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555 General Specifications 
Vcc 
  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)
3.3-V
6.6-V
10.0-V
   Nominal
 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 
Graphic:--> 

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 groundplanes---***NO PROTO_BOARD!!! 

 

       ** MORE SUGGESTIONS: **

 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 .