Photo-gate timer

Specifications

• Run on a battery.
• Include a multi-character display, at least four digits.
• Include a power switch to save the battery.
• Use a breadboard so it can be modified.
• Timing of periods of at least 10 s accurate to 0.01s

Circuit

Circuit diagram for a bicycle speedometer, as a placeholder for an apropos diagram once I make one

Power supply

We can use three AAA or AA cells which would provide 3.6V to 5.1V, or one lithium 3.7V cell. Or, we can use a 9V battery with a voltage regulator. A coin battery would not last a day if we left the switch on.

Program

1. Clear the display
2. Loop.
   1. Watch for a break in the first gate. If so, reset counter and continue to next loop.
3. Loop.
   1. increment a counter periodically. Use a period of 10ms or less.
   2. Watch for a break in the second gate. If so, continue to the next section.
4. Convert the counter to a proper number, if necessary.
5. Display the number
6. Return to the idling loop.

See also

Circuits for teaching physics