Astable multivibrator using 555 vs transistors

[ay215]

If I'm not mistaken, an astable multivibrator can be built with a 555 ic, or using transistors, capacitors, and resistors. I was wondering if there are any advantages or disadvantages of using one approach over the other? Like cost or effectiveness? Thanks.

 

[BradtheRad]

My observation is that 555 output is designed to make rapid square-ish transitions. It must be a chief factor letting it oscillate near 1/2 MHz or faster. I believe the design of logic IC's is similar: High gain snap-action, good at pulse generating.

The classic astable multivibratpr is not quite so square-ish. However it does give you two alternating clocks, each one's phase inverted from the other. This can be convenient for certain uses.

 

[barry]

cost, reliability, thermal stability and PCB area are also considerations.

 

[crutschow]

If the 555 astable meets your output design requirements, then it is likely the cheaper, simpler choice.
If not, then you can design one using discrete devices, or logic gates, which would likely be a more complex circuit.

 

[dick_freebird]

Do not overlook the simple goodness of a CD4000 series phase shift oscillator (or if you need over 1MHz, 74xx at various lower voltages.

 

[FvM]

555 has initial timing accuracy in low percent range, hard to achieve with logic gates or discrete transistors.

 

[crutschow]

555 has initial timing accuracy in low percent range, hard to achieve with logic gates or discrete transistors.

Also consider the much higher output current rating of the 555 as compared to most logic gates.

 

[KlausST]

Hi,

if I´m not mistaken "logic gates" never are mentioned by the OP.
The question was NE555 vs discrete..

Klaus

 

[danadakk]

@ay215, what are target goals for accuracy, drift ?

 

[dick_freebird]

A 555 will work on dirty cig lighter power all by itself, a transistor bodge likewise unless you pick something too fancy. CD4000 has two subfamilies and one rates at 18V but that's weak for automotive IGN power quality.

You can also make a real sloppy astable with a single Schmitt inverter, or less sloppy with a comparator inverting signal with positive feedback.