety levelLevel B

✱4 of 7 pages in this part need a darkroom, a UV source or mains-powered equipment, marked below. Each says what can be improvised and, where one exists, gives an alternative route.

0 / 7 lessons in this part completed

• LessonF-Stop Timing and the Mathematics of Print Exposure60 min
• LessonSwitching Lamps Safely: Isolation, Certified Modules and the Mains BoundaryB60 min
• BuildBuild: The Timer HardwareA✱ Darkroom240 min
• BuildBuild: The Timer FirmwareA240 min
• BuildBuild: The Switched OutputB✱ Darkroom · UV180 min
• ExperimentExperiment: Calibrating the TimerB✱ Darkroom150 min
• Break/fixBreak/Fix: The Timer That LiesB✱ Darkroom90 min