EXPERIMENT 15

Transistor Switches and 555 Timer Operation

In this experiment we consider how transistors can be used as switches. The transistor switch is the only component internal to the 555 timer chip that we have not looked at yet in detail. All the other components … comparators, flip-flop, and inverter … we have already seen. Once we have studied transistor switches, we will address the internal operation of the 555 timer chip.

Part A Transistor Switches

There is quite a bit of discussion in Gingrich about the transistor. We do not need to consider most of this discussion because the operational amplifier is a better device for us to build circuits with. However, you should at least take a quick look at figures like 5.6 which show the transistor in a common-emitter configuration. This is one configuration that we can consider as a switch.

Using PSpice, set up the circuit shown below. For analysis, set up a DC SWEEP from 0.2 to 9 volts (step = 0.005V). The transistor Q1 is acting as a switch in the loop with resistor R2 and voltage V1. The voltage Vs and resistor R1 are used to turn the switch ON or OFF. Do not print your plot for this case.

Draw a simplified circuit diagram that includes just V1, R2 and a simple switch to represent the transistor. When the switch is open (OFF), how much voltage will be across it? When the switch is closed (ON), how much voltage will be across it?

The transistor switch will not work exactly like an ideal, simple switch. However, it can be a good approximation to such a switch AND, more importantly, will switch states by applying a voltage rather than by mechanically flipping something. (It works something like a relay.) From your PROBE plot of the voltages in the transistor circuit, determine the range of voltages Vs for which the switch will be OFF and the range of voltages Vs for which the switch will be ON.

Now we will consider this switch in a configuration that you ordinarily might not think about, but does a good job of switching the voltage across a load. Add the resistor R5 shown below to your circuit.

The transistor switch, when open, allows the maximum voltage to occur across R5. When the switch is closed, the voltage across R5 goes near zero. Run your simulation again and print your output this time. Again determine the range of voltages Vs for which the switch is ON and OFF. What a typical voltage across R5 when the switch is OFF? What a typical voltage across R5 when the switch is ON? Why do you think that these values make sense?

Now we want to take a look at the range of Vs for which the switch is neither ON nor OFF. Replot your PROBE result for Vs from 0.5V to 1V. Now, rather than plotting voltages, plot the collector, base and emitter currents for Q1, each normalized by the current through resistor R1. (Actually, plot the negative of the emitter current so that all three current ratios are positive.) Just for reference purposes, label the collector C, emitter E, and base B on the two circuit diagrams above. You should be able to identify a small range of voltages Vs for which the magnitude of the collector and emitter currents are more that 170 times the base current. Use the cursors to find this range. Print out the plot with the cursors and indicate on the plot where this ratio is 170 or better. This is the range of Vs for which the transistor circuit acts like a very good amplifier. Here it has a current gain of much more than 100. The gain is not a simple constant, nor is it as large as we can obtain with an op-amp.

By looking at the operation of a simple transistor circuit, we have seen that there is a set of input voltages for which is looks like a switch that is OFF, an amplifier, and a switch that is ON.

Part B Back to the 555 Timer

The circuit above is the one we simulated previously. Now that we have seen how a transistor switch works, we will look inside the 555 Timer chip to see what makes it work. You previously received a handout on timers. However, you can also look at one of the datasheets or application notes from a manufacturer to see what is inside this chip. For example, check out the Motorola website:

http://mot-sps.com/sps/General/chips-nav.html which has a half page description of timing circuits.

You will see that this chip contains three resistors, two comparators, an RS flip-flop, an inverter and a transistor switch. Either simulate the 555 circuit again and produce a new PROBE plot or use the correct plot for the astable multivibrator problem in Quiz 3. These plots show the voltages at pins 2, 3, 5, 6, and 7. You do not really need 5, but it shows when the device is working correctly. Also, 2 and 6 are the same in this circuit. We have seen that the state of the 555 changes when 2 and 6 go above (2/3)V_CC or below (1/3)V_CC. The logical devices (flip-flop, inverter, and switch) change state at these voltages. Remember that for logical devices, switches are ON or OFF and inputs and outputs are HIGH or LOW. Determine the states of these devices during the time that the voltage at 2 and 6 is increasing and when it is decreasing. Consider only the steady state time period, not the initial startup time. If you wish, you can look at the 555 tutorial at http://www.academy.rpi.edu/html/555%20timer.html for inspiration. This latter simulation has an additional switch in it, so that the initial turn on time period starts from zero volts rather than from V_CC as in our PSpice simulation. It does look the same during steady state. After working out the states of the logical devices, explain in your own words how the 555 timer works.