THE 555 IC PINS | 555 IC PIN CONNECTION AND FUNCTION When drawing a circuit diagram, always draw the 555 as a building block, as shown below with the
555 IC PIN CONNECTION AND FUNCTION
 |
| 555 IC PIN CONNECTION AND FUNCTION |
When
drawing a circuit diagram, always draw the 555 as a building block, as shown
below with the
pins in the following locations. This will help you instantly recognize the
function of each pin:
Pin
1 GROUND. Connects to the 0v rail.
Pin 2 TRIGGER. Detects 1/3 of rail voltage to make output HIGH. Pin 2 has
control over pin 6. If pin
2 is LOW, and pin 6 LOW, output goes and stays HIGH. If pin 6 HIGH, and pin 2
goes LOW, output
goes LOW while pin 2 LOW. This pin has a very high impedance (about 10M) and
will trigger with
about 1uA.
Pin 3 OUTPUT. (Pins 3 and 7 are "in phase.") Goes HIGH (about 2v
less than rail) and LOW (about
0.5v less than 0v) and will deliver up to 200mA.
Pin 4 RESET. Internally connected HIGH via 100k. Must be taken below 0.8v to
reset the chip.
Pin 5 CONTROL. A voltage applied to this pin will vary the timing of the RC
network (quite
considerably).
Pin 6 THRESHOLD. Detects 2/3 of rail voltage to make output LOW only if pin 2
is HIGH. This pin
has a very high impedance (about 10M) and will trigger with about 0.2uA.
Pin 7 DISCHARGE. Goes LOW when pin 6 detects 2/3 rail voltage but pin 2 must
be HIGH. If pin 2
is HIGH, pin 6 can be HIGH or LOW and pin 7 remains LOW. Goes OPEN (HIGH) and
stays HIGH
when pin 2 detects 1/3 rail voltage (even as a LOW pulse) when pin 6 is LOW.
(Pins 7 and 3 are "in
phase.") Pin 7 is equal to pin 3 but pin 7 does not go high - it goes
OPEN. But it goes LOW and will
sink about 200mA. You can connect pin 7
to pin 3 to get a slightly better SINK capability from the chip.
Pin 8 SUPPLY. Connects to the positive rail.
555 in a circuit - note the circle
on the chip to identify pin 1
This is
sometimes called a "push-out-pin" (hole) and sometimes
it has no importance. But in this case it represents pin 1.
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