The Triggering process starts when
the negative differentiated pulse edge "dips" below 1/3 Vcc,
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.
A Word or Two about using 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 --or your money back.
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.
5) Don't be shy about using several
NEW parts--you could have a BAD PART(s)! This includes resistors and CAPACITORS.
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
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 .