Published:2011/8/8 1:04:00 Author:Li xiao na From:SeekIC
By Burkhard Kainka
Filter
The output signal supplied by the second opamp drives the LED controls by way of simple filters. Each LED driver stage consists of a transistor (T2, T3 and T4). To prevent the transistor bases from being charged with negative levels, anti-parallel diodes (D3, D5, D7) are used on each base terminal. Each transistor is only actuated on the positive half cycle of the drive voltage. Overall, however, the higher frequencies do cause an impression of average brightness.
The filters consist of simple RC networks broadly dimensioned for a cutoff frequency using the formula
fc = 1 / (2πR C) |Hz|
For example, the low-pass section R12/C9 is dimensioned for about 160 Hz using 10 kΩ (R12) and 100 nF (C9). All lower frequencies in the music signal will therefore pass through this ’channel’ The mid-tone channels contains a combined high-pass / low-pass filter R14/C10+C11. The treble channel is driven by a simple high-pass R16/C12. If necessary the cut-off frequencies may be changed to suit individual requirements and that’s easiest done by making small changes to the capacitor values.
Printed circuit board
If the circuit of a relatively simple design, the actual construction of the sound to light unit is made even simpler by a printed circuit board (Figure 2). The two opamps we’ve discussed are contained in a single IC type LM385, which is best fitted in an IC socket (look at the notch in the IC body). The single-sided PCB has no wire links. No problems are expected to arise if you watch the polarity of the diodes, electrolytic capacitors and LEDs and work carefully all the way. The electret (or ’condenser’) microphone insert can be almost any available type as long as it has two terminals.
The maximum LED current amounts to about 18 mA so the circuit is suitable for LEDs described as ’standard’, ’bright’ and ’superbright’. Using the latter the effect of the 3-way filtering is more pronounced; while for a relatively small room with not too much light ordinary LEDs will be found to be perfectly suitable. If you decide to use low-current LEDs, the value of series resistors R13, R15 and R17 should be increased to 3.3 k or even 3.9 k. The resulting reduction in current consumption will enable your 9-V battery to last much longer.
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/08/Personal_Sound_to_Light_Unit__small_but_sophisticated_(2).html
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