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The 555 Timer chip

Terry Allen Steen, EE says

f = 1.44/[C*(Ra+2Rb)]


  1. Ra is between Vcc (Pin #8) and Pin #7 (Discharge)
  2. Rb is between Pin #7 (Discharge) and Pin #6 (Threshold)
  3. C is between Pin #6 (Threshold) and Ground (Pin #1)

ensure Pin #4 (Reset) and #8 (Vcc) are tied together, and Pin 5 (Control) needs a filter cap if the circuit has noise problems.

ALWAYS A L W A Y S ALWAYS! Start with the cap. Set it by pulse width. Use this table:

Cap(uFd)    Pulse width(Seconds)
10.0             90m -  1.2
 1.0              9m - 120m
 0.1            900u -  12m
 0.01            90u - 1.2m
 0.001            9u - 120u

Select your cap, THEN select the resistors. If you want to make it easy, ASSuME the resistors are the same value, then you can use:

f = 0.48/R*C <==> R=0.48/f*C

If your resistor is abnormal in size, you have either chosen the wrong cap, or you are doing some kind of real odd freq.

Mark Willis says:

[The] original 555's which were sort of power hogs; Use a low-power CMOS version, and make sure you have that version's datasheet if you use one, if you go the 555 route. Also, [you] could drive a 555 at a higher frequency and divide it's frequency down by a proportional amount.

Electrolytics have huge temperature coefficients, using one for timing a 555, you want something like a Mylar or other low-variance cap. Those gre REALLY pricey IIRC by the time you reach 100uF, so use smaller C, larger R, and watch the construction, leakage on those pins 6 and 7 will change your timing.

Roman Black says:

I've been playing with 555s since 10 years old. You can also use a single inverter from a logic chip to replace a 555 in 95% of circuits. For a 14pin chip you can get 6 oscillators vs 2 in a 556.

{see: look at the gated astable circuit}


See also:


file: /Techref/logic/555s.htm, 4KB, , updated: 2011/5/3 05:43, local time: 2018/1/23 10:02,

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