quite close to the calculated value.
The second section of the 556 is wired as a monostable multivibrator and should have a quite short output
pulse. The formula for the pulse width of the monostable
section is PW = 1.1*R3*C3 = 110 µs. When the trigger
signal goes low, the 555 output immediately switches high
and the capacitor starts to charge. However, since the
trigger is low for longer than the calculated pulse width,
the capacitor does not charge completely. It only charges
to about 0.7V due to Q1 being on.
Picture 5 shows the results with and without Q1. A is
the triggering pulse from the astable; the actual triggering
edge is off the screen to the left. B is the voltage across
C3 when the transistor is in place.
You can see that the capacitor voltage is clamped at
about 0.7V until the triggering signal goes high, at which
time the capacitor is allowed to complete its charging. The
capacitor charges only to the threshold voltage, which
then enables the internal discharge circuit.
C shows the capacitor voltage with Q1 removed. It
charges to the power supply voltage since the
discharge circuit is disabled, as long as the trigger is
low. The slope of the discharge is due to the finite
amount of current the discharge circuit can pull
from the capacitor. Using the formula cv=it, the
current calculates to about 14 ma: .01 µF 5V/3.55
µs. The only difference between having the
transistor in the circuit vs. out of the circuit is that
with the transistor, the output pulse is about 97 µs
wider that without it. This is somewhat shorter than
the 110 µs calculated previously due to the
capacitor starting from .7V rather than 0.
Since the output pulse width on pin 9 is
basically the same as the trigger pulse from pin 5 —
except that it’s inverted — the same effect can be
achieved with a 555 using the same timing
components as Section 1 of the 556 by connecting
the piezo element between the output and Vcc. If
you use the circuit as a warning device, you can use the
Reset input to enable and disable the oscillator.
To use the PIC
replacement, you can work
the circuit shown in
Schematic 7. The
schematic shows Range 3
(1 ms to 1 sec) and mode
5 which are probably the
best for this application.
You can adjust R6 to give
you the desired tone/chirp
duration, and R5 to set the
repetition rate/period. If
you use mode 4, then R5
would adjust the time
Mims Circuit 18
Circuit 18 in the Mims book (Schematic 8) — called a
February 2018 41
SCHEMATIC 6. 556 chirp generator.
SCHEMATIC 7. PIC 555
PICTURE 5. 556 chirp generator capacitor charge and