A bit about the 555 IC.
This is a very basic introduction for a friend who is learning electronics.
I'm building a NLC 555 resonator which uses five of these. This module provided the
prefect excuse to discuss it.
The 555 is the most popular integrated circuit ever manufactured.
It was developed by Signetics in 1972 and is found in timer, pulse generation, and oscillator applications.
It's commonly known as the 555 timer, but it has many more applications than the name suggests.
There are a few variations of the original 555 ic.
The original were TTL (Transistor-Transistor Logic) or bipolar. These preceded modern CMOS chips
which use much less power. The TTLs use bipolar transistors.
The advantage of TTL is that they are hard to damage and have powerful outputs, but they can generate voltage spikes. In comparison, the CMOS versions don't spike, have less powerful outputs.
The 556 has two timing circuits.
The 558 has four timing circuits in one package.
There is very little standardization among manufacturers.
Sometimes you will see 7555s used instead of 555s
The Intersil ICM7555 is a low-power CMOS version but there are other CMOS versions with no clear
identifing marks.
The 555 IC has 4 operating modes:
1. Astable (free-running) mode
2. Monostable (one-shot) mode
3. Bistable (flip-flop) mode
4. Smitt Trigger (inverter) mode.
Astable ModeAn Astable Circuit has no stable state - hence the name "astable". ... It can also be used to flash lamps and LEDs, and is useful as a 'clock' pulse for other digital ICs and circuitsHere the 555 timer puts out a continuous stream of rectangular pulses at pin 3.
It's output is continually alternating between high (1) and low (0) and it never has a stable state.
The frequency of the pulses at the output is controlled by an RC network (resistor capacitor)
made up of R1, R2 & C.
The frequency can be calculated with the formula:
F = 1.443/(C(R1+2R2))
Thus the larger the values of R1,R2 & C, the lower the frequency.
The 10nF capacitor connected to pin 5 is a decoupling capacitor to shunt electrical noise
MonostableA Monostable Circuit produces one pulse of a set length in response to a trigger input (such as a push button). Here the 555 works as a timer (Monostable Multivibrator).
It's output has a stable logic state that only changes when the 555 is triggered or activated.
Normally, the output at pin 3 is low (0). It remains at this state until a low (0) is applied to pin 2 (trigger).
When this happens, the timer is activated and pin 3 goes high (1)...... but only for a certain amount
of time.
Notice that pins 7 and 6 are connected to each other and to C & R.
The time period when the pulse exiting pin 3 is high is determined by the values of R & C.
This time period can be calculated using the following formula:
Time ON = (1.1)(R)(C)
Thus , the larger the values of R & C, the longer the time ON.
The 10nF capacitor connected to pin 5 is a decoupling capacitor to shunt electrical noise, to avoid instability and false triggering.
Bistable (flip-flop) mode.The full name is a Bistable Multivibrator.
Flipflops are really impt as they are the building blocks of sequential circuits such as counters, registers, decoders, latches, memory, etc etc....
Basically, they are memory circuits that remember the logic state they were set
You can read more about flip flops here:
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Flip Flops - an introduction The 555 timer can act as a SR flip-flop.
You need to put two pull up resistors on pins 4 & 2 to hold the voltage high.
The 10nF capacitor connected to pin 5 is a decoupling capacitor to shunt electrical noise, to avoid instability and false triggering.
For computer applications, the 555 is not very useful as a flip-flop as its output is relatively slow.
Computers are driven by very high speed clocks, and the trigger and reset response times can't keep up.
But it's ok for noncomputer applications where high-speed response isn’t necessary.
Smitt Trigger (inverter) mode.This converts a noisy (analog) input into a clean digital output.
It's a comparator circuit.
The input signal is connected through a capacitor which onnects to the trigger and threshold pins.
The 10nF capacitor connected to pin 5 is a decoupling capacitor to shunt electrical noise, to avoid instability and false triggering.
Links+
TTL - transistor-transistor logic +
DIY Index+
CMOS+
How did NASA steer the saturn V - the computer & data storage in the 1960s (memory module)
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LVDC - The saturn V memory module
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