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Proximity switch for 240VAC mains lights Do you have a table lamp or standard lamp without an in­built mains switch? How would you like to turn the lamp on and off without even touching it? Now you can do it with this compact mains-operated proximity switch. Wave your hand near it to turn it on or off. Article By LEO SIMPSON Design By ALLAN BONNARD M AINS-OPERATED touch switches for light dimmers and table lamps are not new but up till now they have all involved a metal plate or exposed metal work which you need to touch to operate. This Proximity Switch circuit is different – there is no exposed metal plate; you just wave your hand near a concealed plate and hand capacitance does the rest, turning the circuit on and off. In practice the circuit is built into a small plastic case in series with the mains cord to the lamp. Alternatively, if the lamp base has space inside and is not made of metal, you could build the Proximity Switch right inside it. Before we go any further, this circuit 62  Silicon Chip design is not suit­able for permanent installation as a light switch in your house wiring. This is because it is a 4-wire circuit and a light switch is normally a 2-wire circuit. So how do you produce a Proximity Switch for 240VAC lights? It has been made possible by a new IC which is designed to work with a proximity sensor. The sensor is located behind the light switch pad, fully protected by a dielectric (ie, insulating) barrier. Thus the unit offers increased safety. The sensor works on a principle of “charge transfer sens­ing”, which has its origins back in the 1700s, when investiga­tions where first being made into electricity. A study of this was made by Mr William Watson in England and also by the renowned Ben­jamin Franklin. Charge transfer sensing In effect, the IC measures the charge on the sensor plate by transferring its charge to a charge detector with a known ca­pacitance value (Cs). This transferring is done using Mosfet switches internal to the IC. Now if the charge detector detects capacitance above a certain level, the output is triggered. This sounds relatively easy but there are some complica­tions. Firstly, the effective capacitance of the sensor plate can vary widely depending on the location and wiring configuration, so having a fixed threshold level for WARNING: ALL PARTS OPERATE AT 240VAC Fig.1: the circuit is based on a proximity sensor IC (QT116) which turns the Triac on or off in response to change in capacitance of the sensor plate. Note that all parts of the circuit run at 240VAC – they are live and dangerous. circuit switching would be useless. Instead the IC calculates the sensor plate capacitance at that particular location and then looks for sudden changes in this level to control the output triggering. Another problem that comes with this type of circuit is noise so the IC uses a burst mode to acquire the signal and check the level. For a valid detection, a series of four consecutive confirmations will be required before a detection is registered. Within the IC is a 14-bit single-slope switched capacitor Analog-to-Digital Converter (ADC). This optimis­es the burst length according to the buildup on Cs so that the circuit will not become swamped when high capacitance values are encountered. Circuit details Fig.1 shows the complete circuit. It is directly powered from the 240VAC mains supply via the 0.1µF 250VAC capacitor (C1) and 100Ω limiting resistor (R1). The reduced supply is rectified and regulated to 9.1V by zener diode D1 and then fed via diode D2 to the 100µF capacitor to provide a supply voltage of about 8.5V. This supplies transistor Q1 which provides the gate current to the Triac. The 8.5V supply is then fed via a 2kΩ resistor (R4) to zener diode D3 which provides a well-regulated 5.1V supply for IC1. The small sensor plate is connected directly to pin 7 of IC1 and also via a .047µF capacitor to pin 6. Each time the circuit is triggered, the output at pin 2 goes high to turn on Warning! All parts in this circuit (including the sensor plate) operate at mains potential (ie, 240VAC) and must be fully isolated from the user. Do not operate this device unless it is fully enclosed in a plastic case. Similarly, do not work on the circuit or touch ANY part (sensor plate included) while it is plugged into a mains outlet. We recommend that only experienced people build this design. Q1 and the Triac Q2, or low to switch both devices off. The Triac is turned almost fully on and so there is relatively little switching hash produced by it, which would not be the case if it was a conventional phase-controlled Triac circuit as used in a light dimmer. The Triac is rated up to 10A but this circuit is only suit­able for lamp loads up to about 300W since there is no heatsink for Q2 and the PC board tracks are not suitable for high cur­ rents. The 56Ω 1W resistor and .01µF capacitor across the Triac form a snubber network to protect it from switching voltage transients. PC board assembly There are two boards involved for this project, one for the sensor plate which does not have any components on it and one for the main circuit and this comes with a screen-printed comTo ensure safety, the circuit must be fully enclosed in a plastic case with no exposed metal parts. August 2000  63 Fig.2: the wiring diagram and component overlay. The whole assem­bly, including the sense plate, must be mounted in a plastic box for safety. ponent overlay which is also shown in the wiring diagram of Fig.2. First mount the IC socket and then the resistors, diodes, transistor and Triac and make sure that the semiconductors are installed the right way around. The capacitors can go in either way around except for the 100µF electrolytic which must be in­stalled with correct polarity. Do not place IC1 in the socket yet, as we will need to test the circuit first (see below). Next, attach the 4-way insulated terminal block to the PC board with two 6BA screws and nuts. Then use brown and blue 250VAC hookup wire to make the Active & Neutral connections from the PC board to the insulated terminal block. You then need to connect the sensor plate. This is done by soldering a small length of tinned copper wire to the appropriate pad on the PC board. This wire passes through the single hole in the sensor board when it is mated up to the main PC board and soldered. The sensor board is attached to the PC board with four 6BA screws and eight nuts, with nuts being used as spacers, as shown in the photos. Testing There are several ways to test the circuit and it is pre­ferable to do it at low voltage rather than at 240VAC. The pre­ferred method is to use a DC power supply set to an output of between 12V and 15V or thereabouts. Connect the negative lead from the supply to the Active 240VAC line in terminal. Connect the positive lead to the C1 capacitor side of R1 (100Ω 1W). Now use your multimeter to check the DC voltage between pin 1 Table 2: Capacitor Codes      Value IEC Code EIA Code 0.1µF 100n   104 .047µF  47n  473 .01µF  10n  103 22pF  22p   22 Table 1: Resistor Colour Codes  No.   1   1   1   1   1 64  Silicon Chip Value 3.9MΩ 2kΩ 470Ω 100Ω 56Ω 4-Band Code (1%) orange white green brown red black red brown yellow violet brown brown brown black brown brown green blue black brown 5-Band Code (1%) orange white black yellow brown red black black brown brown yellow violet black black brown brown black black black brown green blue black gold brown Parts List 1 PC board, 72 x 33mm 1 PC board (sensor), 52 x 34mm 2 2-way insulated terminal blocks 1 double-screw BP connector 1 plastic utility box, 85 x 56 x 40mm, DSE Cat. H-2874 or equivalent. 1 250VAC 3-core flex and moulded 3-pin plug 2 cordgrip grommets to suit power flex 6 6BA screws and 10 6BA nuts Semiconductors 1 QT116-D proximity sensor and Triac trigger (IC1) 1 C9013 NPN transistor (Q1) 1 BTA06 600C 600V Triac (Q2) 1 9.1V 1W zener diode (D1) 1 1N4004 1A 400V diode (D2) 1 1N4733 5.1V 1W Zener diode (D3) Capacitors 1 100µF 25VW electrolytic 1 0.1µF 250VAC metallised polyester 1 0.1µF monolithic 1 .01µF 250VAC metallised polyester 1 .047µF monolithic 1 22pF ceramic Above: the PC board assembly, complete with the sensor plate, is attached to the lid of the case using contact cement or a similar adhesive. This view shows how the sensor plate is mounted on the main PC board. The entire assembly is then glued to the case lid. DO NOT TOUCH the sensor plate or any other parts while the unit is plugged into the mains. and pin 8 of IC1. It should be close to 5V. Similarly, the voltage across C1 should be close to 8.4V. If these checks are not correct, disconnect the supply and recheck all the components and orientation. If everything is OK, place IC1 in the circuit and connect the power cord as shown in the wiring diagram of Fig.2. The two cords must be secured to the case using appropriately-sized cord­grip grommets. Note that the mains earth wires must be twisted together and secured using BOTH screws in the BP connector. The PC board assembly can be attached to the lid of the case using contact cement or other adhesive, after which the lid can be fastened to the case. Now connect a table lamp and plug in to the 240VAC mains. The lamp should initially be off and you should be able to turn it on by waving your Resistors 1 3.9MΩ 0.5W 1 2kΩ 1 470Ω 1 100Ω 1W 1 56Ω 1W WHERE TO BUY PARTS The design copyright for the Prox­ imity Detector is owned by Futurlec who can supply the PC boards plus all on-board parts (but not the case). The price is $19 plus $3 packing and postage within Australia. Orders may be placed via their website at www.futurlec.com using Bankcard, Visa Card or Master­card. Alternatively, orders can be sent with a credit card authorisation, cheque or money order to Futurlec, 24 William St, Paterson, NSW 2421. hand over the plastic case. To turn it off, just wave your hand over the box again or tap the case briefly. The unit could be used with other small appliances, such as radios or small TV sets and is ideal for the elderly or disabled who may have trouble SC with small switches or knobs. August 2000  65