A working example · For Mrs Froggy, who asked.

Mrs Froggy asked what a proper specification looks like — one that passes the three-implementer test. Here is a real specification for a small garden pond pump controller. Give this to three different engineers and you should get three functionally identical devices.

1. Purpose

Automate the operation of a submersible pond pump to maintain water circulation, aeration, and filter function in a garden pond. The controller must protect the pump from running dry and must allow the pondkeeper to set operating hours.

2. Scope

This specification covers the electronic controller only — not the pump, the plumbing, or the pond liner. The controller interfaces with any standard 240V submersible pump rated up to 1.5A (360W).

3. Functional Requirements

3.1 Power and Safety

• The controller shall accept 220–240V AC, 50Hz mains input via a IEC C14 socket.
• The controller shall provide a grounded (Class I) IEC C13 outlet for the pump.
• A replaceable 2A fuse shall be fitted in the live line, accessible from the exterior.
• The controller shall have a physical on/off switch that disconnects both live and neutral.
• All exposed metal shall be bonded to earth.

3.2 Water Sensing

• Two stainless steel probes shall be provided, 300mm cable length, terminating in screw terminals marked PROBE_H and PROBE_L.
• When the resistance between the two probes is below 50kΩ, the controller shall consider the pond "full" and permit pump operation.
• When the resistance exceeds 200kΩ (probes dry or pond empty) the controller shall cut power to the pump within 2 seconds and enter Dry Protection mode.
• The probe circuit shall use a 5V DC sense voltage (not mains) with no exposed live parts.

3.3 Timer Operation

• The controller shall support two timed periods per day: ON_1 and ON_2.
• Each period shall have a configurable start time (hour and minute, 24h format) and duration (1–480 minutes, in 1-minute increments).
• Default: ON_1 at 08:00 for 120 minutes; ON_2 at 18:00 for 120 minutes.
• Settings shall persist across power loss (non-volatile memory).

3.4 Manual Override

• A momentary push button labelled MANUAL shall start the pump immediately for 30 minutes, overriding the timer but not disabling it.
• Pressing MANUAL while the pump is already running in manual mode shall reset the 30-minute countdown.
• Dry Protection shall override Manual mode at any time.

3.5 Indicators

• Green LED labelled POWER: illuminated when mains power is present and the fuse is intact.
• Blue LED labelled RUNNING: illuminated when the pump outlet is energised.
• Red LED labelled DRY: illuminated when Dry Protection mode is active. Shall flash once per second.

4. Physical Specifications

• Enclosure: IP54 minimum, polycarbonate, wall-mountable, with four fixing holes on a 100mm x 60mm grid.
• Dimensions: no larger than 180mm x 120mm x 80mm.
• Cable entry: two PG9 glands on the bottom face — one for mains in, one for pump out.
• Probe cable entry: one PG7 gland adjacent to the mains glands.
• Operating temperature: 0°C to 50°C.

5. User Interface

• Two push buttons (SET and ADJUST) plus a 16x2 character LCD with backlight.
• Normal display shows current time, next scheduled ON time, and pump status (RUNNING / IDLE / DRY).
• Pressing SET enters configuration mode. The display cycles through: set clock → set ON_1 start → set ON_1 duration → set ON_2 start → set ON_2 duration.
• In configuration mode, ADJUST increments the selected value. Holding ADJUST for 2 seconds accelerates the increment.
• Configuration mode times out and saves after 30 seconds of inactivity.

6. Non-Functional Requirements

• Standby power consumption: less than 0.5W (pump off, display on, no backlight).
• Relay switching life: minimum 100,000 cycles at rated load.
• The controller shall withstand a 30-minute continuous pump run at 40°C ambient without enclosure temperature exceeding 60°C.

7. Acceptance Criteria

1. Connect a 60W incandescent lamp to the pump outlet. Set ON_1 to start 2 minutes from now for 5 minutes. The lamp shall illuminate at the set time and extinguish after 5 minutes.
2. Short the probe terminals with a 47kΩ resistor. The DRY LED shall not illuminate. Remove the resistor. The DRY LED shall illuminate and the lamp shall extinguish within 2 seconds.
3. Cycle mains power. All timer settings shall be retained.
4. Press MANUAL. The lamp shall illuminate for 30 minutes. Press MANUAL again after 10 minutes. The lamp shall remain illuminated for a further 30 minutes from the second press.

8. The Three-Implementer Test

Give this specification to three engineers: a hardware designer, a firmware developer, and a hobbyist with a soldering iron. If all three build you a controller with the same LEDs, the same timing behaviour, the same dry-protection response, and the same connector pinout — the specification passes.

If one of them asks "what colour should the enclosure be?" or "can I use RJ45 instead of screw terminals for the probes?" — the specification was incomplete, and the gap was in the exact place you can now go back and fill.