Difference between revisions of "Photo-gate timer"
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===Specifications=== | ===Specifications=== | ||
| − | * | + | *Connect up to 5 gates for four times (first gate zeros timer). |
| − | *Include a | + | *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. | + | *Include a power switch to save the battery, and/or a reset switch. |
*Use a breadboard so it can be modified. | *Use a breadboard so it can be modified. | ||
| − | * | + | *Allow timing of periods up to at least 10s, accurate to 0.01s. |
===Circuit=== | ===Circuit=== | ||
Revision as of 17:40, 9 March 2014
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 modified.
- Allow timing of periods up to at least 10s, accurate to 0.01s.
Circuit
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
- Clear the display
- Loop.
- Watch for a break in the first gate. If so, reset counter and continue to next loop.
- Loop.
- increment a counter periodically. Use a period of 10ms or less.
- Watch for a break in the second gate. If so, continue to the next section.
- Convert the counter to a proper number, if necessary.
- Display the number
- 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
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
