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There will inevitably be some DC offset at the output of IC1, particularly at higher gains. This is blocked by C3. IC2 is a conventional op-amp and is configures as a non-inverting amplifier with a gain of 11 at lower frequencies. C6 reduces the gain progressively at higher frequencies. This is intentional since the effect works better at lower frequencies - in particular the bass beat. The value of C6 can of course be adjusted to suit your preferences.

The output of IC2 feeds a rectifier circuit comprising D1 and D2. C7 is a DC blocking component and C5 is the smoothing component. C5 is charges via R10 and discharges via R8, R60 and the base of TR1. The component values are arranged to give a gentle attack and slow delay characteristic.

R6 passes sufficient current to bias IC1 to give maximum gain. If the signal level on the output of IC2 is too great the output of the rectifier will rise, causing TR1 to pass more current. This diverts current from the control pin of IC1, reducing the gain. This automatic level control circuit will maintain a consistent output level from IC2 despite a large change in audio input level. From my measurements, a variation of at least 30dB can be accommodated.

The output of IC1 also drives another rectifier circuit comprising R11, C8, C9, D3 and D4. This circuit has a much faster response, due to the low values of R11 and C9. The track of VR1 is the discharge path for C9. A portion of the DC level from this rectifier circuit is tapped off by VR1 and buffered by IC3. A CA3160 op-amp was chosen because it's output can be driven to within 0.1V of the positive and negative supply rails.

The op-amp circuitry mid supply rail is derived from a potential divider (R57 and R57), decoupled by C11.

IC4 (LM3914) is a ten channel bar-graph LED driver IC. The simplified block diagram (figure *) shows how the working of the device more clearly than a long-winded description from me!. This figure and the text in the next paragraph are taken from the LM3914 data sheet, which is copyright National Semiconductor.

"The LM3914 IC senses analogue voltage levels and drives ten LED's, providing a linear analogue display. A single pin changes the display from a moving dot to a bar-graph. Current drive to the LED's is regulated and programmable, eliminating the need for resistors. The circuit contains its own adjustable reference and accurate ten step divider. The low bias current input buffer accepts signals down to ground or V-, yet needs no protection against inputs of 35V above or below ground. The buffer drives 10 individual comparators referenced to the precision divider."

Referring to the circuit diagram, you will notice that I have added resistors in series with the LED's, which appears to contradict the description in the previous paragraph. If the resistors were omitted the outputs would still drive the LED's correctly, but the voltage on the output pins would vary from +14V to +11V, which is not sufficient variation to drive the inputs of CMOS logic devices. The resistors drop an additional 8V approx, giving a level which is low enough to register as a CMOS logic 0, whilst allowing the current regulation to operate. The resistors also reduce the power dissipation within the IC. An output is low when the appropriate LED is lit.

The output current is controlled by the voltage reference circuit, and is about ten times the current flowing from the Ref-Out pin. In this case the reference is set to 3.5V (see formula on figure *), and the reference current is 1.3mA, giving an LED current of about 13mA. The reference voltage is applied across the internal divider resistor chain.

The bottom of the chain is lifted above 0V by the addition of R14. This reduces the dynamic range required, enhancing the effect. The value of this component may be modified to suit your preferences and the style of music played. Modern dance music has a low dynamic range and the value of 4K7 is about right. If you play music from the 60's or 70's, 1K5 would be more suitable. If you play a selection of music, 3K3 is a good compromise. You could replace R14 with a 5K0 preset, and adjust it to your liking.

The ten triac output circuits are identical - I will use the first stage (TR3, TR13) in this description. When the LED1 line and the Z-CROSS lines are both low, the output of the 4001 gate will be high. This will turn on transistor TR3, which drives the gate of triac TR13.

Since the Z-CROSS line is low for only a brief period as the mains cycle passes through 0V, the triac can only be switched on at this point. Once the triac is on, it will remain in this state until the current passing through it drops below a low holding current. With a resistive load this occurs as the mains cycle approaches the next zero crossing point. Thus the load is driven for complete half cycles. No switching occurs at points in the cycle where the switching current would be high.

Because the triac is triggered only momentarily at the zero crossing point, the output is not suitable for driving inductive loads. With an inductive load the current and voltage are out of phase, so this simple triac driving arrangement will not operate correctly. Since the unit is only intended to drive normal light bulbs, this restriction should not cause any problems in practice.

The Z-CROSS signal is derived from the mains transformer. A normal two diode (D15 and D16) full wave rectifier circuit is used with the centre tapped transformer (X2). However an additional diode (D17) is added between the rectifier and the smoothing capacitor (C10). The signal at the junction of the three diodes is a full-wave rectified sine wave, which drops to zero at the zero crossing points. This signal drives TR2, such that it turns off when the voltage at the junction of the diodes is below about 3V. The output on the collector of TR2 is inverted by a gate in IC5. The inputs of the unused gate in IC5 (pins 5 and 6) are connected to 0V.