Fullscreen HTML5Med Res (??MB)HTML4

A 12V Digital Timer using a 240V Timer Module 240V timers are very cheap these days. 12V timers are not so cheap. So why not use the “innards” out of a 240V timer and make a very flexible 12V timer? by Bill De Rose* and Ross Tester W both are used in the project. components from the PC board (unith the price of imported This is quite simple. First, remove fortunately the 24V relay is unsuitable electrical and electronics the three case screws and open the for our timer project). goods these days, often it timer case You’ll need a fairly fine simply isn’t economic to build. But The Circuit Phillips screwdriver to do this. there are times when those same imNext, remove power from the timer The circuit for the Digital Clock Timports can yield components which by carefully de-soldering the back-up er is shown in Fig.1. It comprises the make other do-it-yourself projects battery. In the process be careful not digital clock module, current-limiting viable. to short out the battery terminals – circuitry for the rechargeable battery Such is the case with this project. It’s this will either flatten or destroy the (also removed above) and a handful a 12V DC timer, based around a clock battery. of other components module that’s found in some comused for power supply mercial 240V mains timers. decoupling, limiting The module then becomes the and driving the output heart of the timer enabling the and use relay. user the flexibility to program • Simple to program ting set day 7 weekday, weekend or The clock module is the unit with ease. Its output • Timer for – 24hrs daily, s ent 1 minute increm the heart of the project, controls a relay which in turn • 10 off & 10 on programs with providing all the timing can be used to switch a low • Random setting ction fun e anc adv and program functions. voltage such as 12 VDC. ing sav t ligh Day • There are five connecThe project consists of the • Override switch tions to the module: timer module, removed from n ctio fun n ow ntd Cou • power (+ and 0V), the an Arlec PC697 digital mains • Battery back-up LED indicator (anode timer, which is then mounted off LED indicator & cathode) and signal in a Zippy box along with a • On/ – 12VDC output. simple power supply and relay • Power ks) trac ed ken • 5A rating (10A with thic Let’s start with the interface. module. We’ll assume the board is The SPDT relay contacts assembled. Inserting jumper switch Once the battery is removed, deare rated at 10AC (120V) but we’d S1 will switch power directly from the solder the ribbon cable from the main be loathe to try to switch this sort of back-up battery to pin 3 of the module. relay PC board. Carefully remove the current and expect any sort of longevAt this point, the display should come silicone glue around the base of the ity. A higher-rated relay should be to life and allow the user to program cable with a craft knife or similar. The substituted if heavy currents are to be the clock functions. cable should now pull free. switched. The PC board tracks should In order for the module to operate Other then the clock module and also be thickened with solder and/or the relay (once programmed), external battery, the remaining components are wire for higher current. power (12V DC) will be needed. Diode not used in this project. We haven’t The timer components D1 protects the circuit from reverse thrown ours away – they look too good polarity connection while zener diode for that and we’re sure that another The Arlec timer has to be partially ZD1 limits transient voltages to 15V. project will suggest itself. dismantled in order to retrieve the The 10W series resistor limits the curYou can also salvage a few other clock module and back-up battery; Features 72  Silicon Chip siliconchip.com.au ... rent flow if the zener diode becomes clamped. Both the 100mF and 100nF capacitors decouple the supply. Pin 5 of the clock module is internally connected to the anode side of the LED indicator (pin 4 is the cathode [K]). The 1.2kW resistor limits the LED current to about 7mA, more than enough for adequate LED brightness and at the same time helps to keep the overall standby current to a minimum. When the clock module is in standby mode – in other words all programs and functions in the ‘off’ state – pin 1 is held low, pulling the cathode of D4 low and thus turning it on. This removes LED(A) 5 drive from the Darlington transistor (Q1), switching it off In turn, the relay (RLY1) remains in its rest position and the normally closed (NC) and common contacts are closed. 1.2k 10 Ω 1W + 8.2k 100nF 100 µF 25V D1 1N4004 K A ZD1 15V 1W +12V 0V A +VE 3 12V CLOCK MODULE FROM ARLEC PC697 TIMER LED(K) D3 1N4148 K 2.7k S1 (HEADER PIN SET) 22k RLY 1 K D2 1N4004 A 1.2V CELL COM NO 4 D4 1N4148 OUT Here’s how the timer module starts out in life – as the “works” in an Arlec PC697 digital timer. The module, which is a separate assembly at the top of the timer (as seen here) is easily removed from the “case”. The rest of the device isn’t used – but we’re sure a use will turn up for it shortly! siliconchip.com.au –VE 1 K C A B SC Q1 MPSA14 B MPSA14 2 C 2006 NC E ZD1 + 1N4148 E 12V DIGITAL CLOCK TIMER A K A K 1N4004 Fig.1: the circuit diagram is essentially a power supply and relay driver with all the smart work being done by the commercial timer module. October 2006  73 S1 1S (HEADER PIN SET) + + TO TIMER MODULE D2 0V +12V V21+ C/ N Q1 4148 D4 DNG D1 22k 1.2k + 10 Ω 1W 9 1 0 0A Z 11 2 3 4 5 100 µF ZD1 8.2k 4148 D3 100nF 2.7k _ BATTERY c NC C b e RELAY1 COMM NO O/ N When the timer module output switches on (depending on the programs and functions set), pin 1 will go high. As diode D4 is now reverse-biased, current will flow through the 22kW resistor and switch Darlington transistor Q1 on, which pulls in the relay. Contacts “NO” and common now connect. Diode D2 suppresses the voltage spike which occurs when the relay switches off, protecting Q1. Back-up power from the on-board battery is useful if external power is disconnected (eg, through power failure) or if the unit is stored away when not in use. This allows the clock module to retain program set-up information. The voltage divider network formed by the 8.2kW and 2.7kW resistors provide trickle charging through diode D3 and jumper switch S1 to the rechargeable battery. Diode D3 also stops the battery from discharging back into the circuit when external power is switched off. Fig.2, left, shows the component overlay for the main PC board. No overlay is shown for the display board because it has no components on it – in the photo below, you can see both boards, with the timer module, “opened out” – immediately before being assembled, as shown in the photos below. The top photo shows the completed project, from the front, ready for the front panel (actually the box lid!) to be attached, while the bottom photo is complete with the front panel on, ready to be placed inside its Zippy box. In the kit version, a silkscreened metal panel will be supplied. Jumper switch S1 isolates the battery from the circuit during assembly. Once all is complete and the construction is thoroughly checked, jumper switch S1 can be installed. Note: do not connect external power to the circuit unless the back-up battery is installed and jumper switch S1 is in position. Construction The project has two PC boards, only one of which has components on it. The second is used to hold the timer in place on/through a cutout in the front panel. It has a single large hole for the timer module cable to pass through. Luckily for you, in the DSE kit the cutout for the timer module and the mounting holes will already be punched. In fact, the front panel will be metal (the prototype was 74  Silicon Chip siliconchip.com.au Parts List – 12V Digital Timer 1 Arlec 240V AC digital timer 1 main PC board, 78 x 57mm, code ZA0019 1 display PC board, 73 x 52mm, code ZA0020 1 UB3 Zippy Box (44 x 68 x 130mm) 1 front panel/case lid 1 10A SPDT relay 1 pin header (2-way) 1 jumper shunt (2-way) 4 M3 x 6mm csk screws (black) 4 6mm x No. 4 csk screws (black) 2 M3 x 6mm pan-head screws 3 M3 x 25mm pan-head screws 12 M3 flat washers 3 M3 shakeproof washers 3 4mm Nylon spacers 3 12mm Nylon spacers 4 M3 x 9mm tapped spacers 1 2-way terminal block 1 3-way terminal block Here are a few more shots showing how the boards go together. In these three, the front panel is attached, holding the timer module in place. The drawing below, Fig. 3, also shows how it all goes together. No4 x 6mm BLACK CSK SELF-TAPPING SCREW M3 x 6mm BLACK CSK SCREW CLOCK MODULE 9mm TAPPED SPACER 3 x M3 FLAT WASHERS 12mm NYLON SPACER M3 nut & battery jumper 4mm NYLON SPACER 12mm NYLON SPACER 3 x M3 FLAT WASHERS Capacitors 1 100mF 25V electrolytic 1 100nF MKT Resistors (0.25W, 1%) 1 22kW 1 8.2kW 1 2.7kW 1 1.2kW 1 10W 1W 9mm TAPPED SPACER M3 x 25mm PANHEAD SCREW built on the standard Zippy box ABS lid) – black powdercoated with white printing (similar to the prototype). Final assembly is like a sandwich, with the main PC board on one side, the second PC board and the timer module in the middle and the case lid on the other side.The photographs and drawing (Fig.3) give a good idea of the construction method. The timer module connects to the main PC board via a 5-way ribbon cable. The two PC boards are mounted together via three 25mm-long screws passing through two spacers (Nylon siliconchip.com.au FRONT PANEL SHAKEPROOF WASHER 4mm NYLON SPACER M3 x 6mm PANHEAD SCREW No4 x 6mm BLACK CSK SELF-TAPPING SCREW M3 x 6mm BLACK CSK SCREW Semiconductors 2 1N4004 diodes (D1, D2) 2 1N4148 diodes (D3, D4) 1 15V 1W zener diode (ZD1) 1 MPSA14 Darlington transistor (Q1) RELAY Assembly MAIN PCB CASE standoffs) – one 12mm and one 4mm. The main board actually overhangs the second board to allow room for the relay. Two of the three 25mm screws pass through the second board and into M3 tapped 9mm standoffs. These in turn are fixed to the front panel via countersunk 3mm black screws (the black to match the panel colour). The opposite end of the second board also has 9mm M3 tapped spacers between it and the case lid but has 6mm M3 screws holding it down. There is nothing particularly tricky about assembling the PC board. Of course, you need to watch out for electrolytic capacitor, diode, transistor and battery polarities (the latter must mount with its positive side (marked with a red “+”) towards the middle of the board. If in doubt as to the value of the resistors, use a digital multimeter to check them. The last components to be fitted should be the relay, terminal blocks and header pin set (which forms S1). Carefully check that you have the components in the right spots and, where appropriate, the right way around and that you haven’t bridged October 2006  75 You’ll need to drill a couple of holes in one end of the case for the power wiring and switched (relay) wiring to emerge. The holes don’t need to be this big – just enough to accommodate the wire you use. over any tracks or left any components unsoldered or improperly soldered. We made mention before of the limited current capacity of the relay. The same comment applies to the PC board tracks. While these are much wider than other tracks, they are still not capable of high current. If the intention is to use the timer to switch high currents, we’d be inclined to run a coat of solder over the entire tracks (three of them) from the PC board terminals back to the relay pins. Even better, three pieces of stout tinned copper wire each bent to the same shape as their respective tracks and soldered to those tracks will allow higher current flow. Finally, solder the five wires in the ribbon cable from the timer module to their respective positions on the PC board. Note that this cable is not marked in any way so you need to be careful that the right wires go to the right positions. The easiest way is to lay the boards out as in our photograph – then the wires end up in the right spots. We’d advise not flexing the ribbon cable too much: its wires are single strands, not designed to be moved after the original timer was manufactured. Finishing off Use the photos and assembly diagram (Fig.3) to put the boards together in the right order and position. When yours agrees with our photos and diagram, you’re almost ready to put it in the Zippy box. But not quite! Two holes need to be drilled in the case opposite the on-board terminal blocks. In most cases, the holes need only be big enough for two wires each (supply one side, switched device the other). However, as the relay is a changeover type, your application might require all three wires to be used. It’s up to you. Programming the timer In the kit, you will be supplied with the complete Arlec PC697 timer, complete with instructions. As the basic function of the timer hasn’t changed, you program the timer in accordance with those instructions. There is little point in repeating the SC instructions here. Where from, how much: This project was designed by Dick Smith Electronics, who also retain the copyright. A complete kit (Cat K-3582), including the Arlec Digital Timer and screened front panel, is available from Dick Smith Electronics stores and www.dse.com.au for $49.40 *Dick Smith Electronics kit department From the publishers of SILICON CHIP PERFORMANCE ELECTRONICS FOR CARS NOT A REPRINT: More than 160 pages of new and exciting projects never published before – all designed to get top performance from your car. FASCINATING ARTICLES: 7 chapters explaining your car – engine management, car electronics systems, etc ADVANCED PROJECTS: You’ll build controllers for turbo boost, nitrous, fuel injection and much more! We explain the why as well as the how to! Available direct from the Publisher ($22.50 inc postage): Silicon Chip Publications, PO Box 139, Collaroy NSW 2097. Ph (02) 9939 3295; Fax (02) 9939 2648; email silchip<at>siliconchip.com.au or via our website: www.siliconchip.com.au 76  Silicon Chip siliconchip.com.au