PLL Provides Ratiometric Capacitive Touch Switch For Appliances

This article is part of the Ideas for Design Series: Vol. 3, No. 1

Capacitive-based touch-sensor switches are gaining popularity for appliance, automotive, and industrial applications for many reasons. They’re aesthetically appealing, flexible, easy to manufacture, and easy to use. They avoid the need for buttons, sliders, and other parts needed in mechanical switches. They’re also durable, since they don’t have any moving parts.

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Further, the absence of holes and other openings lowers the manufacturing cost. Surface contaminants and moisture have no effect on the quality of the switches. The front surface can be back-lit to indicate the switch status. And, the copper pads of the printed-circuit board (PCB) can be made to function as touch sensing plates.

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The circuit in the figure uses two identical metal plates or copper pads on one side of a double-sided PCB as the sensing pads (SP_On and SP_Off). Sensing capacitors C_On and C_Off are formed by placing these sensing pads behind a thin, decorative overlaying cover film or plate of the switch. The overlay isolates the user from the mains voltage. Components C4, R9, Z1, D1, and C3 form a compact and low-cost transformerless power supply.

The capacitors act as the timing components of the voltage controlled oscillator (VCO) of a commonly used and inexpensive CMOS phase-locked loop (PLL) (CD4046). ON period T_On and OFF period T_Off of the square wave at the output of the VCO are determined by the values of the sense capacitors C_On and C_Off, respectively. Integrator R4C1 develops dc voltage V_dc proportional to the duty cycle (η) of the square-wave output:

V_dc = duty cycle × amplitude of the square wave

where:

η = duty cycle = T_On/(T_On +T_Off)

and:

amplitude of the square wave = 12 V

Since the sensing pads are identical, the capacitors have equal values when the sensing pads are not touched. Hence, T_On =T_Off, duty cycle = 0.5, and V_dc = 6 V. The voltage divider formed by R6, P1, and R7 provides reference voltages V_Ref-hi and V_Ref-lo to voltage comparators U_2-a and U_2-b.

When the sensing pad (SP_On) used for switching the appliance on is touched, the user’s finger capacitance will be added to the value of C_On. This increases the values of T_On, the duty cycle, and the integrator output. Preset potentiometer P1 is adjusted so:

V_Ref-hi = (V_dc + V_dc-on)/2

where V_dc and V_dc-on are the integrator outputs when SP_On is untouched and touched respectively.

Thus, when SP_On is touched, the integrator output exceeds V_Ref-hi, and the output of comparator U_2-a output goes high, setting the set-reset flip-flop, which is the phase comparator (PC-2) of the PLL IC.  

Similarly, when sensing pad SP_off used for switching the appliance off is touched, the user’s finger capacitance will be added to the value of C_Off. This increases the value of T_Off and decreases the values of duty cycle and the integrator output.

As R6 = R7, the reference voltage V_Ref-lo tracks V_Ref-hi such that:

(V_Ref-hi - V_dc) = (V_dc - V_Ref-lo)

Hence, when SP_Off is touched, the integrator output falls below V_Ref-lo, and the output of comparator U_2-b output goes high and resets the set-reset flip-flop of the PLL IC. A high level on the flip-flop output drives triac T2 to switch on the electrical appliance connected to it. A low level at the flip-flop output switches off the appliance.

The variation in the ambient temperature causes equal percentage variation in the values of C_On and C_Off and hence in values of T_On and T_Off. The values of duty cycle and V_dc are not affected, so the touch sensitivity is not altered. Also, as V_Ref-hi, V_Ref-lo, and V_dc are all ratiometric to the supply voltage V_dd, the touch switch action is immune to the supply-voltage variations.

For better touch sensitivity, the area of the sensing pads should not be more than the area of contact of the finger with the sensor, as the finger capacitance (C_f) should be comparable to the untouched sensor capacitance (C_un). In addition, a thinner overlaying cover material produces a greater difference between the touched and untouched sensor pads.