semiconductor process used for the part: MOS versus
bipolar. Using an MOS part (input current about 10 pa)
allows you to pick a photoresistor with higher on and off
resistances. If you are using a bipolar part, you will need a
photoresistor with lower resistance values and you may
need to make R3 smaller.
You can use the circuit of Schematic 2 for this
application as well, with either astable mode ( 4 or 5). The
same photoresistor circuit can also be utilized to drive the
gate/trigger (RA3) input.
Mims Circuit 25
Schematic 9A is an infrared security
alarm. U1 is configured as an astable
multivibrator with a frequency of about 6
Hz, period = 166 ms. The LED will be
turned on when the output is low; about
3. 2 ms. U2 is configured as a monostable
multivibrator (one-shot) with an on-time of
almost 52 ms.
With the insert in place, Q1 will be
off which causes Q2 to be held off and
the trigger input of U2 is held high. With
the trigger high, the output will be low
keeping the buzzer turned off. When the
insert is removed, U2 will be triggered as
well as Q2 turning on synchronously with
the output of U1 causing the one-shot
(U2) output to go high.
Capacitor C2 will charge until it
reaches the threshold voltage at which
time the output will turn off and the
discharge circuit will turn on. The buzzer
in this circuit must be of the internally
Since I was unsure of the function of
Q2, I tested the operation of this circuit
by driving the U2 trigger circuit directly
from a PIC 555 in mode 4, range 2, and off-time of about
When the trigger goes low (simulating the insert being
removed and LED1 turning on), U2 is triggered, Q2 turns
on, the 555 discharge circuit turns off, and the buzzer
turns on for the duration of the C2 charge cycle.
With the component values shown, the removal of
Q2 has no impact on the circuit. The output low time of
U1 is short enough ( 3. 3 ms) that Q2 does not affect the
charging of C2. If the U1 output low time is increased to 6
ms or greater, then Q2 begins to have an effect on the
charge time of C2; see Picture 1.
The area circled in red shows the plateau caused by
Q2 which is clamping the capacitor to about 0.5V.
Since the plateau does not start until about 5 ms
after the trigger, it will not occur with a trigger
pulse width less than 5 ms.
Once the trigger is removed, Q2 will turn off
allowing C2 to finish charging to the threshold
voltage and turn off the output.
Schematic 9B shows another possible
implementation. This one is similar to FM- 24 in that
the Reset input is used as a gate for the astable
I also changed the operation such that the
frequency of the 555 is about 500 Hz so that the
buzzer needs to be of the externally driven type. In
this case, the buzzer will sound continuously when
the insert is removed.
62 July/August 2018
SCHEMATIC 9. 555 infrared security alarm.
PICTURE 1. 555 astable capacitor charge cycle.