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). I use a Lite-On LTE-302-M, which has a black dot to identify itself as an emitter; it also allows up to 50mA of constant current, or even 1A if pulsed 300 10µs pulses per second, so it can be used at a greater distance than 20mA emitters. The cathode is the short lead, on the left as you're facing the lens.
  • Phototransistor, infrared (~$0.20). I use a Lite-On LTR-301, which has a red dot to identify itself as a receiver. The collector is the long lead, on the right as you're facing the lens, the short lead being the emitter of the transistor.
  • Switching transistor, 2N2222 or equivalent NPN (~$0.10). I use an R3309, whose leads are emitter, collector, and base, left to right, as seen leading down when looking at the flat face.
  • Resistors
    • LED limiting resistor (4V/40mA=100Ω; 4V/20mA=200Ω; 2V/40mA=50Ω) (~$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