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Multivibrators
Now that we know what flip flops are, let’s look into some other variations along the
same theme. Flip Flops fall into a category of circuits named Multivibrators. Specifically
Bi-Stable Multivibrators, because both states are stable. Once Set, it will stay Set until we
deliberately Clear it. In contrast, there are other members of the Multivibrator family
called Astable Multivibrators. Astable implying that they have no stable state. We still
have two possible states, but the circuit stays in neither condition. Oscillators are Astable
Multivibrators. In between these two extremes we have circuits called Monostable
Multivibrators, meaning they have only one stable state. Normally this quiescent state is
called the Clear state. When it gets a Clock pulse the Monostable Multivbrator goes into
the Set state for a short while, then reverts back to the Clear state on its own. Monostable
Multivibrators are often called Single Shots, One Short, Pulse Generators or many similar
names. A Resistor and Capacitor normally determines the length of time it stays in the
Set state. The Time Constant for the circuit is a matter of multiplying Resistance (in
Ohms) times Capacitance (in Farads) and getting the results in seconds. Some chips may
vary from this slightly (R x C x 0.7 for instance).
Clocks and Triggers
The clock pulse that triggers our multivibrator is usually an edge triggered clock. Many
circuits have two triggers. One active low edge, the other active high edge. You disable
the one you don’t want to use. Others may have an Enable (high or low level) that must
be present before the trigger is applied. Some chips have one input with a Schmitt Trigger
design to the circuit that allows clean triggering from dirty or slowly changing input
signals.
Retriggerable
Normally, once a multivibrator is triggered a second trigger will be ignored if it is applied
before the circuit gets a chance to time out. Not all multivibrators are this way.
Retriggerable devices restart the timing period if another clock is applied.
Resettable
Many devices have a Reset input. In some cases the Reset interrupts the present timing
cycle and Clears the multivibrator right away. Some devices will time out normally and
Reset only prohibits the starting of a new cycle.
CMOS or Bi-polar
Not all monostable multivibrators are members of a logic family. They may be either
CMOS and usable with CMOS logic if run from the same supply voltage. They may be
bi-polar (NPN and PNP transistors) but may not readily work with TTL family logic.
CMOS and TTL themselves both have members in their families that are designed to
work with the logic gates. Some devices are designed to work across broad power supply
ranges and are not intended to be compatible with logic devices.
Real Parts
LM555
The 555 is a very popular device. Most every electronics book has this as an example of
an IC. It can be used as an Astable Multivibrator, Monostable Multivibrator, or Bi-Stable
Multivibrator. This is a bipolar device, meaning it is made up of NPN and PNP type
transistors.
LM555CN (Commercial temperature Range, DIP package)
60-0052-00 (Happ Controls)
32011790 (IGT)
E-00620-0004 (Bally)
512-LM555CN (Mouser)
LM555M (Surface Mount package, SOIC-8)
512-LM555CM (Mouser)
556 and 558
The LM556 is a dual version of the LM555. The LM558 is a Quad version of the LM555.
7555
The 7555 is a CMOS version of the 555. The inner parts are made of CMOS transistors.
As such it operates on much lower current than the 555, and can use much higher
resistance timing resistors and larger timing capacitors.
LM3909
The LM3909 is designed to be an LED flasher. Using a single AA battery, the 3909 can
blink an LED (that requires 2 Volts to flash) for months at a time. This is one of the
popularly used chips used to make flashing jewelry, grocery store shelf tags, birthday
cards, things like that. The 3909 is a bipolar device, designed for low voltage and low
current operation.
3905
The LM3905 is similar to the LM555 in many functions.
The “LM” prefix has been used primarily for popularity. The prefix added before the
basic part number (registration number) is tagged on by the manufacturer of the device.
National Semiconductors may name it an LM555. RCA may name it a CA555. Either are
still a 555. The suffix after the registration number usually indicates the case the chip has
been put in. “N” usually means a DIP package. “D” or “M” means a Surface Mount
package, and so on. Each manufacturer has their terminology. Consult the data sheet for
the device if you have any doubts.
TTL devices
74121 Monostable Multivibrator.
74122
74123 Retriggerable monostable multivibrator, with Clear. Dual.
74221 Monostable Multivibrator (non-retriggerable). Dual
74423 Monostable Multivibrator, with Clear. Dual.
CMOS devices
4047 Monostable / Astable Multivibrator
4098 Monostable Multivibrator. Dual.
4538 Precision Monostable Multivibrator. Dual.
Available sources of parts:
Radio Shack
Mouser Electronics
IGT p/n’s
Williams p/n’s