Tinker, Learn And Do Engineering With NI myDAQ - Lab 4: Photogate Fun

Publish Date: Apr 08, 2014 | 1 Ratings | 5.00 out of 5 | Print | Submit your review


As part of the National Instruments Tinker, Learn, and Do Engineering with NI myDAQ course ware, the hands on experiments demonstrate the wide range of possibilities of using NI myDAQ hardware.

The 10 lab experiments cover a wide range of topics including analog and digital circuits, sensors and signal processing of audio sound tracks.

View all of the labs for the Tinker, Learn, And Do Engineering With NI myDAQ course ware.

Table of Contents


Surging through the heart of microprocessors are digital pulses, single pulses, bursts of pulses, and wave trains of pulses. Pulses are the life- giving blood of all digital systems and of NI myDAQ.  Simple high or low signals transmit data based on their state, timing, rate, and pattern.  They are the simplest measurement a computer can make.

Fig. 1 Tachometer Built from Old CD, DC Motor, and Photogate

This Lab uses myDAQ to measure pulse counts, counts/second, or the pulse length. An infrared photogate will be built and used to measure the speed and acceleration of a small toy race car hurtling down a track.

Table of Contents

  1. Downloads and Required Components
  2. Background
  3. Exercise 4-1: Building the Photogate Circuit
  4. Exercise 4-2: Measuring Speed With the Photogate
  5. Exercise 4-3: Photogate Fun
  6. Related Links

Downloads and Required Components


Download the files above to access the courseware pertaining to this lesson that illustrate how the mDAQ can be used with NI LabVIEW covering a range of tools from introductory level exercises to design challenges.


  • NI myDAQ
  • TL072 or Equivalent Op Amp
  • 10 W Resistor
  • Solderless Breadboard
  • Optional: Matchbox toy race car and 3 cm x 3 cm card
  • Counter Circuits
    • Infrared LED SI5312-H or Equivalent
    • Phototransistor SI5811 or Equivalent (lens is black)
    • Resistors: Two 22 kW , One 4.7 kW, One 100 W, and One 68 kΩ
    • Transistor 2N3904

Prerequisite Reference Materials:


IR Photogates

An IR photogate uses a phototransistor to measure the amount of light from an optical beam originating from an IR LED. Infrared light is chosen so as to minimize any signal from ambient light sources often found in our normal working environment. We cannot see infrared light, but a phototransistor can. When the optical beam is blocked, the phototransistor changes from a high to a low state. This transition signals the start of a pulse. When the optical beam is unblocked, the phototransistor changes back to its normal state (i.e., from a low state to a high state). The low to high signals the end of a pulse. The pulse described is a negative pulse (HI-LO-HI). It is often inverted to (LO-HI-LO) by a transistor amplifier. The time between the start and end of pulse is the pulse length or width (Fig. 2).


Fig. 2 Pulse Definitions

Infrared Light-Emitting Diode (LED) (Optional)

The I-V characteristic curve of an LED is most informative. Build an I-V curve tracer following the circuit found in “V-I Characteristic II–Nonlinear Devices” by E. Doering: I-V Curve Tracer Video

This uses an op amp configured as a voltage follower (current booster), a 10 W resistor, a breadboard, and myDAQ.


Fig. 3 The characteristic I-V curve of an Infrared Light-Emitting Diode

In the forward-biased region, the current rises exponentially for V > 1.0 volts (Fig. 3). The IR LED can take a maximum current of 100 mA. In our circuit, the forward-biased current is about 40 mA {(5 V-1.0 V)/100 W}.

End of Background

Exercise 4-1: Building the Photogate Circuit

Assemble the circuit components for the counter circuit. Measure and record all of the resistors using the DMM(W). Build the following photogate circuit, as shown in Fig. 4.

Fig. 4 Photogate Circuit

MyDAQ provides the power supply (5 V and DGND) to bias the LED, the photodiode, and the single-transistor amplifier circuit.

Measure the voltage at the output points x and y using DMM(V), and use those voltages to fill in the chart:


  Vx Vy

Optical beam 



Optical beam 




Note: The range of the output Vy falls within the digital TTL logic levels (0 or 1).

The photogate output signal (DIO 0/1) can be input into the myDAQ digital input lines.

End of Exercise 4-1

Exercise 4-2: Measuring Speed with the Photogate

Building the Test Car

Find an old Matchbox or toy race car and secure a card (3 cm x 3 cm) to its roof (Fig. 5). Plastic airplane glue, silicon glue, or a hot glue gun can be used.  If no car is available, one can simply move the card manually over the photodetector.


Fig. 5 Small Racing Car with Card Glued to Roof

Align the photogate perpendicular to the car’s line of motion at a height so that when the car passes through the photogate, the optical beam will be blocked by the card. Just in case you don’t have racetrack lying around, you can build a ramp out of angled wood or cardboard. Adding sides to the ramp is helpful to guide the car down the track and through the sensor gate.


Measuring the Car’s Speed

Connect the photogate output to the myDAQ socket (AI 0+) and the circuit ground to (AI 0–-).

Use the oscilloscope Scope with the following settings:

  • Photogate Output to AI 0
  • Channel 0  Source AI 0, 1 v/div
  • Time Base 200 msec/div
  • Trigger-Edge, Channel 0 source, Level 0.2 V
  • Horizontal Position 10%
  • Acquisition Mode [Run Once]

Run the car past the sensor to view the photogate pulse.

Note: The intrinsic ‘Scope Timeout’ feature limits our ability to see the pulse on an expanded time scale.


Another approach uses NI ELVISmx Oscilloscope VI and provides more flexibility.

Launch and View the LabVIEW program Pulse Logger.vi.


    Fig. 6 Sequence of Pulses Displayed on a LabVIEW Graph

The sample rate has been set to 10 k samples per second in (Fig. 6). The record length has been set to 50 k samples. Thus, the recording time is 50 k/10 k (samples/samples per sec) or 5 seconds. This provides a window of time to catch the car hurtling down the ramp.

You can change the record length to suit your application.

Set the car on top of the ramp, let it go, and press [Run].  Observe the signal on oscilloscope, as in Fig. 7.

To expand the scale to better see the pulse, pick a time a little earlier than the start of the pulse. Replace the origin value of the photogate signal graph with this value.  The y-axis (amplitude) may also need to be adjusted.

Pick a time a little later than the end of the pulse time and replace the maximum time (5 seconds) with this value.

Note: Make sure AutoScale x-axis is unchecked.


Fig. 7 Zoom in on the x axis of the Pulse by Modifying the First and Last Values


Measure the start time, stop time, pulse width, and pulse amplitude from the trace.

To calculate the speed of the car, use the formula: length of the card (3 cm)/(pulse width).


Here is Another Way:

The DAQ Assistant Express VI can be configured to measure pulse width directly.

Open a blank LabVIEW VI and go to the block diagram panel.

Select the DAQ Assistant VI and place it on a block diagram.

Choose the following options:

  • Acquire Signals/Counter Input/Pulse Width/ctro/ [Finish].

Verify the final settings:

  • Starting Edge: Rising Acquisition
  • Mode: 1 Sample (on Demand)

Click [OK]

Note: Connect the Photogate output to myDAQ socket DIO 1 and circuit ground back to DGND

Load and view the LabVIEW program Pulse Width.vi to see how to finish your program. Click on [Run] and send the car down the ramp.  The program gives both the measured pulse width and the speed of the car as it zooms past the photogate.

Wow! That was easy.

End of Exercise 4-2

Exercise 4-3: Photogate Fun

Now that we have an idea of pulses and how to measure them, here are a few project ideas:

Events:  An event is signaled by a rising edge or a falling edge.

Hook your photogate up to detect the opening of a window or the refrigerator door (great for those on a diet). Once you have detected an event, sound the alarm: play a weird sound, activate a buzzer, play a recorded message or your favorite song.

Note: Sound generation is covered in Labs 7, 8, and 9.

Load and view the LabVIEW program Pulse Counter.vi to see how easy it is to count events.


Pulse:  A pulse is signaled by a rising edge followed by a falling edge (positive pulse), while the reverse order forms a negative pulse.

Have a race night. Build an interesting track. See who has the fastest car.

Extend the width of the photogate by replacing the LED source with a laser pointer. Now you can have a human race. Add a camera at the finish line for a photo-finish or sound a checkered flag.


Pulse Trains:  A sequence of uniform pulses (pulse + delay, pulse + delay, etc.) forms a rectangular wave or a square wave (if pulse width = delay). Pulse waveforms can be described with parameters like frequency, period, ‘On’ time, and ‘Off’ time.

A tachometer is a measurement of the angular speed of a wheel expressed in the number of pulses per second (frequency) or revolutions per minute (rpm). Attach a CD to a motor (Fig. 8). Cut a hole in the edge of the CD the width of the optical beam. As the disc spins, a wave train is produced by the photogate that is proportional to the angular speed.                             

End of Exercise 4-3

Related Links

  1. Go back to Lab  3: Touchless Electronic Lock, part of the Tinker, Learn, And Do Engineering With NI myDAQ course ware.
  2. Back to the Tinker, Learn, And Do Engineering With NI myDAQ course ware
  3. Watch this introductory NI myDAQ video to learn more about the educational design platform
  4. Find more courseware on NI ELVIS II, NI LabVIEW, and NI Multisim
  5. Reference: “Introduction to NI ELVIS II,” B.E. Paton, Lab 10 Mechanical Motion.


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