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----555 Timing Plots---Signetics Application Note 170----PDF version AN170---NE555.pdf

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555 General Specifications 
 5-Volts  10-Volts 15-Volts   Notes: 
Max Frequency (Astable)
 500-kHz to 2-MHz 
  Varies with Mfg & Layout
 Vc Frequency Range  +/- 25%  +/- 25%  +/- 25%   Linear range
 Vc Frequency Range  +/- 90%  +/- 90% +/- 90%   Max Deviation
 Vc Voltage Level (center)
 Vc Voltage Range          
Frequency Error (Astable)
 ~ 5%
 ~ 5%
 ~ 5%
   Temp 25° C
Timing Error (Mono)
~ 1%
~ 1%
~ 1%
   Temp 25° C
Max Value  Ra +Rb  3.4-Meg  6.2-Meg  10-Meg    
Min Value   Ra   5-K  5-K  5-K     
Min Value   Rb  3-K   3-K   3-K     
Reset VH/VL (pin-4)  >0.4/<0.3   >0.4/<0.3  >0.4/<0.3    
Output Current (pin-3)  ~200ma  ~200ma  ~200ma    

555 Timing:  Ra Rb
Duty Cycle <50%
Duty Cycle >50%
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 

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

Some Hints on Prototyping the 555
When building your prototype circuit, you can avoid confusion if YOU MAKE SURE OF THE FOLLOWING:

1) Double check (triple check) the schematic against the DATA SHEET! 

2) Draw a pictorial--drawing--of the circuit from the schematic, that is, draw the dip on its back--numbering the pins clockwise--use a socket that is larger than the IC dip, using the extra positions for resistors and caps. 

3) Now draw--using pencil--the connections between the pins. 

4) Don't Forget the BYPASS CAPS!!!

 ** Refer to the Prototyping Page **

 5) Don't be shy about using several NEW parts--you could have a BAD PART(s)! 
This includes resistors and CAPACITORS. ** DO NOT use electrolytics for the timing caps 
(if at all possible)!

 6) Check the DATA SHEET, you may be using a resistor for Ra or Rb that
is too low in value, e.g., the 555 doesn't like Ra values below ~5K.

 7) Pin 5--the Voltage Control pin of a 555 timer must be A.C. coupled!
If you Direct Couple pin 5, you force a bias on it that could prevent it from ever charging the timing cap to 2/3 Vcc thus rettriggering!

 8) When troubleshooting, remember more than 80% of all problems could have been found using your EYES! "--gee, I didn't see that the power cord was out of the wall..."

 9) To get the best performance from the 555 in question, use the highest allowable supply voltage--15Volts--the timing capacitor should be in the picofarads and the "R" the very minimum value: you should get better than 500 kHz to ~2MHz.

 10) Bypass the device and use ground planes---***NO PROTO_BOARD!!! 


**CHECK-it-OUT:  the following pages 
Prototyping, R.F., Layout, Bypassing, Decoupling, Shielding, EMC,

 1) BYPASS pin 8 (Vcc) to 1 (grd.) with at least .1 ufd ceramic cap--shortest leads possible!!!

 2) Use the smallest timing capacitor--pins 6&2 to grd.

 3) Try different 555 chips, and/or different manufacturers 555s they vary!

Finally, Don't hesitate to "PLAY"--to experiment with this device; the 555 timer has many uses--some yet to be discovered; who knows you may be the one to find even more uses for it.

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 .  

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