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About 555 mode of operation



Jan 1, 1970
Jamie said:
astable = free running.

Yes i know that. I want to know what is that that determines how a 555
works. If it is triggered by itself or if it is triggered externally

Bob Eldred

Jan 1, 1970
A 555 has two internal comparators a high one called the "threshold" and a
low one called the "trigger" It also has an internal flip-flop and a
discharge transistor. When the voltage on the trigger goes above its trigger
point, approx. 1/3 of Vcc., the f-f flips high and the discharge transistor
is off. When the voltage on the threshold goes above it's value, approx. 2/3
of Vcc., the f-f flips low and the discharge transistor turns on. It can be
hooked up in many ways with these inputs.

In the monostable mode, the trigger is hooked up to an external circuit and
each time it pulses high, the discharge transistor goes off, a cap connected
to the threshold and discharge is allowed to charge to the 2/3 threshold
point then the discharge dumps the cap. The f-f goes through one cycle timed
by the cap and a resistor to Vcc. Simplified timing part of curcuit: R1a to
Vcc; R1b to pin 7 disch. and pin 6 thresh. and cap C1a; cap C1b to gnd;
Input to pin 2 trig. Note, 6 and 7 are hooked together in this mode. A
resistor here would slow the discharge and recovery time for the next pulse.

In the astable mode, the trigger is hooked to the threshold and the two of
them connected through a resistor to the discharge transistor with another
resistor connected to Vcc. The capacitor charges through the two resistors
until the threshold is reached then the discharge transistor turns on and
the cap discharges down to the trigger level through the resistor between
the discharge and threshold. The thing oscillate continuoulsy. Simplified
timing part of circuit: R1a to Vcc; R1b to pin 7 disch.; R2a to pin 7
disch.; R2b to pin 6 thresh. and pin 2 trig.; C1a cap to pin 6 thresh. and
pin 2 trig.; C1b cap to gnd. Note: 2 and 6 are hooked together and there is
a resistor, R2 between 2-6 and 7. The resistor is require for the discharge