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-Signetics Application Note 170-----NE555.pdf

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555 Timing:  Ra Rb
Duty Cycle >50%
Duty Cycle <50%
 Normally the 555 timer is unable to produce a duty cycle of 50% or less. This is due to the fact that the first half of the cycle both Ra and Rb determine the charging interval (T1); where Rb alone determines the discharge interval (T2).
 To allow a Duty Cycle of 50% or less, a Diode D1 is placed in parallel with Rb such that during the charging cycle (T1) Rb is bypassed. This allows Ra and Rb to act independently, allowing a duty cycle of nearly 0% to nearly 100%.
Triggering of a 555
The Triggering process starts when the negative differentiated pulse edge "dips" below 1/3 Vcc, the capacitor starts charging.

If the trigger is held below 1/3 Vcc longer than the charge time, the output will remain high even though the capacitor charging cycle is complete; and then only goes low when the trigger rises above 1/3 Vcc

It can be seen, therefore, that it is desirable to have the negative going trigger pulse to be shorter than the charge time.

Using two 555s as an AM Transmitter
A Word  or Two about using Pulse Width Modulation for AM broadcasting:
Always keep the Pulse Width of the modulation between
> 50% & < 100%;
OR between
> 00% & < 50%.

At first blush, the reasoning for this may not be obvious: A transmitter is not broadcasting a varying voltage but a varying POWER. That is, the varying modulation voltage is controlling how much Carrier POWER is transmitted. Although the modulating voltage may have a polarity +/-, the RF power leaving the antenna has no polarity.

At the receiver the varying power is demodulated/ which is saying it is converted from a varying power to a varying voltage, and if you "AC Couple" the detector's output you again have a varying voltage that has a +/- polarity. --Huh?

NOW! At this point you should not have experienced an "Ah Ha!" --yet.
--BUT,  Stand-By YOU are about to Experience a Genuine "Ah Ha!"  --or your money back.

 Of course, we will need a  Graphic:-->

Notice anything Funny? Right, the second harmonic of the modulation signal increases if the [ > 50%, < 50% ] rule is violated!

 Example:  Generating a delay,  using Dual 555s (or one NE556) Triggering a 555 requires that the "Trigger" input be A.C. coupled. Typical values can be found in the 555's data sheet & app notes. However, some values: Rp = 10k, Cc = .001ufd. ______________ Car Tachometer The timer receives pulses from the distributor points. Meter M receives a calibrated current thru R 6 when the timer output is high. After time-out, the meter receives no current for that part of the duty cycle. Integration of the variable duty cycle by the meter movement provides a visible indication of engine speed. 555 Circuit using 60 Hz as clocking source Using Countdown circuit for LONG delays. Note relay driving circuit with Protection. Layout of 555 Circuit 555 as  F.M. Xmitter 445 kHz Oscillator frequency modulated by a 1-kHz tone. Method for  50% Duty Cycle Copyright 1999 -- 2016  Comments or questions about this site webmaster Suggestions are Solicited, P l e a s e ! .