DRIVING A RELAY Circuit
The 555 will
activate a relay. When pins 2 and 6 are connected as an input, the
chip requires only about 1uA to activate the output. This is
equivalent to a gain of about 200,000,000 (200 million) and
represents about 4 stages of amplification via transistors.
In the first circuit, the output will be opposite to the
input. The relay can be connected "high" or "low" as show in the
second
diagram. One point to note: The input must be higher than 2/3V for
the output to be low and below 1/3V for the output to be high. This
is called HYSTERESIS and prevents any noise on the input creating
"relay chatter."
NEGATIVE LOGIC
An interesting point to remember.
In the first diagram above, the relay is connected so that it is
active when the output is low. This is called NEGATIVE or NEGATIVE
LOGIC. It has the same reasoning as
-5 - (-5) = 0.
Or in
English: "I am not NOT going."
When the input is low in the first diagram, the output is HIGH and
the relay is OFF. The circuitry creates two reversals and makes it
easy to see that when the input is LOW, the relay is OFF.
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