and debugging. Once the devices are programmed
for an application, pin 7 may be left floating but pin 4
should be tied high. RA3 can only be a digital input
when the unit is not connected to a programmer.
Circuit 16 (Schematic 7) is a gated oscillator
which uses an FET in series with the timing resistors
as the gate. The FET can be replaced by directly
connecting the output of the logic device to the high end
of R1 if you use a device from one of the many CMOS
families such as 74HC.
Either solution requires that the supply voltage be 5V
or lower. If you use a device from one of the original
CMOS logic families — such as the 4000 series — you can
use a supply of up to 18V.
Another solution for the 555 is shown in the
schematic as an option. You can simply use the Reset
input to gate the oscillator. You’ll need to remove Q1 and
connect R5 directly to R1.
All three astable modes of the PIC replacement for
the 555 are gated. For most audio applications, mode 4 is
appropriate (Schematic 5) with pins 5 and 6 tied together
for a 50% duty cycle. Pin 4 is the gate input; it’s shown
tied high for normal astable operation. Grounding it turns
off the oscillator.
Circuit 20 (Schematic 8) shows a 555 wired as a tone
burst generator. It uses an RC network (R3, R4, C2) on the
Reset input in order to enable and disable the output.
When the switch is closed, the output is enabled and
C2 charges up to above the threshold voltage of the Reset
input. When the switch is opened, C2 will discharge
through R4 until it’s below the Reset threshold, which
then turns off the output.
The time interval that the output remains on after the
switch is opened is dependent on the R4*C2 time
constant. The circuit will respond more quickly to the
switch if the value of R3 is reduced; 100 ohms will work
I modified the original PIC replacement program so
that it now has gated astable multivibrator modes using
pin 4 as the gate input. The same circuit used by the 555
in Schematic 8 can be used on the gate input of the PIC.
It will operate identically to the way in which the 555
operates, except the 555 has a 0.7V threshold whereas
the PIC inputs are set up for Schmitt trigger levels with a
somewhat higher threshold.
You can also use the same speaker circuit on the PIC
as is used by the 555. However, I prefer the speaker
circuit shown in Schematics 2 and 5.
All of the sample programs, schematics, diagrams, and
pictures for this series can be found on my website
( www.qsl.net/k3pto), as well as the article link on the
Nuts & Volts website ( www.nutsvolts.com).
Happy PICings until next time! NV
December 2017 45
SCHEMATIC 8. 555 tone
burst generator. Resources
All schematics are drawn using Dip Trace
All parts purchased are from Digi-Key
Answers to the Acronyms Quiz
on Page 6:
1. Double Pole Double Throw. 2. Graphical User
Interface. 3. Real Time Clock. 4. Musical Instrument
Digital Interface. 5. Liquid Crystal Display. 6. Bayonet
Neill-Concelman. 7. Personal Digital Assistant. 8. Ultra
High Frequency. 9. Reduced Instruction Set Computer.
10. Random Access Memory. 11. Garbage In Garbage
Out. 12. Digital-To-Analog Converter. 13. Normally
Closed. 14. Computer-aided Design. 15. Root Mean
Square. 16. Field Effect Transistor. 17. Universal Serial
Bus. 18. Reverse Polish Notation. 19. Pulse-Width
Modulation. 20. picoFarad. 21. Vacuum Tube
Voltmeter. 22. Lithium-ion Polymer. 23. American Radio
Relay League. 24. Inter-Integrated Circuit. 25. Dual InLine Package. 26. Exclusive OR. 27. Cathode Ray Tube.
28. Surface-Mounted Device.