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D/A Conversion/Sampling Circuits > APP
4620
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App notes
> Display
Drivers > APP 4620

Oct 14, 2009

Keywords: ADCs, A/D converters, analog-digital
converters, shift registers, parallel-output registers,
transformers, transformer drivers, TRIACs
Related
parts

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APPLICATION NOTE 4620

Set Your Holiday Lights to Music

By: Hanif
Saeed

Abstract: This
circuit digitizes music with a 12-bit A/D converter, monitors
the resulting digital outputs, and produces a logarithmic
thermometer-code format to control multiple power outlets.

Attend
this brief webcast by Maxim on
TechOnline

This
design idea appeared in the August 6, 2009 issue of EDN
magazine.

As one of many ways you can implement a light
show, this one selectively activates various subsets in a
group of six strings of Christmas tree lights, causing them to
flash on and off according to the level and tempo of music you
are playing. The stand-alone circuit (Figure 1)
requires no microcontroller, no software, and no
trimming.

Figure
1. Driven by a 0-to-2.048V music signal applied at IC1 pins 12
and 1, this circuit activates the six AC outlets according to
the music amplitude, in a logarithmic thermometer-code
format.

The audio signal to be displayed is applied
to a 12-bit ADC
(MAX1276). The
signal ranges from zero to +2.048V in this circuit, causing
the first string of lights to come on at +2mV. Though the
circuit shown controls six AC outlets, it can be expanded to
control 12 outlets. When triggered by a short positive pulse
at CNVST, IC1 initiates a conversion clocked by the signal at
SCLK.
Its output (DOUT), clocked by rising edges of SCLK, consists
of four leading zeros followed by the 12-bit conversion
result, MSB
first. Thus, one conversion requires 16 clock pulses at
SCLK.

The display is powered by a vertical stack of six
switched outlets, in which the top outlet represents the MSB.
You might, for example, plug a separate string of lights into
each outlet. During operation, the circuit scans each
conversion result as it is generated (MSB first, as described
above) and notes the first bit to assume a value of "1." It
then turns on the corresponding outlet and all those below it
in the stack. The result is a logarithmic column, in which the
change of input voltage
necessary to move the column one step up or down (a 12.04dB
increment) is either quadruple or one fourth of the immediate
value. Although the number of steps available equals the ADC
resolution (12), this circuit only uses every other one to
drive the six outlets.

At DOUT, the first output bit
with a value of "1" charges C5 (via D1) to the logic-one
level. The voltage on C5 connects to the data input (DS) of
the first of two cascaded 74HC595 ICs, which together form a
16-bit shift
register. The signal that clocks the ADC, slightly
delayed, also clocks this shift register, and thereby inserts
into the shift register the value presented at its input. At
the end of a conversion, all bits following the first one that
exhibits a value of "1" are also forced to "1" by the voltage
stored on C5.

At the completion of each conversion, a
negative pulse applied to the SC_TP inputs of both 74HC595 ICs
transfers these shift-register contents to a parallel-output
register (IC6). The same pulse discharges the storage
capacitor through diode
D2, leaving the circuit ready for the next conversion scan.
The parallel-register outputs then serve as drivers for the
12-bit logarithmic column (with the MSB driving the top
outlet).

The 74HC4060 IC serves as a clock and
timing-sequence generator and the 74HC132 provides some
necessary glue logic. For each 74HC595 output connected, the
signal (inverted by the 74C901) activates the corresponding MAX253 transformer
driver (IC7 in one of the six Power Blocks). Isolated by a 1:1
transformer, this driver signal then triggers the TRIAC (Q1, a
solid-state TRIode for Alternating Current)
to its ON state. For the component values shown, the circuit
has a display sampling
rate of about 2.5kHz, and uses the 12th,
10th, 8th, 6th,
4th, and 2nd bits to control the six
outlets. The resulting light show adds an extra dazzle to the
music you are playing.

This circuit operates at lethal
voltages and requires proper handling (note that the
transformer must withstand a line level of 120VAC). It’s
designed to operate with incandescent light bulbs; no other
type of light bulb should be used. Even though the outlets are
shown as standard 120VAC outputs for use with commercial
incandescent lights, fast switching in the TRIACs makes them
unsuitable for driving other types of load such as appliances,
electronics, or AC adapters. Transformer T1 is a TGM-350NA
from HALO Electronics, Inc., and TRIAC Q1 is a
T1235-T from STMicroelectronics®.

STMicroelectronics
is a registered trademark of STMicroelectronics, Inc.

Related parts

MAX1276

1.8Msps, Single-Supply,
Low-Power, True-Differential, 12-Bit ADCs with
Internal Reference

Free
samples

MAX253

1W Primary-Side
Transformer H-Bridge Driver for Isolated
Supplies

Free
samples

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Download,
PDF format (58kB)
APP 4620: Oct 14, 2009

AN4620, AN 4620, APP4620,
Appnote4620, Appnote 4620

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