Electronics

There are two main parts to this project - an oscillator (based on IC1) and a LED driver (based on IC2). In a nutshell, when you press the pushbutton, power is supplied to the circuit and the "reservoir" capacitor on the main supply rail charges to the battery voltage (3V). At the same time, the resistor and capacitor around one of the Schmitt NAND gates (IC1c) cause it to oscillate. The word NAND is a contraction of NOT & AND. The "AND" part means that both inputs to the gate need to be a logic "high" for the gate to operate and the "NOT" means the output is opposite, or inverted, to the input. There is a "truth table" shown in Table 1 which shows what happens to the output, depending on what is occurring at the input. The "Schmitt" part of the name refers to a feature of the threshold points, or triggering, of the gate. The voltage levels at which it triggers, either low or high, are quite precise but more importantly, are widely separated. This makes a Schmitt trigger more immune to noisy triggering waveforms. As you can see, the two inputs to the gate are connected together, effectively turning it into an inverter. As such, the input and output can never be the same state - when the input is high, the output must be low and vice versa.

When you press and hold the button, the IC is powered up but at that instant the inputs are in a low state (because the 1μF capacitor is not charged). Therefore the output is high. The capacitor then starts to charge via the 68kΩ resistor from output to input. When the capacitor voltage passes the gate's upper threshold voltage (ie, the input goes high), the output goes low. The capacitor then starts to discharge, the voltage eventually dropping below the gate's lower threshold voltage. The output then goes high again. This keeps happening as long as power is applied to the circuit. It's called a "relaxation oscillator" and is a very easy way to make any form of pulse generator. The frequency at which it operates is determined by the values of the resistor and capacitor. The formula is 1/0.55 x RC, where R is in ohms and C is in Farads (note that - Farads, not microfarads). Therefore if the resistor is exactly 68,000 ohms (unlikely!) and the capacitor is exactly 1μF (even more unlikely!), the frequency of this oscillator circuit will be 1/ 0.55 x 68,000 x 0.000001, or 1/0.0374, or approximately 26Hz (actually 26.7Hz). Why did we say it was unlikely that the resistor and capacitor wouldn't be exactly what their marked value said? If the resistor has a 1% tolerance, its actual value could be anywhere from 99% of 68,000 ohms (67,320Ω) to 101% (68,680Ω). And capacitors normally have a much wider tolerance - as much as 20% or more. So you can see we are not talking exact values in a simple circuit such as this.

A race condition is an undesirable situation that occurs
when a device or system attempts to perform two or more
operations at the same time, but because of the nature of
the device or system, the operations must be done in the
proper sequence in order to be done correctly.
In computer memory or storage, a race condition may occur
if commands to read and write a large amount of data are
received at almost the same instant, and the machine
attempts to overwrite some or all of the old data while
that old data is still being read. The result may be one or
more of the following: a computer crash, an "illegal
operation," notification and shutdown of the program,
errors reading the old data, or errors writing the new
data. This can be prevented by serialization of memory or
storage access, such that if read and write commands are
received close together, the read command is executed and
completed first, by default.

In a network, a race condition may occur if two users
attempt to access an available channel at the same instant,
and neither computer receives notification that the channel
is occupied before the system grants access. Statistically,
this sort of coincidence is most likely to occur in
networks having long lag times, such as those that use
geostationary satellites. To prevent such a race condition
from developing, a priority scheme must be devised. For
example, the subscriber whose username begins with the
earlier letter of the alphabet (or the lower numeral) may
get priority by default when two subscribers attempt to
access the system within a prescribed increment of time.
Hackers can take advantage of race-condition
vulnerabilities to gain unauthorized access to networks.

Race conditions occasionally occur in logic gates when
certain inputs come into conflict. Because the gate output
state takes a finite, nonzero amount of time to react to
any change in input states, sensitive circuits or devices
following the gate may be fooled by the state of the
output, and thereby caused to not operate properly.

source : http://www.allinterview.com/showanswers/10617.html

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