■ FIGURE 4
first pulse loads a one into the first cell
of the register and at the same time
resets the flip-flop (IC1, A and B). Take
a look at Figure 4. I used all four of the
NOR gates because they were there
and also to give some added delay to
insure that the J/K input is one when
the clock goes high. The capacitors, C1
and C2, are to debounce the mechanical switches. The resistors, R1 and R10,
provide an initial low level to the inputs.
The first pulse lights LED D1; subse-
quent pulses shift the lighted LED to D2,
D3, and D4. The last pulse leaves one
of the LEDs lighted and it stays that way
until the reset button is pressed. Almost
any NPN will work for the transistors;
2N3904 or PN2222, for example.
A fault with this circuit is that if a
second alarm comes in and the reset
button has not been pressed, then the
lighted LED will be shifted giving an
erroneous indication. It may be shifted
out of the register, leaving all the LEDs
unlighted. For that reason, I added an
LED to the IC1D output which
indicates that an alarm happened and
the reset button needs to be pressed.
If desired, a time delay could be
initiated from the IC1D output which
would automatically reset the flip-flop.
QCould you explain the
theory of operation of
motion detectors (sensors)?
Is it based on infrared or
■ FIGURE 5
ABoth infrared and ultrasonic
motion detectors are
available. The infrared detects
the heat of the human body,
so motion is not actually required.
Ultrasonic systems use the Doppler
Effect where the frequency of the
reflected wave increases when an
object moves closer and decreases
when the object moves away. A disad-