Frequency meter

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 1 Hall effect switch to watch a magnet passing once per cycle.
• Include a display with at least 4 characters.
• 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

• 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 5V voltage regulator, capacitor(s), resistor(s), or
  • 120V AC to 5V DC adapter, or
  • Three alkaline AAA or AA batteries.

Program

For an explicit example, see Frequency meter with a PICAXE 08M2.

1. Initiate the display
2. Set an interrupt on the pin attached to the first gate. Will interrupt the program wherever execution is and go to Interrupt.
3. Loop.
   1. Increment time counter.
   2. Go back to Loop.
4. Interrupt

See also

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