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15
Sound to Light
Zero-crossing detector
Transistor: general purpose pnp
This circuit gives out a short pulse each time the 220v supply passes through
zero.
in
7809
out
+
gnd
33k
470µ
9v supply for
modulator
220v
3k3
3k9
-
N.B.There is only one cut on the copper side of this board; you should be
able to see where it must be !
Modulator
The pulses are used to set (start) a monostable (made using a CMOS 555
timer). When the timed period is finished, the monostable resets and gives a
pulse which causes the triac to start conducting. The time period of the
monostable is varied using a rectified and smoothed voltage taken from the
music source. Opto-isolator: MOC3020 or similar. Op-amp: 081.
+
470k
Ge
2k2
4
100k
8
X 1M
555
2
1
6k8
500k
7
5
C
fuse
6
100n
4k7
3
220v a.c.
2
6
1
4
47n
2
-
1
g
1
The capacitor C determines the speed of response of the system; try about
1µF.
If a low pass filter is connected to the input, the light will be modulated by the
bass notes in the music. With a high pass filter, the light will respond the
treble notes.
22n
10k
input
330n
input
output
Low pass filter
10k
output
High pass filter
Vero board for circuit using an opto-isolator (as shown in the circuit diagram).
If you use an isolating transformer instead of an opto-isolator, the vero board
will be as shown below.
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The diagrams below are to illustrate how the triac is used to vary the
brightness of the light bulb. The first diagram represents a graph of voltage
against time for the 220v supply.
The next two diagrams represent graphs of current flowing through the triac
(and light bulb) against time. In these diagrams, t1 is the time at which the
monostable is triggered and t2 is the time (a few milli-seconds later) when it
reaches the end of its timed period and gives a pulse to start the triac
conducting.
N.B.when the current through a triac falls to zero, it returns to its nonconducting state until it receives another pulse into its gate.
3
t1
t2
Light at about half power
t1
t2
Light at less than half power
© David Hoult 2001
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