This circuit modulates the brightness of a small incandescent lamp in a continuous rising and dimming cycle, with the objective of optimizing the aesthetic appearance of the intensity changes. The lamp used in this application is the miniature low-voltage type 1892, rated at 14.4V, 0.12A, and 0.75 mean spherical candle power. The lamp's modest level of brightness allows the direct observation of the bulb without eye discomfort. Control of lamp intensity is achieved by phase-modulation of the AC line voltage, greatly reducing waste power, as compared with circuits employing DC techniques. The circuit may easily be adapted for driving a multitude of paralleled 1892 lamps, or with the appropriate selection of transformer T2, many other types of incandescent lamps.

Operation is as follows:  Transformer T1, rectifiers CR1, CR2, filter capacitor C1, and voltage regulator VR1 provide +12V of direct current for the circuit. C2 improves the regulator's transient response. T1's secondary also drives diodes CR3 and CR4 to provide a full-wave rectified pulsating DC waveform synchronized with the AC line frequency (60Hz). Resistor R2 provides a pull-down for CR3 and CR4, and R1 and C3 provide noise filtering to remove high frequency transient disturbances on the AC line. The pulsating waveform at the junction of R1 and C3 is fed to the negative input of operational amplifier U1A, causing its output to go high when the voltage falls below the reference threshold of 380mV at the comparator's positive input, set by voltage divider R3 and R4.

Operational amplifier U1B, in conjunction with resistor R8 and capacitor C6, comprise an integrator. Resistor string R5, R6, and R7 provides U1B's positive input with a reference voltage of +6V. Analog switch U2A resets the the output of the integrator to the reference voltage each time U1A'S output goes high. The resulting waveform, present at U1B's output, is a positive-going, line-synchronized, 120Hz sawtooth with amplitude limits of +6V and +9V. A second integrator, comprised of operational amplifier U1C, resistor R9 and capacitor C7, converts the sawtooth to a negative-going, line synchronized, parabolic waveform with amplitude limits of +3V and +6V. Analog switch U2B resets the the output of U1C to the +6V reference voltage each time the output of U1A goes high. The parabolic waveform is fed to the negative input of operational amplifier U1D.

Operational amplifier U3A, in conjunction with R12 and C9, form an integrator referenced to +6V via R5, R6 and R7, with noise decoupling provided by C8. U3A drives operational amplifier U3B, configured as a hysteretic comparator, referenced to +5V via R5, R6, and R7. When the voltage at U3A's output is sufficiently positive with respect to U3B's reference, U3B's output goes from a low to high state, casing the slope of the waveform at integrator U3A's output to reverse direction. When the voltage at U3A's output is sufficiently negative with respect to U3B's reference, U3B's output reverts to a low state, again causing the U3A's integrator slope to reverse direction. The cycle continues, providing a triangular wave with a 2.8 second period and amplitude excursions of +5.8V and +3.9V at U3A's output. This waveform is fed to the positive input of operational amplifier U1D.

Operational amplifier U1D, configured as a comparator, provides a slowly-changing rectangular waveform that controls the firing angle of triac Q1 via optical coupler OC1. The composite effect of the parabolic and low-frequency triangle waveforms provides the appropriate aesthetic appearance of the lamp's intensity changes. The rate of change is determined by the frequency of the triangle wave.

Since optical coupler OC1 provides galvanic isolation between the control and load circuit, T2 could be eliminated for applications with line-voltage lamps. However, the use of T2 and the low-voltage type 1892 lamp considerably improves safety by reducing the amount of high-voltage wiring.

June 10, 2007
Text and images ©2007 by Arthur Harrison
Source documents dated December 30, 1989


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