Photo-gate timer

A photo-gate timer featuring an LCD display, showing 3 of 5 possible gates at left. In a working lab, the LEDs and photo-transistors would have to be attached to the board via long wires so they could be placed usefully.

Specifications

• Connect up to 5 gates for four times (first gate zeros timer).
• Include a display with enough characters to show the time past each connected gate, 8x2 or 16x2.
• Include a power switch to save the battery, and/or a reset switch.
• Use a breadboard so it can be built by students, or modified.
• Allow timing of periods up to at least 10s, accurate to 0.01s.

Circuit

Circuit diagram for a photo-gate timer featuring an LCD display.

Components

• Photogates
  • LED, infrared (~$0.20). I use a Lite-On LTE-302-M, which has a black or gold 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. I use a PICAXE 20M2 (software timing) or a 20X2 (dedicated hardware timing for better accuracy); both have plenty of inputs and outputs for multiple gates and any type of display. ($4-$6)
  • Resonator circuit if necessary. Not available for PICAXE 20M2. (~$1)
• Display
  • LCD Display, serial capable. I use Newhaven's NHD-C0216CiZ-FSW-FBW-3V3 LCD with I2C ($10)
    • Two 1µF capacitors to aid voltage booster circuit.
    • Two 5kΩ resistors to pull up serial clock and data lines while idle.
    • One 50kΩ resistor to pull up reset.
  • Alternatively, a TTL-to-USB or TTL-to-serial circuit for sharing data with computer ($18)
• Hook-up wires ($2)
• Power supply
  • 9V or a quartet of AAA with a 3V voltage regulator, capacitor(s), resistor(s), or
  • 3V AC adapter, or
  • Two alkaline AAA or AA batteries.

Saving power

The LCD and micro-controller use little power compared to the infrared emitters, which are constantly on in this circuit as initially designed. So, one way to significantly reduce power would be to power only the next-expected-to-close gate. Another way to reduce power would be to pulse the emitters, which would allow for very bright IR light—This way would require periodic pulses of narrow width accompanied by polling of the receiving circuit.

Program

1. Clear the display
2. Set an interrupt on the pin attached to the first gate.
3. Loop.
   1. Watch for a break in the activated gate. Go to interrupt routine when broken.
   2. Watch special button for reset. If pressed, start over.
4. Interrupt
   1. If first gate, reset counter
   2. if other gate, save counter as time for that gate, display that value in the appropriate place.

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

See more discussion and other circuits at Circuits for teaching physics.

For a discussion of the circuit around the photo-transistor, read An Arduino-Controlled Photogate by Calin Galeriu of Becker College, Worcester, Massachusetts, in The Physics Teacher, v51:n3:pp156-158. http://dx.doi.org/10.1119/1.4792011