The operation of this circuit can easily be obtained
using a 555 replacement in either mode 4 or mode 5.
Schematic 5 shows the wiring of the 555 replacement for
this equivalent circuit (the resistor values are for mode 5).
Note that the PIC uses a transistor buffer to drive the
The PIC output current is limited to 20 mA, so a
buffer is going to be required for most relays. Select mode
4 if you want to control the off-time of the relay and its
on-time. Select mode 5 if you want to control the period
of the relay operation and its on-time. This is a subtle
difference but may be important depending on the
Also, note that the 555 circuit uses +12V while the
PIC should use no more than +5V. A higher voltage relay
can be used if desired simply by connecting the “top” side
of the relay and cathode of the diode to the required
Mims Circuits 12, 22, and 23
The circuits in Schematic 6 use the 555 as a
simple pulse generator. For FM- 22 and FM- 23, the
only differences are the loads the 555 is driving
and the timing components.
Its astable operation has been discussed in
previous articles of this series, so I won’t repeat it
here. The frequency range of FM- 12 is roughly
1.4 Hz to 300 kHz depending on the capacitor. I
have included a spreadsheet with the article
downloads (555 Astable.ods) which calculates the
time values based on the three timing
components. Keep in mind that as R1 becomes
small, the circuit will stop operating correctly (if
R7 = 0).
I found that with C1 = .01 µF or .1 µF, I
needed to make R7 about 390 ohms for the
circuit to work properly with R1 = 0 and VCC =
5V. Each capacitor value yields about a 400:1
With this circuit, the output is a very narrow
low level pulse. For any given range
determined by the capacitor, the output
pulse width will be fixed unless you vary the
value of R2. The following formulas apply:
LowPulse Width = ln( 2) x R2 x C1 = .693 x
R2 x C1
Period = ln( 2) x (R1 + R7 + ( 2 x R2)) x C1
Due to component tolerances —
especially the capacitor — you can use
either 0.66 (2/3) or 0.7 instead of the
constant shown in the equations, which will
get you close enough for the vast majority of applications.
The values shown in the schematic are specifically for
FM- 12, the pulse generator. For FM- 22 (an LED flasher),
R1 is 100K and C1 is 47 µF. The flash rate can be varied
between 0.2 Hz and 8. 3 Hz. The 2N2222 inverts the
signal and causes the LED to flash on for a short time
(when pin 5 is low) of approximately 32 ms.
Since the 555 has plenty of drive capability that can
easily handle an LED directly, it’s possible to get the same
results without a transistor using the alternate LED circuit
shown in the box. The value of R6 (as well as R4) should
be calculated based on the supply voltage and the current
required by the LED. The values shown for R6 cause about
5 mA through LED2 at the two voltage extremes.
Just as a note of interest, the original circuit with the
transistor draws roughly the same amount of current
independent of the state of the LED. The alternative circuit
(without the transistor) draws very little current when the
LED is off.
60 July/August 2018
SCHEMATIC 6. 555 pulse generator.
SCHEMATIC 5. PIC 555 replacement for the intervalometer.