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Do-It-Your Garage Do This view shows the motor drive with the control box mounted close by. Behind is the vane which is moved up or down when the door contacts the cord stops at top and bottom of the door. 54  Silicon Chip rself Automatic oor Opener; Pt.2 Last month we featured the electronic circuitry to con­trol the windscreen wiper motor which powers this chain drive system. This month we complete the job with the presentation of the mechanical details. By RICK WALTERS While there is a fair amount of mechanical work involved in the drive system it should not be beyond the ability of the average handyman with a reasonable array of tools. The garage door we fitted the drive to was about 4 years old but it should be suitable for any roll-up style door. Our door is apparently designed to take a drive pulley as the three bolts Fig.1: the mounting bracket for the motor was made out of a piece of angle iron, 40 x 40 x 200mm. A 15-tooth rear-wheel bicycle sprocket (62mm diameter) was mounted on the wiper motor shaft (see text). you can see in the photos are inserted directly into exist­ing holes in the door spider. Doors of this type always have a three-legged spider which rotates on the central support shaft which is typically a 2-inch pipe. Our first attempt at this drive system was to use a V-belt and pulleys; a large pulley attached to the door spider and a small pulley attached to the windscreen wiper motor. Modern windscreen wiper motors are ideal for this task as they use a ferrite permanent magnet with a very high flux density which gives lots of torque without needing lots of current. They also have a built-in worm-drive reduction gear which prevents the motor from being rotated by the output shaft. This effectively locks the door in position when the motor stops. The third benefit is the fact that the permanent magnet motor can be readily reversed by swapping the leads to the bat­tery; ie, by swapping the supply polarity. Unfortunately this original setup did not work satisfacto­rily. The belt needed a lot of tension and was still prone to slip when the going got tough. Our answer was to go to plan B which uses bike gears and a bike chain for the drive and this has proved to be entirely satisfactory. The pulley can still be seen fixed to the door spider in the photos and it was left there to provide a rigid support for the 46-tooth pedal sprocket. These days pedal sprockets are made from pressed steel and while they are adequate for pushbike use, when the central section is cut out to clear the central door support shaft, they are little on the flimsy side. Mind you, we certainly do not May 1998  55 still in place, with the bike sprocket attached to it. The small sprocket can be bolted or welded to the 60mm plate. It depends on the type you obtain. Drill the motor mounting bracket holes to suit your motor and mount the motor on it. The ends of the chain were joined to establish its length, then the motor bracket was secured to the door frame using two coach screws, allowing just a slight amount of slack in the chain. Make sure the sprockets are in vertical alignment. Just a trace of grease was applied to the chain, once everything was adjusted, to keep it running smoothly. Make sure that the chain you buy matches the sprockets as there are two different sizes. Initial tests Fig.2: a 46-tooth pedal sprocket (about 190mm in diameter) is bolted to a stiffening plate and to the door spider. Spacers are necessary to allow the bike sprocket and chain to clear the end of the roller door but they should not let the bolt threads and nuts get too close to the adjacent mounting bracket. advocate using a large pulley if you are building this project from scratch since the central section will have be cut out. A cheaper and easier solution would be to make a support plate from a round or hexagonal metal plate, say 3mm or thicker. The 46-tooth pedal sprocket (about 190mm in diameter) was bolted to the pulley with 10mm standoffs to give a little clearance for the chain. Fig.1 shows the general concept of how the support disc and bike sprocket is attached. Spacers are necessary to allow the bike sprocket and chain to clear the end of the roller door but they should 56  Silicon Chip not let the bolt threads and nuts get too close to the mounting bracket. The mounting bracket for the motor was made out of a piece of angle iron, 40 x 40 x 200mm. A 15-tooth rear- wheel bicycle sprocket (62mm dia­meter) was mounted on the wiper motor (see Fig.1). The wiper motor shaft was originally fitted with an angled bracket to drive the wiper arms. The bracket was keyed to the gearbox shaft with a D-shaped hole and this took a bit of drill­ing and filing to reproduce in the centre of a 60mm plate. This plate is retained on the shaft with a locking nut. Our photo shows the V-belt pulley Power the motor and run the door up and down a few times to ensure that everything is working smoothly. If the motor tends to labour excessively towards the top or bottom of the door’s travel, the door balance spring may need adjustment. Speaking from experience, this is definitely a two-person job. Disconnect the chain and then run the door up and down by hand to see whether the spring is set to pull the door up or drive it down. Ideally, you want the spring set so that it has no bias and the door is equally easy to push up or pull down. To adjust the spring you need two people, each with a pair of Stillsons to hold each end of the door shaft. Loosen the pipe U-bolts and twist the pipe so that the spring tension just bal­ ances the door. Re-tighten the U-bolts and then run the door up and down by hand to confirm that it is balanced. When you are satisfied, reconnect the chain and run the motor again to confirm your adjustment. Should the door drive motor or electronics fail for any reason, the nut holding the small sprocket to the motor shaft can be undone and the sprocket removed, allowing the door to be manually operated. Limit switches The item that caused the most brain strain was the limit switch system. As you will have noted from last month’s article, the circuit has provision for one or two limit switches. These may be microswitches, reed switches or any other type that comes to mind. For our prototype, we initially tried This view of the windscreen wiper motor shows the small pulley still fitted, in addition to the rear wheel bike sprocket. This general view of the chain drive shows the arrange­ment of the sprockets. The large pulley attached to the door spider is not necessary although you will need a circular plate to add rigidity to the bike sprocket. A chain guard is recommended, as a safety measure. microswitches that are readily available from the usual suppliers but they were found to be too flimsy for this type of application. After several trials and many impolite words the system shown in Fig.3 was evolved. Two pulleys were mount­ ed, top and bottom, on the door guide rail using a 25mm bolt and a 10mm unthreaded spacer. An aluminium bracket was mounted on the bottom rail of the door. The limit switch cord is threaded through this bracket and at the top and bottom of its travel it contacts the respective limit stop. The limit stops are single pieces of the connector strip with the cord threaded through them. The two screws are gently tightened once the correct limit position is found. Depending on which limit stop is contacted by the door bracket, the cord moves the aluminium blade up or down, to oper­ate a single reed switch. A steel strip, which is attached to the aluminium blade, acts as a magnetic shunt when it moves between the magnet and the reed switch, allowing the switch to open. The limit switch setup may have to be adapted slightly to suit your door or you may find it simpler to use an upper and a lower limit switch as shown in the circuit. These can be wired in parallel at the terminal block or looped at the door. Obviously if you use a single switch as we did, only one set of limit inputs will be used. The size of the magnetic shunt will have to be determined by trial and error, as it will depend on the strength of the magnet. When the blade is horizontal the reed switch should be open but as the blade moves up or down it should close. Too small a piece of steel will not let the switch open and too large a piece will never let it close. The piece we used was the width of the blade and 25mm long. This should be a good starting point. Limit switch indicator One additional feature which we found we needed when we were adjusting the limit stops was an indication of when the limit switch actually operated. This was easily added to the controller by soldering a wire to pin 2 of IC1 and another to pin 16, the +12V supply. A 5mm high brightness LED was mounted in a retaining clip on May 1998  57 Fig.3; details of the cord-operated limit switch devised for the door. Two pulleys are mounted, top and bottom, on the door guide rail using a 25mm bolt and a 10mm unthreaded spacer. An aluminium bracket is mounted on the bottom rail of the door. The limit switch cord is threaded through this bracket and at the top and bottom of its travel it contacts the respective limit stop. 58  Silicon Chip Parts List Mechanical 1 12V windscreen wiper motor and gearbox (available from most car wreckers) 1 3mm thick 220mm diameter round or hexagonal steel plate (refer text) 1 3mm thick 75mm diameter steel plate (refer text) 1 46-tooth pedal sprocket (refer text) 1 15-tooth rear wheel sprocket (refer text) 1 bike chain to suit sprockets 1 200mm x 40mm x 40mm x 10G steel angle 2 25mm curtain pulleys 7M curtain cord (for limit switch) Miscellaneous 16-gauge aluminium, bolts, nuts, washers, spacers etc. the front panel below the light. A 3.3kΩ resistor was soldered to the supply wire and the anode of the LED. Its cathode was soldered to the pin 2 wire. Whenever the motor is running pin 1 of IC1 goes high and consequently pin 2 goes low, lighting the LED. When the flipflop toggles, the motor stops and the LED goes out. This is a simple but useful addition. Installation You will need to mount the control box in a suitable loca­tion on the garage wall and connect the external leads. We fur­ther suggest that the control box be as close as possible to the drive motor and the battery should be close by as well, to mini­mise the length of the power leads. Ideally, it would be good to have a 240VAC power point nearby, to plug in the 12V DC plugpack to be used as the trickle charger for the battery. By the way, do not mount the battery on the garage floor and this especially applies if you are using a car battery. Car batteries deteriorate quick­ ly if they are left on a cold concrete floor. The battery should be mounted above ground on its own shelf and should have a cover over it to prevent any possibility of accidental shorts. The limit switch cord runs over a pulley at the top and bottom of the door. The cord is the same as used for curtain and blind pulls. The Local switch should be located in a hidden but conveni­ent position. You don’t want any burglars opening the door to let themselves out. The door should be set to the half open position. The first time the door is operated after power is initially applied it should open. If it closes, reverse the leads to the motor. Overcurrent setting The overcurrent control is wired so that it is most sensi­tive in the fully clockwise position. VR1 should be set anti­clockwise so that the door closes without reversing, but if additional restraint (placing your hand under it) is experienced, it will reverse. Chain guard Finally, we recommend that you fit a chain guard to cover the chain and sprockets to ensure safety. You can either fabricate this yourself from sheet metal or perhaps purchase a ready-made unit from a bicycle shop. Reference Remote Controlled Gates For Your Home, SILICON CHIP, August 1997. An aluminium bracket was mounted on the bottom rail of the door. The limit switch cord is threaded through this bracket and at the top and bottom of its travel it contacts the respective limit stop. May 1998  59