1/3The next level of sophistication (with Relaxation Oscillators) involves the use of timer or waveform-generator ICs as relaxation oscillators. The most popular chip around is the 555 (and its successors).
It is also a misunderstood chip, and we intend to set the record straight with the
equivalent circuit shown in the figure above. Some of the symbols belong to the digital world , so you won't become a 555 expert for a while yet.
But the operation is simple enough: The output goes HIGH (near Vcc) when the
555 receives a TRIGGER' input, and it stays there until the THRESHOLD input is driven, at which time the output goes LOW (near ground) and the DISCHARGE
transistor is turned on. The TRIGGER' input is activated by an input level below
1/3Vcc, and the THRESHOLD is activated by an input level above 2/3Vcc.
The easiest way to understand the workings of the 555 is to look at an example
below. When power is applied, the capacitor is discharged; so the 555 is triggered,
causing the output to go HIGH, the discharge transistor Q1 to turn off, and the capacitor to begin charging toward 10 volts through RA + RB. When it has reached 2/3Vcc, the THRESHOLD input is triggered, causing the output to go LOW and Q1 to turn on, discharging C toward ground through RB. Operation is now cyclic, with C's voltage going between 1/3Vcc and 2/3Vcc, with period T = 0.693(RA + 2RB)C.
The output you generally use is the square wave at the output. The 555 makes a respectable oscillator, with stability approaching 1%. It can run from a single positive supply of 4.5 to 16 volts, maintaining good frequency stability with supply voltage variations because the thresholds track the supply fluctuations.
The 555 can also be used to generate single pulses of arbitrary width, as well as a bunch of other things. It is really a small kit, containing comparators, gates, and flip-flops. It has become a game in the electronics industry to try to think of new
uses for the 555. Suffice it to say that many succeed at this new form of entertainment.
A caution about the 555: The 555, along with some other timer chips, generates a grid big supply-current glitch during each output transition. Be sure to use a hefty bypass capacitor near the chip. 6.8k Even so, the 555 may have a tendency to generate double output transitions.
|Engineer.Labs | Hardware Design Engineer (Freelancer)||
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