Warren Wilson Physics Activity Guide

Photogate Circuit

INTRODUCTION A photogate circuit is a circuit which sends a signal to an electronic counter or computer when an optical beam is blocked. Such a device is useful for precision timing of masses in motion to determine speeds to a fairly high precision. It is also referred to as an "electric eye". The photogate consists of a light-emitting-diode (LED) and a phototransistor. A phototransistor is a transistor with a transparent package and is turned ON by light illuminating the base. In order to use the photogate in ambient room light, the photogate circuit usually uses infrared light - light invisible to the eye and absent in ambient room light.

LED. The LED light source is a special PN junction which has a turn-on voltage corresponding to the energy of visible light. An infrared LED (IRLED) emits invisible infrared radiation (wavelength ~ 1000 nm). The wavelength is set by the manufacturer by controlling the energy band gap in the diode. Please look-up Light Emitting Diode in your text.

Phototransistor. A phototransistor is a transistor in which the base current is created by light shining on the base. Light energy creates electron-hole pairs by excitation of the electron bonds in the semiconductor. The photo-electrons in the P-type base are minority carriers. Their presence turns on the transistor - i.e. cause it to begin to conduct from collector to emitter. The action is similar to last week's transistor circuit except that last week's transistor is controlled or "turned-on" by means of base current. The phototransistor, on the other hand, is controlled or turned-on by light instead of current.

Complete Circuit. The output of the phototransistor will be amplified by last week's transistor. The complete circuit is shown in the accompanying diagram. The logic of the circuit follows: When the IRLED emits radiation which illuminates the IR photo transistor, this transistor conducts (turns-on) and its collector voltage drops. The collector voltage is connected to the base of the 2nd transistor (Q₂) (2N3904) through the R_B2 resistance. Thus the base of Q₂ drops in voltage. The drop in base voltage turns off Q₂ and the Q₂ collector voltage rises. When the light path is blocked, the phototransistor turns OFF, and the collector of the phototransistor rises.  With the large (or Hi) value for voltage at the collector of Q₁, determine what the current into the base of Q₂ becomes - thus complete the truth table below:
 

If you have completed the truth table properly, only one transistor conducts at a time. If not try to find the fault in your logic.

PROCEDURE

Visible LED.

• Connect a red LED to the circuit as shown. Make sure that the 100 Ohm resistor is connected in series with the 5 Volt voltage source, otherwise the LED will burn out from too much current.

• Connect the LED in both directions and note what happens.

• Predict the voltage across the LED in the series circuit in the

• Forward direction

• Reverse direction.

• Measure the voltages and tabulate the results.

• Measure the LED with the Ommeter on diode mode. Explain the results. Remember: the diode must be un-biased when using the Ohmmeter, otherwise the Ohmmeter results are meaningless.

IRLED.

• Connect the IRLED to a bias circuit as similar to the visible LED.

• Verify that the IRLED works by photographing it with the IR sensitive video camera and monitor. You may have to experiment with the leads of the LED.

• Measure the voltage across the LED when activated. Since voltage represents the ENERGY per unit charge, what can you say about the energy needed to produce visible light compared to the energy to produce infrared light.

Couple IRLED radiation to Phototransistor

Attach the IRLED and the IR Phototransistor to the photogate yoke by soldering.  The yoke consists of an insulated perforated board and terminals to which the components may be soldered.  In soldering the circuit, it is important to apply heat to the joint, then apply solder.  Bend the leads of the diode and transistor so that the diode and transistor face each other.
  Verify your predicted truth talbe by measuring the V_c1 when the photogate is blocked and unblocked. If no change is detected, you should adjust the wires so that better optical coupling is established. This is critical! Compare with your prediction in 2A above.

Transistor amplifier. Connect the collector of the phototransistor to the 2nd transistor (the 2N3904) according to the diagram on Page 1.. Notice that the base resistance of the 2nd transistor should be 22 K Ohm instead of the 100K Ohm used in the transistor lab. Measure the collector voltage of the 2nd transistor when the IR beam is blocked and unblocked. Explain your results. If the results don't match the introductory notes, make sure you have the circuit wired correctly, then consult the instructor.

Summarize your results in a table listing the two photo-states (blocked/unblocked) and the collector voltage of each of the two transistors. See marker board for suggestions.

Explain your results on the basis of transistor action and by thinking of the transistors as switches.

Oscilloscope measurement.

1. Mount the gate on a ring stand and connect an oscilloscope to the collector of the phototransistor.  Insert a segmented pulley as a sector wheel in the gate and spin the wheel while monitoring the oscilloscope trace.  The oscilloscope is a graphical voltmeter which displays the voltage vs. time.  Note the pattern as the spinning sector wheel slows down.

2. Attach the oscilloscope probe to the output of the second transistor and compare the pattern with that from the first transistor.  Explain what happens in terms of the amplification qualities of the second transistor.

vocabulary list
 

Questions (due Thursday)

Why does a current through a LED emit light as it goes through the junction?

When no light shines on the phototransistor, explain why it does not conduct. Look-up the transistor activity.

What happens in the base of the phototransistor when light of the proper energy strikes it?

Explain why a photocurrent is produced in the biased phototransistor when light illuminates the base.

Follow the logic of the complete circuit to explain the output voltage of the 2^nd transistor for both states of the phototransistor: illuminated and blocked. Place results in a “truth table”.