Touch dimming circuit based on LS7232

LS7232 is a touch-type stepless dimming ASIC produced by LSI Corporation of the United States. It has the characteristics of few peripheral components, low power consumption and gradual light-off. Since the chip is made by D-type PMOS process, it requires a higher power supply voltage of 12~18V.

The 220V AC mains power is stepped down and limited by resistor R1, half-wave rectified by diode D1, stabilized by voltage regulator diode D2, and filtered by capacitor C1, providing U1 with a working voltage of about 15V. When a person touches the metal sheet M1, the human body sensing signal is added to the (5) pin of U1 through resistors R3 and R4, making the internal circuit of U1 work.

The human body is charged at the same frequency as the mains. When a hand touches the touch pad, it is clipped, amplified, and shaped by the input buffer stage of U1 to become a standard MOS level. When the hand touches the pad for 39ms~399ms, U1 only controls the bidirectional thyristor to complete the switch action. When the hand touches M1 for ≥399ms, TRIAC phase shift dimming is performed. The (8) pin outputs a pulse width of 40μs, which can absorb a negative jump pulse with a trigger current of 25mA. For every 1° lag in the pulse phase, the brightness of the bulb decreases by one level. The light gradually dims from the brightest (159°) to slightly bright (41°), and then gradually changes to the brightest. This change takes 7.64s. When the hand stops touching, the light maintains the brightness of that moment. The chip has a memory function. When the light is turned off and then on again, it still maintains the brightness when the light was turned off last time, but the light brightness change is opposite to the last dimming state, that is, reverse dimming.