User's Guide for the Dangerpants Labs Darktimer

Written by Laura Johnston
Posted August 29, 2015

Introduction

Congratulations on your construction of the Dangerpants Labs Darktimer! This specialist timer allows you to set a large number of sequential timers, then trigger them in order without requiring any interaction other than pressing the GO button. It provides you with audio feedback to help determine timing of a variety of events within the duration of a timer's countdown.

Overview

The Darktimer's exact construction will depend on how you have configured your device. However, all Darktimers will share the same basic set of features:

LCD: The LCD is used to show the current state of the timer, display menu options, and assist when setting up timers.

The LCD normally shows the Timer number on the top line, and details of that timer on the bottom line. The bottom line also shows menu options.

GO button: This button is used to start timers, as well as acting as a kind of "enter" or "execute" button.

NEXT button: The NEXT button is used to move the cursor among different options. While a timer is running, it also acts as a "pause" and optionally as a "reset" button to set a timer back to the start.

PLUS button: This button increments values. It will repeat to quickly advance a number.

MINUS button: This button decrements values. It will repeat to quickly decrement a number.

Speaker: The speaker is used to generate the tones which provide darkroom-friendly feedback as to the operation of the timer.

Power switch: To turn the timer on or off and preserve battery life.

Normal Operation

Normal operation consists of two distinct actions: programming timers, and using timers.

Programming Timers

On initial power-up or after deleting all timers, press NEXT to select ">Add", and press GO to start creating a new timer.

Press PLUS or MINUS to modify the countdown time.

Press the NEXT button followed by PLUS or MINUS to modify the double-beep interval.

Press GO at any time to save the timer. Pressing NEXT until the cursor is cleared also saves the timer.

Once at least one timer is saved, the Edit and Delete functions are available. Edit and Add work as described above. Edit modifies the current timer, and Add adds a new timer after the current one. Delete deletes the current timer, asking for confirmation before making the change.

Press NEXT once to select the timer number, and the PLUS and MINUS keys to select a particular timer. Timer specifics are shown on the bottom line.

Running Timers

Press the NEXT button to move the cursor to the timer number, and press PLUS or MINUS to select the timer you wish to run. Once the timer is selected, press GO to start it. If the correct timer is already shown, press GO to start.

Once a timer is running, press the NEXT button to pause. GO will resume, and NEXT again will reset the timer back to its initial value.

Audio Feedback

The Darktimer provides extensive feedback via the speaker, to help you keep track of the time without having to look at the display.

1 second marker: Once per second, a short beep is sounded.

Interval marker: Each timer can be configured with a double-beep interval marker. It will double-beep when the countdown is a multiple of the interval from done. That is, if the timer is 1 minute and 12 seconds long, and the interval is set to 10 seconds, it will double-beep at the 1:10 mark.

Minute marker: At the top of every minute (when the display is showing 00 seconds), a longer beep is sounded. If it's the final minute, a longer beep with a higher pitch is sounded, to indicate the timer is nearly finished.

Final 5 second marker: The final 5 seconds will be marked with a higher pitched triple-beep.

End of timer marker: The end of the timer is marked with three long beeps.

Keypress marker: Pressing a key results in a beep, to help confirm the press was received. If a function can't be done (such as pressing GO when there are no timers, or trying to advance beyond the final timer), a triple-beep will sound to indicate the problem.

Troubleshooting

The most likely problem in a system this simple is low battery. The timer will continue to operate (responding to keypresses, sounding timer beeps) long after the LCD has stopped displaying characters. If the LCD is difficult to see, that's your indicator to recharge or replace the batteries. As of v1, the system doesn't have a way to measure battery voltage, and will not explicitly warn of low battery. A 2000 mAh NiMH cell should last for at least 20 hours of continuous operation.

The next most likely problem is a poor connection, so if buttons fail to respond, or the LCD doesn't seem to be working right, ensure that the solder joints are sound (test with a multimeter if you're unsure).

Accuracy

The Dangerpants Labs Darktimer uses the Arduino's CPU crystal as its timer source. This is usually pretty accurate for short durations such as the Darktimer uses, but this is by no means a precision timer. A typical crystal in this service can be out by as much as 100 ppm, according to manufacturer's specs. Not much, but enough to disqualify the Darktimer as a precision instrument.

The Darktimer should compare favorably with most commercial timers of this quality level (kitchen timers, primarily), and should be accurate enough for darkroom use.

However, before you commit film to this timer, check it against a known-good timer, such as on a smartphone, or a normal electronic kichen timer. Inexpensive Arduino clones, while amazing values, have been known to cut corners on components, and the crystal is one of those which may suffer from this lack of quality, resulting in a timer which runs noticeably fast or slow.

See the construction guide for information on tuning your timer, if it's noticeably inaccurate.