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kitchen table Personal Sound to Light Unit small but sophisticated Burkhard Kainka Disco nights are great fun from time to time but you don’t want to overdo it in regard of the sound volumes you’re exposed to for a couple of hours. Arguably there’s no less pleasure in enjoying dance music in the privacy of your home, study or student digs. However, the true disco feeling is not obtained without a matching sound to light unit, so here’s a really small version. 58 elektor electronics - 6/2004 BT1 R3 8 100k 10k R1 C13 R13 T2 9V IC1 grün green R12 22µ 16V 4 D4 390Ω 10k C9 D3 BC548C 100n R2 10k C1 R11 1N4148 100k R6 10µ 16V IC1 = LM385 100k R15 T3 5 C2 3 IC1.A IC1.B 6 10k 2 C4 x 100 1k MIC1 x 10 R10 2µ2 16V R5 T1 7 C8 R14 22n D5 10µ 16V BC548C BC548C C7 1N4148 100n D1 R9 R17 T4 R16 R8 10k 100k C3 C11 10n 10k R4 yellow gelb C10 10k 1µ 16V 10k 100n 1 R7 D6 390Ω D2 C5 C6 22n 4µ7 16V D8 390Ω rot red C12 10k 4n7 2x 1N4148 D7 1N4148 BC548C 030019 - 11 Figure 1. The circuit of the sound to light unit consists of a controlled-gain amplifier followed by a 3-way frequency splitter. A sound to light unit converts music signals into light pulses. In most cases, three channels are used to cover different frequency ranges. The bass channel will then indicate the ‘beat’ of the music by a more or less rhythmical flash, while the two other channels represent the higher frequency ranges of the music channel. The ‘private’ version of a sound to light unit discussed in this article employs three coloured LEDs instead of powerful flashing lights or floodlights as used in most discos. For the rest, it has practically the same functionality as the ‘real thing’ experienced on Saturday nights. However, a direct connection to the music amplifier is not necessary as the unit we’ve in mind has been designed to pick up the sound information through a microphone. Another peculiarity of the circuit is the automatic sensitivity adaptation to the music volume detected. In this way the circuit can work without any controls. The circuit draws an average current of just 20 mA and works happily off a 9volt PP3 block battery. However, the low current requires super bright LEDs to be used to ensure sufficient brightness. Amplify it High amplification is required to enable the tiny signals produced by an 6/2004 - elektor electronics electret microphone to be turned into bright flashes from LEDs. The circuit diagram in Figure 1 shows that we employ two operational amplifiers with a total gain of about 1000 times. The input of the first opamp (IC1.A) is connected to the electret microphone capsule via coupling capacitor C2. The microphone is given a certain DC bias level obtained from the 9-V supply rail by resistors R1 and R2. R1 together with electrolytic capacitor C1 decouple the supply voltage for the sensitive microphone while R2 equals the microphone impedance. Capacitor C2, then, ensures that the microphone bias level does not appear at the opamp input. In other words, it will only pass the alternating component, which is caused by sound picked up by the microphone. The + input of the opamp has its own bias voltage supplied by potential divider R3-R4. Because the two resistors have the same value, the supply voltage is effectively halved, i.e., 4.5 V exists at the junction of R3 and R4 (assuming a nominal 9 V supply). This bias voltage will also exist at the output of the first opamp and, because of R7, at the input of the second one (IC1.B) whose output will also copy this dc level. In this way, R3 and R4 keep both opamps biased at half the supply voltage. Both opamps are used in the non-inverting configuration hence provide unity (_1) dc gain. For alternating signals, however, the gain is much greater. In the case of the first opamp, the gain is determined by the ratio between resistors R6 and R5, or R11/R10 for the second opamp. Just look at the relevant resistor values and you’ll discover that IC1.A is configured for a gain of 100 and IC1.B for a gain of 10. The signal level at the output of IC1.B is rectified by D1 and D2, smoothed by C6 and then used to drive n-p-n transistor T1. To the signal voltage at the output of IC1.A, the combination of R7 and T1 looks like a voltage divider. With rising signal levels, the rectified voltage on C6 also rises and the transistor is driven harder because of the larger base current supplied by R9. The result is a lower resistance in the transistor and consequently a lower AF signal behind R7. You may wonder why we did not use an n-p-n transistor without direct current in the collector circuit. Alternatively you might have expected to see a FET at this position, its drain-source junction acting as a controlled resistance. It is less known that virtually the same function may be obtained from a regular switching/AF transistor like the ubiquitous BC548C. An n-p-n transistor, too, represents a variable resistance that can be controlled wit- 59 D6 = LED, yellow (see text) D8 = LED, red (see text) IC1 = LM385N (with socket) T1-T4 = BC548C Resistors: R1,R2,R7-R10,R12,R14,R16 = 10kΩ R3,R4,R6,R11= 100kΩ R5 = 1kΩ R13,R15,R17 = 390Ω C4 = 2µF2 16V radial C5,C10 = 22nF C6 = 4µF7 16V radial C8 = 1µF 16V radial C11 = 10nF C12 =4nF7 C13 = 22µF 16V radial Capacitors: C1,C3 = 10µF 16V radial C2,C7,C9 = 100nF Semiconductors: D1,D2,D3,D5,D7 = 1N4148 D4 = LED, green (see text) 1-910030 R1 C13 030019-1 C5 R16 R11 R10 R2 R3 R6 R7 ROTKELE )C( T4 D8 C12 C2 MIC1 C7 D7 R14 R17 C10 R15 R4 R5 C1 C3 C6 R8 R9 D1 D2 D5 C4 T1 + D6 C8 T hin a certain range. However, for a low-distortion volume control, only a tiny signal level (of the order of millivolts) may be applied to the collector. This condition is not satisfied here as the output voltage is regulated to about 1 Vpp. If the second stage has a gain of 10, about 100 mVpp can be found at the collector. At such a level, distortion occurs that will not be acceptable in other applications. No problem for the sound to light unit, however, because the output signal is used to control LEDs than drive an audio amplifier. If you do want to use such a ‘volume control’ for audio applications, you should make sure a much smaller signal level is handled, which is probably easiest realised by moving the automatic volume control towards the circuit input. Miscelllaneous: BT1 = 9V PP3 (6F22) battery with clip-on lead MIC1 = electret microphone PCB, available from The PCBShop IC1 Components list R12 D3 C11 T3 C9 D4 - R13 T2 Figure 2. The single-sided PCB has no wire links. 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 / (2 π R 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 60 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. (030019-1) elektor electronics - 6/2004