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Workout timer for aerobics classes Are you into a strenuous exercise program involving aerobics? This Workout Timer will tell you when to change exercises by flashing a light and sounding a buzzer. By STEVE CALDER We have a confession to make. None of the slobs at SILICON CHIP are actually into aerobics - as such. But if you're one of those people who gets dressed up in tight leotards with contrasting (preferably vertically striped) leg warmers, then this Workout Timer is for you. The whole idea behind aerobics (or so we're told) is to do a series of 44 SILICON CHIP different exercises in quick succession. These exercises are usually accompanied by ear-splitting music, which probably serves some purpose although we're not too sure what. Maybe it dulls the pain of the exercises. Anyway, the idea is to get nice and sweaty and to give those muscles a real workout. And to do that, you've got to do lots of dif- ferent exercises that would put a contortionist to shame. So assuming that you're an aerobics type person, how much time should you give to each exercise and when should you switch to the next? Our Workout Timer solves this vexing problem. It sounds a buzzer and flashes a light when it is time to change exercises. No more do you have to rely on the instructor. You can vary the time between "beeps" (or flashes), and thus the time for each exercise, from 10-75 seconds by means of a rotary control on the front panel of the unit. Each beep lasts about 2 seconds and then the timer automatically begins the next cycle. For example, let's say that you've PUSH-ON PUSH-OFF T 2x1N4004 01 I A---0 +12V s, 1000 + 25VW+ 240VAC 10 ,. o.,I + 16VWr LED1 BUZZER .,. 150mA 03 1N4004 MAX .,. IC1 555 56k RLY1 47 + 16VWI LL . GNO WORKOUT TIMER Fig.2: the circuit uses a 555 timer IC which has been wired as an astable oscillator with a long duty cycle. Its output (pin 3) drives a 12V buzzer and relay RLY1 to flash the lamp. VR1 adjusts the period between "beeps" from 10-75 seconds but you can vary this if you wish simply by changing the 47µF timing capacitor. set the rotary control to 20 seconds. This means that the buzzer will briefly sound every 20 seconds for as long as power is applied to the unit. When you hear the buzzer, it's time to change to the next exercise (or collapse in a heap!). A mains socket mounted on one end of the case allows a 240V lamp to be plugged into the unit and this flashes when the buzzer sounds. After all, you might not be able to hear the buzzer above all that loud music! How it works Fig.1 shows the circuit details. As can be seen, the Workout Timer is based on a 555 timer IC and this has been wired as an astable oscillator with a high duty cycle. VRl and the 270k0 and 56k0 resistors determine the charging period for the 47 µF timing capacitor, while the 56k0 resistor sets the discharge period. These periods correspond to the exercise time (ie, the time between beeps) and the buzzer time respectively. Let's see how the circuit works. At switch on, the 47 µF capacitor begins charging via VRl. During this time, pin 3 of ICl is high and so the buzzer and the relay (RLY1) are both off. When the voltage across the 47 µF timing capacitor reaches 2/3Vcc, pin 3 immediately switches low and the buzzer and relay turn on. This closes the relay contacts which are wired in series between the mains active lead and the power socket, and so the lamp also turns on. The 47µF capacitor now discharges via the 56k0 resistor until it reaches 1/3Vcc. When it reaches this voltage, pin 3 switches high again and the buzzer and relay are turned off. The 47µF capacitor now starts charging again and so the cycle is repeated continuously while ever power is applied. By varying VRl between its minimum and maximum settings, we can vary the charge time from 10 seconds to 75 seconds. The discharge time (Td) is about 1.8 seconds, as determined by the equation Td = 0.693RC, where R = 56k0 and C = 47µF. Power for the circuit is derived PARTS LIST 1 PC board, code SC08102901, 132 x 87mm 1 plastic zippy box, 1 50 x 90 x 50mm (Jaycar HB-6021 , Altronics H-01 51 ) 1 surface-mount mains socket 1 24V centre-tapped mains transformer (Altronics M-7124) 1 12V buzzer (Arista AB2, available from David Reid Electronics; Jaycar AB-3454) 1 mains cord and 3-pin plug 1 cord grip grommet 1 2MO linear potentiometer 2 25mm-long spacers 1 pushbutton mains on/off switch 1 12V DPDT relay 3 plastic cable ties 10 PC stakes 1 large solder lug 4 3 x 1 0mm machine screws 2 3 x 1 5mm machine screws 4 3mm nuts Semiconductors 1 3 1 1 555 timer IC (IC 1 ) 1 N4004 diodes (01 -03) red LED (LED 1) 7812 3-terminal regulator Capacitors 1 1 1 1 1 1OOOµF 25VW electrolytic 4 7µF 16VW LL electrolytic 1 OµF 16VW electrolytic 0. 1µF polyester .01 µF polyester Resistors (0.25W, 5%) 1 270k0 1 56k0 1 1k0 Miscellaneous Heatshrink tubing, 240VAC rated hookup wire MARCH 1990 45 Most of the parts are mounted directly on the PC board but note that the buzzer is stood off the board on 25mm pillars. Use plastic cable ties to bind the 240V AC wiring and sleeve all exposed terminals with heatshrink tubing. from the mains via a 24V centretapped transformer. This feeds a full-wave rectifier circuit consisting of Dl and D2 to produce an unregulated supply rail of about 17V DC. This rail is then filtered by a 1000µF electrolytic capacitor and regulated to + 12V using a 7812 3-terminal regulator. A 10µF capacitor is used to filter the regulator output while LED 1 provides power on/off indication. A 1k0 resistor is used to limit the LED current to 12mA. Construction An earth lead must be run from the power socket to a large solder lug which fits over the pot shaft. This ensures correct earthing of the pot metalwork. As an additional precaution, fit the pot with a plastic knob. 46 SILICON CHIP A printed circuit board has been designed to accommodate most of the parts, including the mains transformer. This board is coded SC08102901, measures 132 x 87mm and fits into a standard plastic zippy box. Fig.2 shows the wiring details. Begin construction by installing PC stakes at all the external wiring points (10 in all), then mount the parts on the PC board. It's best to install the small components first, before moving on to the buzzer, relay and power transformer. Note that many of the parts are polarised so be sure to install them the right way around. It's easy to POLYSTYRENE CAPACITORS -----,~.......- - - - CORO CLAMP GROMMET E (GREE YELLO A (BROWN) POWER TRANSFORMER Made here in Australia? Made Special to Type? Polystryrene Capacitors - 10pF to 1mF Voltage Range - 63VDCW to 10,000 VDCW Tolerance - 0.25% to 10% Allied Capacitors Australia manufactures capacitors to the specification of the customers using high quality, imported polystyrene 240VAC POWER SOCKET Fig.2: be sure to use 240V AC rated cable for all connections between the PCB, switch St and the power socket. The 7812 regulator is installed with its metal tab adjacent to the lO00µF capacitor. identify pin 1 of the IC - it's adjacent to a dot or notch in one end of the plastic body. Install the 3-terminal regulator with its metal tab towards the 1000µ.F capacitor. The buzzer must be stood off the board using two 25mm spacers and two 3mm nuts (see photo). This is necessary to ensure that the buzzer protrudes slightly through the front panel when the lid is screwed down. The flying leads from the buzzer are then run back to the PCB, with the red lead going to the positive supply rail. At this stage, you should be ready to drill the case to accept the external hardware. You will have to drill holes in one end of the case to accept the mains switch and the surface mount mains socket, and another hole in the other end of the case for the cord clamp grommet. Use the front panel artwork as a guide for drilling the front panel. There are three holes to be drilled here: one for the LED bezel, one for the potentiometer (VR1), and one for the buzzer. Use pilot drills to start the holes for the pot and buzzer, then carefully ream them out to the correct size. and aluminium foil with a tolerance of 1 micron. Our capacitors are manufactured to the system of Total Quality Control. We can provide Just In Time delivery if required, together with a Certificate of Conformance if requested. Specific values between 10pF and : 1uF are no more expensive than standard value components. You can now design circuits to use a single capacitor rather than a number in parallel or series to achieve a desired value. Personalised labelling is available at no extra cost. Your component code or name can be built into the capacitor. Minimum runs of on ly 25 allow you to specify a particular value for a prototype run. Delivery lead time for short runs is 3 to 4 working days, and for longer runs is less than 2 weeks. We are also able to produce capacitor styles for applications such as dual mount, end filled and mini style capacitors . INTERESTED? Call us now on: (02) 938 4690 ALLIED CAPACITORS AUSTRALIA PO Box 740 Brookvale, NSW 2100 M ARCH 1990 47 ....a: ....<Cen 0 0 + ,... 1/1 0 co 0 a, 1/1 N "' 0 ,... ~ 00 0 + CJ CJ) 1/1 0 ,- 1/1 0 N a: w :E - II- ::::, 0 a: 0 L == Fig.3 & 4: here are actual size artworks for the front panel and the PC board. You can use the front panel artwork as a drilling template if you wish. ~ Now check that the PCB clips neatly into the case. File the edges of the board until it fits, if necessary. Don't use metal pillars to mount the PCB. If you do elect to mount the board on pillars, you must use insulated types to provide the necessary isolation from the mains. Next, fit the mains switch, then strip back 180mm of outer insulation from the mains cord and clamp it in position. Connect the earth lead (yellow/green) to the power socket and the active lead (brown) to the switch. It's a good idea to fit heatshrink tubing over the switch terminals to eliminate the possibility of electric shock. The remainder of the wiring can 48 SILICON CHIP 0 now be run from the PCB to the mains socket and S 1 as shown in Fig.2. Be sure to use 240V AC rated cable for these connections. In addition, an earth lead must be run from the power socket to a large solder lug which fits over the pot shaft. This ensures that the pot metalwork is correctly earthed and is a necessary safety feature. Once all the wiring has been run to the power socket, it can be secured to the case using machine screws and nuts. Use plastic cable ties to bind the 240V AC wiring so that if any lead comes adrift, it will not short to the low voltage circuitry. Construction can now be completed by installing the LED and the potentiometer on the front panel and wiring them back to the PCB. Fit a plastic knob to the pot shaft (not metal) and carefully align the indicator before tightening the grub screw. Testing This is the easy part - just switch the unit on and if it doesn't work you've got a fault. After all, there's not much that can go wrong with a 555 timer circuit! If you do strike problems, check for incorrect wiring, faulty soldering and incorrect component orientation. Finally, you can change the timing range of the Workout Timer simply by changing the 47 µF timing capacitor. For example, to double the range to 150s, just increase the capacitor value to lOOµF. ~