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

Components

• Photogates
  • LED, infrared (~$0.20)
  • Phototransistor, infrared (~$0.20)
  • Switching transistor, 2N2222 or equivalent NPN (~$0.10)
  • Resistors
    • LED limiting resistor (~100Ω) (~$0.02)
    • Phototransistor resistor (~5kΩ) (~$0.02)
    • NPN base resistor (~50kΩ) (~$0.02)
    • NPN collector resistor (~1kΩ) (~$0.02)
• Breadboard (~$5)
• Micro-controller ($2-$10)
  • Power circuit if necessary ($1)
  • Resonator circuit if necessary (~$1)
• Display, serial capable ($10), or
• TTL-to-USB or TTL-to-serial for sharing data with computer ($18)
• Hook-up wires ($2)

Power supply

Using a 3V LCD display will require us to use one 3V coin cell or two AA or AAA batteries. Or, we could use a 5V phone charger with a set of enough diodes to bring the voltage down, or find a rarer 3V plug-in power supply.

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.

Datasheets

Newhaven Display liquid crystal display module, with side white backlight and transflective display.

http://www.newhavendisplay.com/specs/NHD-C0216CiZ-FSW-FBW-3V3.pdf

See also

Circuits for teaching physics