Touchless capacitive switch

This post describes a circuit that has a capacitive input sensor and a switching transistor at the output PNP. When we activate the sensor, i.e. when we bring two fingers close to it hand without touching directly sensor electrode, the PNP transistor closes. At rest, this transistor is off. The described capacitive switch is supplied by the company AVT as kit under the order number AVT-1690. To try out the feature and because of more easily reproducible was the sample capacitive switch made on a clean PCB. A photo of the board fitted with components is in Fig. 1. Description of function Wiring diagram of contactless of the capacitive switch is shown in Fig. 2. The switch contains a capacitive sensor, an astable multivibrator with IO1, a low-pass filter with P1 and C4, a high-end rectifier with D1, C5 and R7, a comparator with IO2 and output switching transistor T1. The capacitive switch is powered by a stabilized DC voltage of 12 V, which is supplied from an external source to terminals 3 and 1 screw terminal block K1. The supply current at rest is about 3.5 mA, when T1 is switched on without an external load it is about 12mA. Capacitors C1 prevent rapid fluctuations in the supply voltage and C2 blocks the power bus. The capacitive sensor consists of two copper foil sheets (KC1 and KC2) placed next to each other on the board with printed circuit boards. The pads have only a small mutual capacitance; however, it increases significantly when a conductive object, such as fingers, approaches above them. Capacitive sensor together with resistors R1 and R2 form a timer RC cell in an astable multivibrator with a 555 timer (IO1 ) in CMOS design.

Non-contact capacitive switchboard fitted with components
Fig 1. Non-contact capacitive switchboard fitted with components

At rest, the multivibrator has a frequency of about 15 kHz, after activating the sensor by touching it
two fingers on the plate on the side of the components above the frequency sensor surfaces
drops to about 5 kHz. At a greater distance between the fingers and the sensor, the frequency changes
only slightly.
The rectangular signal alternates approximately 1 : 1 from output 3 IO1
to the integration RC cell with trimmer P1 and capacitor C4. The article changes the shape of the signal from rectangular to exponentially triangular. The task of this cell is to convert a change in the frequency of the signal from IO1 to a change in the amplitude of the signal at the output of the cell. With a set resistance of 200 kQ trimmer P1 and at a frequency of 15 kHz, the triangular signal at the output of the cell has an inter-peak swing of about 4 V. As the frequency decreases, the inter-peak swing of the signal at the cell output increases continuously; at 5kHz is about 8V.
The oscillation of the signal from the output of the integrating element is detected by a high-end rectifier with components D1, C5 and R7. At the output of the rectifier (on capacitor C5) there is a ss voltage which is less than the level of the positive peaks of the signal from the integrator by the voltage drop across D1 (about 0.4 V).
At a multivibrator frequency of 15 (or 5) kHz and at a trimmer resistance of 200 kQ
P1 was measured at C5 ss voltage of 7.4 in the realized switch sample
(or 9.3) V.
The magnitude of the voltage on C5 (and therefore also the frequency of the multivibrator) is evaluated
comparator with operational amplifier (OZ) TL071 (IO2). The OZ is chosen with FETs at the input to have an almost infinite input resistance and not load the peak rectifier. The comparator has no hysteresis. The voltage from C5 is applied to the inverter input OZ, to the non-inverting input OZ, the reference voltage from the runner of the trimmer P2 is fed through the protective resistor R3.
The reference voltage must be set roughly midway between the lowest and highest voltage levels on C5.
At rest, when the multivibrator frequency is 15 kHz, the voltage across C5 is less
than the reference, so the inverting input OZ is more negative than non-inverting input. The OZ output is therefore positive saturation (in high H level). Output PNP transistor T1 excited
from the output OZ is turned off and on its collector, which is brought out to terminal 2 of the terminal board K1, there is a low-level L due to the grounding resistor R6
When activating the capacitive sensor, i.e. after reducing the frequency of the multivibrator to
5 kHz, the voltage at C5 will rise above the reference voltage and the inverting input OZ will thus become more positive than the non-inverting one. The OZ output will therefore go into negative saturation (to a low L level). T1 closes and level H appears on terminal 2 of K1. At the same time, green lights up
LED D2 indicating the closed state of T1.
When testing the function of the switch in the laboratory, it turned out that mains hum is transmitted to the capacitive sensor, which
causing a voltage ripple across C5. Consequently, in a situation where the voltage at C5 is close to the reference, T1 switches intermittently.

Circuit diagram of a non-contact capacitive switch
Fig. 2. Circuit diagram of a non-contact capacitive switch

Construction and revival The capacitive switch is constructed mainly from terminal components on a single-sided printed circuit board. Only R7, which was additionally added to the capacitive switch, is in SMD design to make the additional modification of printed circuit boards as simple as possible. The pattern of connections is in Fig. 3, and the distribution of components on the board is in Fig. 4. The capacitive sensor with plates KC1 and KC2 can be separated from the board and installed in the required place. The sensor is connected to the board by short connections via soldering points J1A to J2B. The surfaces of the sensor may need to be insulated with plastic film so that it is not possible to touch them directly with your hands. First, we solder R7 to the board in SMD design, which is placed on on the connection side, although it is drawn on the components side in Fig. 4. Then we fit the board with outlet components from the lowest to the highest. There is one wire jumper on the board made from a cut-off resistor terminal. There are sockets on the board for both IO1 and IO2 so that they can be used in other designs as well. When reviving, we first set the trimmer P1 so that when the capacitive sensor is activated, the voltage change on C5 is as large as possible. Then we adjust trimmer P2 so that the reference voltage on its runner is roughly midway between the lowest and highest voltage levels on C5.

Circuit board of non-contact capacitive switch
Fig. 3. Circuit board of non-contact capacitive switch
Layout of components on the non-contact capacitive switchboard
Fig. 4. Layout of components on the non-contact capacitive switchboard

Parts list

R1 10 MQ/0.6 W/1%, metal.

R2 33 kQ/0.6 W/1%, metal.

R3, R5 10 kQ/0.6 W/1%, metal.

R4, R6 2.2 kQ/0.6 W/1%, metal.

R7 10 MQ/5%, SMD 1206

P1 1 MQ, horizontal trimmer, 10 mm

P2 50 kQ, horizontal trimmer, 10 mm
C1 1000 pF/25 V, radial

C2 100 nF/J/63 V, foil

C3 2.2 nF/J/100 V, foil

C4 220 pF/NPO, ceramic

05 10 nF/J/100 V, foil

D1 1N4148

D2 LED green, 5 mm

T1 BC557B, TO92

101 NE555 CMOS, DIL8

102 TL071, DIL8

2 precision DIL8 sockets for 101 and 102

K1 ARK210/3, three-pole screw terminal block PCB No. KE02Z5R

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