Example Circuits:
I have placed a couple of 555 circuit examples below for your convenience. Play
with different component values and use the formulas mentioned earlier to
calculate your results. Things to remember: For proper monostable operation with
the 555 timer, the negative-going trigger pulse width should be kept short
compared tot he desired output pulse width. Values for the external timeing
resistor and capacitor can either be determined from the previous formulas.
However, you should stay within the ranges of resistances shown earlier to avoid
the use of large value electrolytic capacitors, since they tend to be leaky.
Otherwise, tantalum or mylar types should be used. (For noise immunity on most
timer circuits I recommend a 0.01uF (10nF) ceramic capacitor between pin 5 and
ground.) In all circuit diagrams below I used the LM555CN timer IC from
National, but the NE555 and others should not give you any problems
Circuits 1 to 10a:
Play with different indicating devices such as bells, horns, lights, relays, or
whatever (if possible). Try different types of LDR's. If for any reason you get
false triggering, connect a ceramic 0.01uF (=10nF) capacitor between pin 5 (555)
and ground. Keeping the basic rules of the 555 timer, try different values for
Ct and Rt (or the C & R over pins 2, 6 & 7) Replace Rt with a 1 megohm
potentiometer if you wish. Make notes of the values used and use the formulas to
calculate timing. Verify your calculations with your timing.
Fig. 1, Dark Detector: It will sound
an alarm if it gets too dark all over sudden. For example, this circuit could be
used to notify when a lamp (or bulb) burns out. The detector used is a regular
cadmium-sulphide Light Dependent Resistor or LDR, for short, to
sense the absense of light and to operate a small speaker. The LDR enables the
alarm when light falls below a certain level.
Fig. 2, Power Alarm: This circuit can
be used as a audible 'Power-out Alarm'. It uses the 555 timer as an oscillator
biased off by the presence of line-based DC voltage. When the line voltage
fails, the bias is removed, and the tone will be heard in the speaker. R1 and C1
provide the DC bias that charges capacitor Ct to over 2/3 voltage, thereby
holding the timer output low (as you learned previously). Diode D1 provides DC
bias to the timer-supply pin and, optionally, charges a rechargeable 9-volt
battery across D2. And when the line power fails, DC is furnished to the timer
through D2.
Fig. 3 Tilt Switch: Actually really a
alarm circuit, it shows how to use a 555 timer and a small glass-encapsulated
mercury switch to indicate 'tilt'.
The switch is mounted in its normal 'open' position, which allows the timer
output to stay low, as established by C1 on startup. When S1 is disturbed,
causing its contacts to be bridged by the mercury blob, the 555 latch is set to
a high output level where it will stay even if the switch is returned to its
starting position. The high output can be used to enable an alarm of the visual
or the audible type. Switch S2 will silent the alarm and reset the latch. C1 is
a ceramic 0.1uF (=100 nano-Farad) capacitor.
Fig. 4, Electric Eye Alarm: The
Electric-Eye Alarm is actually a simular circuit like the Dark Detector of Fig.
1. The same type of LDR is used. The pitch for the speaker can be set with the
500 kilo-ohm potentiometer. Watch for the orientation of the positive (+) of the
10uF capacitor. The '+' goes to pin 3.
Fig. 5, Metronome: A Metronome is a
device used in the music industry. It indicates the ritme by a 'toc-toc' sound
which speed can be adjusted with the 250K potentiometer. Very handy if you
learning to play music and need to keep the correct rhythm up.
Error fixed with thanks to Grant Fair
in regards to the two resistors. (Grant also added a PNP power transistor to
increase the volume and a led for visual as well as sound output).
Fig. 6, CW Practice Oscillator: CW
stands for 'Contineous Wave' or Morse-Code. You can practice the morse-code
with this circuit. The 100K potmeter is for the 'pitch' and the 10K for the
speaker volume. The "Key" is a morse code key.
Fig. 7, CW Monitor: This circuit
monitors the morse code 'on-air' via the tuning circuit hookup to pin 4 and the
short wire antenna. The 100K potmeter controls the tone-pitch.
Fig. 8, Ten-Minute Timer: Can be used
as a time-out warning for Ham Radio. The Federal Communications Commission (FCC)
requires the ham radio operator to identify his station by giving his call-sign
at least every 10 minutes. This can be a problem, especially during lengthy
conversations when it is difficult to keep track of time. The 555 is used as a
one-shot so that a visual warning indicator becomes active after 10-minutes. To
begin the cycle, the reset switch is pressed which causes the 'Green' led
to light up. After 10 minutes, set by the 500K potentiometer R1, the 'Red'
led will light to warn the operator that he must indentify.
Fig. 9, Schmitt Trigger: A very
simple, but effective circuit. It cleans up any noisy input signal in a nice,
clean and square output signal. In radio control (R/C) it will clean up noisy
servo signals caused by rf interference by long servo leads. As long as R1
equals R2, the 555 will automatically be biased for any supply voltage in the 5
to 16 volt range. (Advanced Electronics: It should be noted that there is a
180-degree phase shift.) This circuit also lends itself to condition 60-Hz
sine-wave reference signal taken from a 6.3 volt AC transformer before driving a
series of binary or divide-by-N counters. The major advantage is that, unlike a
conventional multivibrator type of squarer which devides the input frequency by
2, this method simply squeares the 60-Hz sine wave reference signal without
division.
Fig. 10, Better Timing: Better and
more stable timing output is created with the addition of a transistor and a
diode to the R-C timing network. The frequency can be varied over a wide range
while maintaining a constant 50% duty-cycle. When the output is high, the
transistor is biased into saturation by R2 so that the charging current passes
through the transistor and R1 to C. When the output goes low, the
discharge transistor (pin 7) cuts off the transistor and discharges the
capacitor through R1 and the diode. The high & low periods are equal. The value
of the capacitor (C) and the resistor (R1 or potmeter) is not given. It is a
mere example of how to do it and the values are pending on the type of
application, so choose your own values. The diode can be any small signal diode
like the NTE519, 1N4148, 1N914 or 1N3063, but a high conductance Germanium or
Schottky type for the diode will minimize the diode voltage drops in the
transistor and diode. However, the transistor should have a high beta so that R2
can be large and still cause the transistor to saturate. The transistor can be a
TUN (europe), NTE123, 2N3569 and most others.
Fig. 10a, Missing Pulse Detector (Basic):
This transistor can be replaced with a ECG or NTE159. This is just a basic
model but works. Experiment with the values of Resistor and Capacitor. A good
example would be the
'Crashed Aircraft Locator' beacon used in radio control. If there is
no signal it sees it as a missing pulse and sounds buzzer.
The following circuits are examples of how a 555 timer IC assist in combination with another Integrated Circuit. Again, don't be afraid to experiment. Unless you circumvent the min and max parameters of the 555, it is very hard to destroy. Just have fun and learn something doing it.
Circuits 11 to 14:
Play with different indicating devices such as bells, horns, lights, relays, or
whatever (if possible). Try different types of LDR's. If for any reason you get
false triggering, connect a ceramic 0.01uF (=10nF) capacitor between pin 5 (555)
and ground. In all circuit diagrams below I used the LM555CN timer IC from
National. The 555 timer will work with any voltage between 3.5 and 15volt. A
9-volt battery is usually a general choice. Keeping notes is an important aspect
of the learning process.
Fig. 11, Two-Tones: The purpose of
this experiment is to wire two 555 timers together to create a 2-note tone. If
you wish, you can use the dual 556 timer ic.
Fig. 12, Recording Beep: This circuit
is used to keep recording of telephone conversations legal. As you may know,
doing otherwise without consent of the other party is illegal. The output of IC1
is fed to the 2nd 555's pin 3 and made audible via C2 and the speaker. Any 8-ohm
speaker will do.
Fig. 13, Coin Toss: Electronic
'Heads-or-tails' coin toss circuit. Basically a Yes or No decision
maker when you can't make up your mind yourself. The 555 is wired as a Astable
Oscillator, driving in turn, via pin 3, the
7473 flip-flop. When you press S1 it randomly selects the 'Heads' or
'Tails' led. The leds flashrate is about 2Khz (kilo-Hertz), which is much faster
than your eyes can follow, so initially it appears that both leds are 'ON'. As
soon as the switch is released only one led will be lit.
Fig. 14, Logic Probe: Provides you
with three visible indicators; "Logic 1" (+, red led), "Logic 0" (-, green led),
and "Pulse" (yellow led). Good for TTL and CMOS. The yellow or 'pulse' led comes
on for approximately 200 mSec to indicate a pulse without regards to its width.
This feature enables one to observe a short-duration pulse that would otherwise
not be seen on the logic 1 and 0 led's. A small switch (subminiature slide or
momentary push) across the 20K resistor can be used to keep this "pulse" led on
permanently after a pulse occurs.
In operation, for a logic 0 input signal, both the '0' led and the pulse led
will come 'ON', but the 'pulse' led will go off after 200 mSec. The logic levels
are detected via resistor R1 (1K), then amplified by T1 (NPN, Si-AF
Preamplifier/Driver), and selected by the 7400 IC for what they are. Diode D1 is
a small signal diode to protect the
7400 and the leds from excessive inverse voltages during capacitor
discharge.
For a logic '1' input, only the logic '1' led (red) will be 'ON'. With the
switch closed, the circuit will indicate whether a negative-going or
positive-going pulse has occurred. If the pulse is positive-going, both the '0'
and 'pulse' led's will be on. If the pulse is negative-going, the '1' and
'pulse' led's will be on.
Check the listing in Table 2. It shows some variations in the 555 manufacturing
process by two different manufacturers, National Semiconductor and Signetics
Corporation. Since there are other manufacturers then those two I suggest when
you build a circuit to stick with the particular 555 model they specify in the
schematic.
Unless you know what you're doing ofcourse... [grin].
The absolute maximum ratings (in free air) for
NE/SA/SE types are:
Vcc, supply voltage: 18V
Input voltage (CONT, RESET, THRES, TRIG): Vcc
Output current: 225mA (approx)
Operating free-air temp. range: NE555........... 0°C - 70°C
SA555........... -40°C - 85°C
SE555, SE555C... -55°C - 125°C
Storage temperature range: -65°C - 150°C
Case temperature for 60sec. (FK package): 26