Q&A
■ WITH RUSSELL KINCAID
In this column, I answer questions about all
aspects of electronics, including computer hardware,
software, circuits, electronic theory, troubleshooting,
and anything else of interest to the hobbyist. Feel
free to participate with your questions, comments, or
suggestions.Send all questions and comments to:
Q&A@nutsvolts.com
WHAT’S UP:
Join us as we delve into the
basics of electronics as applied
to every day problems, like:
Christmas Tree Lights ✓
● Clock Oscillator Schematic
Noisy Audio
✓
✓
CHRISTMAS TREE
LIGHTS
QI would like to develop or buy a device/circuit that would apply voltage gradually to a string of
Christmas tree lights so when a switch
is turned on, the lights would light
slowly and when turned off they would
go off slowly. In model trains, I believe
it is called a momentum throttle. An
ability to vary the rate would be good.
Thank you in advance for any
help you can provide.
■ FIGURE 1
— Bob Rymer
AChristmas tree lights can vary from tiny milliwatt bulbs to up to two or three watts per bulb, so I will
design for up to 70 watts of power.
In the circuit (Figure 1), the diode
bridge allows the control circuit to
operate at DC while AC flows through
the load. When SW1 is open, the full
168 peak volts are across Q1 and
there is no current in the load. When
SW1 closes, C1 charges to 168 volts;
this puts a spike of current in the
load (probably not noticeable). C2
gradually charges through R1 and R2,
slowly turning on Q1. The AAA
battery, G1, puts an initial charge on
C2 so you don’t have to wait until it
charges up to the Vbe of the
transistors. C2 is a super cap; values
up to one farad are available if you
want an even longer delay. The turn-on time is adjustable from less than
one second to about five seconds via
R2. The turn-off time is determined
by the current draw of Q2 and is not
easily varied. The turn-off time is
expected to be about five seconds,
according to my simulation. Figure 2
that interests me most is the FM
demodulator, such as the LM565. I
have seen basic circuits like that
before on the web or in books, but
could you explain where the
equations come from? Is it common
to treat these chips as black boxes
and just use the equations from the
datasheet? Is it simple or are the
equations fairly complex to derive?
■ FIGURE 2
22 May 2010
TURN ON/TURN OFF DIMMER
PARTS LIST
PART DESCRIPTION
D1 1 amp, 400V bridge
D2 1 amp, 400V diode
Q1 3 amp, 275V NPN
Darlington
Q2 100 mA, 300V NPN 512-KSC2258ASTU
C1 100 µF, 250V, aluminum elect. 140-XRL250V100-RC
C2 22,000 µF, 5.5V supercap 598-EDLSD223V5R5C
G1 1.5V AAA cell 115432
R1 1K, 1/4W 271-1K-RC
R2 100K potentiometer 31VC501-F
All Mouser.com part numbers except **Jameco.com
PART #
512-DF04M
512-1N4004
512-FJP9100TU
PHASE
LOCK LOOP
QCould you explain the
basics of phase
locked loops?
The example
AI believe Z transforms are needed to really analyze a PLL but I know nothing about that, so I use
standard linear analysis which only
goes so far. The following is from an
article that I wrote which has not
previously been published.
There are two types of phase
lock loops (PLL). Type 1 uses a
passive filter and type 2 uses an
operational integrator as a filter.
The type 2 PLL consists of: mixer
or demodulator; integrator; and
oscillator, as shown in Figure 3. This
type has zero phase error in the steady
state and is useful for recovering a
pilot tone or carrier frequency.
The type 1 PLL consists of: mixer
or demodulator, low pass filter, and
oscillator, also shown in Figure 3. This
