Relaxation Oscillator and One-Shot Using 555 Timer
Physics 116B Lab 11
Rev. 3, January, 2011
Introduction
In this experiment you will use an FET version of a 555 timer (TLC555) to
• make a relaxation oscillator (astable multivibrator)
• use the circuit with a thermistor as the timing resistor to make a digital thermometer of sorts (example
of telemetry and analog-to-digital conversion) and
• make a one-shot circuit (monostable multivibrator or timer).
Relaxation Oscillator
Wire the circuit shown in Fig. 1. Note: black dots indicate connections. Lines crossing at right angles
without black dots are not connected. Be sure to use a capacitor with a 10% tolerance for the 0.047 µF
timing capacitor. Use a 10 kΩ resistor for the timing resistor initially. Note that the output is connected to
the trigger and threshold inputs through the timing resistor. Examine the circuit and convince yourself that
the trigger/threshold level will vary between 1/3 and 2/3 of the supply voltage as the output switches from
0 V to a voltage close to the supply voltage and back. Observe (and copy in your report) the waveforms on
the trigger/threshold node and on the output. Record the actual transition voltages at the trigger/threshold
input and the high state output voltage. From these measurements and your analysis of the circuit, estimate
the period of oscillation. Measure the oscillation frequency using the frequency counter in the Tektronix
function generator and compare with what you expect. (Note that the frequency counter is a feature of the
Tektronix function generator, not the pulse generator shown in Fig. 4.)
Digital Thermometer
Now replace the 10 kΩ timing resistor with the thermistor. This is a semiconductor device whose resistance
varies with temperature in a known way. The oscillator frequency readout will now be a measure of the
thermistor temperature. You will observe the frequency to increase if you press the thermistor between
your fingers. Based on the supplied resistance vs. temperature chart, make a chart of expected oscillator
frequency for temperatures between 0 and 50 degrees Celsius (in 5 degree steps). If the frequency were read
into a computer, it would be simple to convert it to Celsius for display or recording.
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Figure 1: Relaxation oscillator diagram with thermistor resistance chart.
One-Shot
This circuit (Fig. 2) produces a pulse of known length following a short trigger. The input is normally held
at VDD = 5 V. In this case, the output is 0 V in the stable state. When the input falls below VDD /3 = 1.67 V
due to the trigger pulse, the output goes to 5 V for a period T determined by the RC time constant, then
returns to 0 V.
Build the circuit close to the coaxial connector so signal leads on the board can be kept short to minimize the
effects of wiring inductance and capacitance on rapidly rising pulses. Note that the coaxial cable has been
terminated in its characteristic impedance (50 Ω) to prevent reflected waves and spurious triggers (not a big
problem here due to the long pulses and long time constants). The .01 µF bypass capacitor is connected
between +5 V and ground at the chip to absorb voltage and current spikes when the output changes rapidly.
The completed circuit is shown in Fig. 3.
Use the Tektronix PG 502 pulse generator (shown in Fig. 4) to produce the desired input pulse. It is probably
easiest to start with a positive going pulse. Begin with the “Complement” button out, rather than in, as
shown in the figure. (“Complement” means applying the logical “NOT” function to the output levels.) The
controls in the top half of the front panel set the pulse width and period. Observe the (now positive) pulse by
connecting the Channel 1 oscilloscope probe to the pulse input on the circuit board, as shown in Fig. 3. Set
the oscilloscope to trigger on a positive going signal on Channel 1. The pulse generator width should be set
to 0.5 ms and the period should be 100 ms. The high and low levels of the pulse are set with the knobs in the
center of the panel. The high level should be 5 V and the low level 0 V. Once you have the positive pulse in
good shape, push in the “Complement” button to give the desired falling pulse and change the oscilloscope
to trigger on a falling edge on Channel 1. Then you can observe the output simultaneously on Channel 2 to
see if the circuit is functioning properly and measure the output pulse width T .
Observe the pulse on the trigger input of the timer and sketch it to include in your report. Observe the
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operation of the timer circuit for timing resistances of 100 kΩ and 1.0 MΩ. Measure the output pulse widths
(T in Fig. 2). Analyze the operation of the circuit and compare the measured widths with what you expect
from the analysis.
You may also want to try increasing the capacitor value to C = 1 µF with R = 1 MΩ and triggering the
circuit manually as shown in the inset of Fig. 2. Observe the output by connecting it to one of the light
emitting diodes on the circuit board.
+5 V
1 k1
0.5 ms
5V
0V
Tektronix
Pulse
Generator
2
Trigger
Push Button
Switch
8
R=100 k1
6
2
+
<
50 1 coaxial cable
C=.047 µF
+/- 10%
.01 µF
4
TLC555
7
Manual Trigger Option
In
Bypass capacitor
connects between
5 V and ground at chip
+5 V
Discharge
3
Out
Threshold
Trigger
1
T
5
.01 µF
Coax termination RT = 50 1
(Two 100 1 1/4 W resistors in parallel)
Figure 2: One-shot (timer) circuit. The output pulse width T is proportional to RC.
Figure 3: One-shot (timer) circuit as constructed.
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Figure 4: Tektronix PG502 pulse generator.
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Figure 5: TLC555 specifications.
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