than the pulse time, the
output will stay high until
the trigger goes high.
If the switch bounces
upon contact, it will
probably bounce upon
release which will also
cause the 555 to trigger.
The best operation of this
circuit occurs when the
switch closure time is
shorter than the pulse
width. The PIC 555
replacement can be
used as a debouncer
(Schematic 5) in much
the same way as the
555. Simply set its
mode to either 0 (
one-shot) or 2
(retriggerable one-shot). The main
the PIC and the 555
are that the PIC’s
output pulse is not
dependent on the
triggering pulse width.
Plus, you can modify
the program to trigger
on either edge.
However, it will still be susceptible to triggering when the
switch opens in the same way as the 555.
If you have a choice of devices to use for debouncing,
a better solution is to use a Set-Reset flip-flop and a two-position switch. I have included a schematic (Schematic
6) which shows three simple switch debounce circuits.
Circuits 1 and 2 are examples using Set-Reset flip-flops;
circuit 3 can use any logic non-inverter.
The purpose of the resistor in the feedback loop is to
minimize the amount of momentary
current the output has to sink or source
when you change the state of the
switch. All three of these circuits require
a two-position switch of break-before-make construction, which is typical of
most toggle switches.
Mims Circuit 4
Circuit 4 (Schematic 7) is a Touch
Activated Switch. It activates when your
body (acting as an antenna) picks up
the 60 Hz power line. With the timing
values listed in the book, the pulse
width will be between 1 ms (C1 = .01
µf) and 1 sec (C1 = 10 µf). If you use
the smaller values of capacitance, you
will get a series of pulses out of the 555
at a 60 Hz rate. Depending on your
particular application, it might be better
to use the 555 as a retriggerable one-shot and set the
pulse width to about 20 ms.
The PIC processors I have been using do not have the
hardware to use a touch sensitive switch in this way.
Several of them do have a capacitive sensing module.
However, the sensing uses the change in frequency of
an oscillator to determine when the switch has been
closed. This means that your finger is acting as a capacitor
to ground instead of an antenna as described above. A
method similar to the one in the book can be implemented
using either an external op-amp (Schematic 8) or one of
the larger PICs which has an integrated op-amp.
May 2017 51
SCHEMATIC 7. Touch activated switch.
SCHEMATIC 8. PIC 555 touch activated switch.
SCHEMATIC 9. The 555 relay driver.
Engineers Mini-Notebook 555 Circuits by Forrest M.
Mimms, III ©1984. Also see www.forrestmims.org.