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N E WARNING: COMPONENTS & WIRING IN SHADED AREA ARE AT 240V MAINS POTENTIAL WHEN THE CIRCUIT IS OPERATING. CONTACT MAY BE LETHAL! SLOW BLOW F1 10A A GPO N A 240V INPUT E S1 T1 12.6V/2VA D1–D4 K 12.6V 240V 6.3V RLY1 20A AC CONTACTS A A K +17V 68  5W* A K 0V K 470 F 25V A 100 F 25V D5 K A A K ACTIVE  LED1 2.2k JP2 * FOR SY-4042 RELAY (20A) USE 47  5W FOR SY-4040 RELAY (30A) BC337 LEDS D1– D5: 1N4004 A SC 2009 K TO REMOTE SWITCH CON3 A 560 B K A E 1k C REMOTE POWER SWITCH B 22k C Q2 BC337 POWER  LED2 K E Fig.1: the circuit is triggered by closing the contacts of a remote switch. When that happens, transistor Q2 turns on and activates the relay which then switches power through to the GPO. LED1 indicates when the relay is on. Remote Mains Relay Mk.2 The PC board used for the USB Sensing Power Switch can also be used to build a remote mains relay. This can be used for switching mains power to lights or appliances using a remote switch (or relay) linked by low-voltage wiring. It can also be used for controlling security lights from the relay contacts on a PIR sensor. By JIM ROWE T HERE ARE many devices with relay outputs that are unable to safely switch mains voltages. Often, the relay contacts will not be rated for 240VAC mains or, even worse, there are exposed tracks on the PC board 38  Silicon Chip which were never designed to carry mains voltages and currents. This Remote Mains Relay Mk.2 neatly solves that problem. It can be activated using a switch, relay contacts or even a switching semiconductor such as an open-collector transistor. Because the remote switch is in the low-voltage section of the circuit, it is completely safe. In addition, the switch leads only carry a couple of milliamps, so light-duty cable can be used to connect the switch. Fig.1 shows the modified circuit for the Remote Mains Relay. As shown, the ICs, transistors and USB sockets used in the USB Sensing Power Switch are deleted. Instead, jumper JP2 is installed and the +2V supply that appears across power indicator LED2 (red) is fed to a remote switch via connector CON3. The remote switch is used to turn transistor Q2 on or off, simply by switching its base current on or off (via a 560Ω resistor). When the switch closes, Q2 turns on. This then turns on relay RLY1 which closes its contacts and switches power through to the mains socket (GPO). Building it This device must be installed in exactly the same type of polycarbonate box used for the USB Sensing Power Switch. The mains wiring to the IEC siliconchip.com.au (EARTH: GRN/YELLOW) 4004 SECONDARY 19010101 4004 2851 (NEUTRAL: BLUE) 4004 CON1 A LED2 9002 C A HCTIWS REWOP M3 x 10mm SCREW WITH LOCK WASHER & NUT LED1 4004 K D5 3 WIRES TO SPRING TERMINAL BLOCK FOR REMOTE SWITCH JP2 2.2k 2 D1-D4 K GNISNES BSU 2 4004 100 F T1 N PRIMARY A 1k GPO (REAR VIEW) E 68  5W 470 F SEE DETAIL DIAGRAM 2 GM & 2 2 1 RLY1 (ACTIVE: BROWN) 2 BC337 22k RLY1 Q2 IEC MAINS CONNECTOR WITH SWITCH AND FUSE (REAR VIEW) MAINS WIRING CONNECTORS: 1: 6.4mm INSULATED SPADE CONNECTORS 2: 4.8mm INSULATED SPADE CONNECTORS 3: 4.8mm PIGGYBACK SPADE CONNECTOR 1 CON2 560 2 CON3 M4 x 10mm SCREWS WITH FLAT & LOCK WASHERS, NUTS CRIMP EYELET M3 NUT STAR WASHERS TRANSFORMER MOUNTING FOOT NOTE: ALL WIRING TO THE IEC CONNECTOR, THE GPO AND THE OUTPUT CONTACTS ON THE RELAY (1) MUST BE RUN USING 250VAC CABLE PC BOARD M3 x 10mm SCREW Fig.2: install the parts on the PC board and complete the wiring as shown here. The device is built into the same case as the USB Sensing Power Switch. Secure all the mains wiring with cable ties as shown in the previous article. siliconchip.com.au TERMINAL BLOCK ATTACHED TO BOX USING TWO M3 x 10mm NYLON SCREWS & NUTS REMOTE SWITCH 1S connector, transformer and relay must also be installed (and secured using cable ties) in exactly the same manner – see previous article. Apart from that, it’s just a matter of installing the parts on the PC board as shown in Fig.2. You also have to run a couple of leads from CON3 to the remote switch. These leads can be connected to the rear of a small 2-way spring (speaker) terminal block (eg, Jaycar PT-3000) which is fastened to the righthand end of the box using M3 x 10mm Nylon screws and nuts. The remote switch or relay contacts can then be connected to the spring terminal block using a suitable length of light-duty figure-8 speaker cable, or similar – see Fig.3. Finally, note that this design supercedes the Remote Mains Relay described in May 2006. That project was exclusive to Dick Smith ElectronSC ics and is no longer available. SPRING TERMINAL BLOCK RIGHT-HAND END OF ENCLOSURE SUITABLE LENGTH OF LIGHT DUTY FIGURE-8 CABLE 1: TRIGGERING USING A TOGGLE SWITCH RELAY IN EQUIPMENT NO COM NC 2: TRIGGERING USING A REMOTE RELAY January 2009  39 Fig.3: the unit can be triggered using a remote toggle switch, the contacts of a remote relay (eg, in a PIR sensor), or a semiconductor switch.