EXPERIMENT 11

Introduction to the 555 Timer, LEDs and Photodiodes

In this experiment, we learn a little about some of the new components which we will use in the next project. The first is the 555 timer. This is a very useful little device that can produce trains of pulses in a controlled manner. We will not be able to address exactly how it works in this experiment, since we need to go further into how op-amps work and digital electronics. However, the basic rules that govern its behavior are not that difficult to figure out. We have been studying diodes, but have not directly considered how we can use them to generate and detect light. We will perform a simple experiment to see how this is done.

Part A Controlling Oscillation Frequency with Resistors and Capacitors

We have seen that when current flows through a series combination of a resistor R and a capacitor C, that the circuit responds with a characteristic time constant = RC. It turns out that we can use this effect with a 555 timer chip to make an oscillator. The circuit we will use is shown below (at least in the form we will use for PSpice simulation). This chip has 8 pins with some relatively strange sounding names (see figure). The DC voltage source connected to pin 8 sets the value of the voltage called VCC. In this case VCC = 12 volts. By selecting just the right values for the resistors and capacitors in this circuit, we can make the voltage at pin 3 (the OUTPUT) go from zero to VCC in a repeated pattern that looks like a square wave. The operation of the 555 timer is as follows. When an applied THRESHOLD voltage (pin 6) is V₆ (2/3)VCC, then the OUTPUT voltage V₃ is low (about equal to zero) and, in addition, a switch (transistor) that is internally connected between the DISCHARGE (pin 7) and GND (pin 1) is turned ON. Conversely, when an applied TRIGGER voltage (pin 2) is V₂ (1/3)VCC, then the OUTPUT V₃ is high (about equal to VCC) and, in addition, the DISCHARGE transistor is turned OFF. When the RESET voltage (pin 4) V₄ is low, the DISCHARGE transistor is ON. However, the reset is disabled when the RESET pin is tied (directly connected) to the supply voltage.

Note that the drawing of the circuit has been split in half. This was the only way to make it large enough so that it can be read easily. Just think of it as being connected.

Simulation of the Astable Multivibrator

Draw the circuit shown.

Perform a transient analysis in increments of 2us up to 10ms.

Use PROBE to plot the control and output voltages. The control voltage is either pin 2 or pin 6 while the output is taken at pin 3.

Print your PROBE plot.

Verify that the timer output changes according to the rules listed above.

Find the time period that the output is ON (about VCC) and the time period that the output is OFF (about zero). One of these times should be equal to .693(R1+R2)C1 while the other should be equal to .693(R2)C1. [(R1+R2)C is the time constant of R1, R2, and C in series, while (R2)C is the time constant of R2 and C in series.] Which is which? What is the total period of this output (the sum of the two times)?

Change the value of R1 to be R2/10. Repeat the transient analysis. Is the time that the output is ON look closer to the time that the output is OFF than it did when the two resistors where the same? What is the total period of the output now?

Design an Oscillator

Using the circuit above, pick some component values that will produce an output with a frequency equal to about 500Hz, making it look as much like a square wave oscillator as possible. Check your component selection with a PSpice simulation.

Part B Diodes and Light

In the circuits shown above, Q1 is a phototransistor and the diode is and LED. We will address the phototransistor circuit first.

Photodetection

Build the first circuit. The TAs will show you which wires to connect. Connect the output (the voltage across R1) to one channel of your scope. The signal you should be seeing is due to the lights in the classroom. Block the light going to the transistor and confirm that the sign at least becomes smaller. You may be also picking up some electrical noise related to the lights. Do not take this circuit apart at this time.

Light Emission from an LED

Assemble the second circuit on your protoboard. Use the function generator as the source. Select a red, yellow or green LED for this circuit. Do not use a white LED.

Adjust the function generator to produce a square wave with a frequency of 1 Hz so you can observe the diode going on and off. Raise the amplitude until the diode lights up. Then change the frequency to 1kHz.

Connect the output of the function generator to the other channel on your scope. The phototransistor circuit should still be connected.

Making sure that the LED is aimed at the phototransistor, observe the coupling between these two components.

At 1kHz, find the minimum squarewave amplitude that produces a signal in the phototransistor circuit.

Last Updated on 1 March 1998