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Driveway Gate Remote Control for sliding and swinging electric Gates Sliding/swinging gate controllers inevitably fail after some years of service. The more poorly made models will die after just a few years, so you will end up repairing or replacing them frequently. The solution is to replace the controller with this much more robust design, and as you build it yourself, it’s easy to fix if it does go wrong. By Dr Hugo Holden W hen I moved into my current home some 20 years ago, I enjoyed the fact that the front fence had a sliding electric driveway gate. However, after about a year, the gate started to malfunction, initially with intermittent behaviour and then total failure. I inspected the gate control module, which was based around a controller CPU. The motor switching relays looked somewhat small for the task, and I could see significant contact burning through their transparent covers. I called the manufacturers for a schematic, but they did not want to provide any assistance. Instead, they directed me to their local repair agents. A fellow at the company seemed quite sympathetic, but it was apparent he ‘wasn’t allowed’ to help a customer to effect their own repairs. As is often the case, the repair agents were unable to make PCB-level repairs and could only replace the whole 76 Silicon Chip control board for hundreds of dollars. Initially, I accepted this. It failed again a year later, and again, I had to buy a new PCB. Further failures appeared after lightning storms on two occasions. After repeated episodes of the system failing, I was getting fed up. I took one of the original boards and replaced the relays, to good effect. I also replaced some aged electrolytic capacitors, but the writing was on the wall. Fortunately, the radio receiver board (a generic third-party product) had always been very reliable, so I kept that and decided to design a new controller board to connect to it. My solution I decided to throw the original controller PCB in the bin and design my own from scratch. Looking around at the parts in my workshop, I had a good supply of 74-series vintage TTL logic ICs (some of which were were used Australia's electronics magazine in a Pong system; see the June 2021 issue) . These are rugged and reliable, also highly resistant to damage from electrostatic discharge (ESD). The task of an electric driveway gate appears simple on its face. But like many automation systems, the devil is in the detail. My sliding gate is powered by a 24V DC bidirectional brush motor. It has two standard micro-switches as motion limit switches. These are mounted close together in the motor drive unit and are mechanically activated at each end of the gate travel, via a spring arm, when the gate is fully closed or fully open. A swinging gate is likely to have a similar arrangement, so my controller could be suitable for that type of gate. However, I have not tested it as such. You would have to check how your gate system works before deciding to use my controller. The controller logic needs to take account of the states of these limit siliconchip.com.au Easy to service; no software and all through-hole parts Triggered by a single remote or local button (or both) High long-term reliability and EMI tolerance Stops the gate if it hits an obstacle Safe power-on reset Power input: 24V AC Motor current limit: adjustable from 0A to 8.33A Power for remote control board: 5V DC or 24V DC Motor drive: 24V DC or rectified AC at up to 8.33A (200W) switches during the use of the gate. It must then control the motor direction appropriately when the gate starts from a fully closed or fully open, or perhaps intermediate position. It also needs to detect the motor current in case the gate strikes an obstacle, to stop the gate motor. The gate is controlled by a handheld remote via a radio receiver board, its output being a momentary closed contact from a small relay on the radio receiver board. But it could also be controlled by a manual pushbutton. Finally, the control logic requires a very effective reset function to ensure that the gate remains in its stopped position with any kind of rapid, slow, or variable mains power cycling. Otherwise, a brownout, blackout or other event could trigger the gate’s motion and maybe open up the gate when you are not home. there are four fundamental modes of operation, cycled through by a button press. Initially ignoring the two limit switches, the remote control needs to cycle the gate through four operational states, shown in Fig.1. Therefore, a two-bit counter is needed, giving four logic states. I achieved that using a 7474 dual D-type flip-flop IC. These flip flops can be preset or cleared, which is required to take account of the gate limit switch conditions. Fig.1: the gate is controlled using a ‘state machine’ with four states: fully open, fully closed, opening or closing. The remote button cycles to the next state in the loop, while the limit switches on the gate force the machine into one of the stopped states. The state machine Considering these requirements, siliconchip.com.au Australia's electronics magazine February 2022  77 Fig.2 shows how the state machine is controlled by a combination of the limit switches and the remote control. For example, when the gate is opening and it reaches the limit switch, a 100ms pulse is gated via the OR gate and the lower AND gate, the state machine changes to the ‘stop before forward’ state, and the gate motor stops. If the control button is then pressed on the remote, upon the button initially being pressed, the ‘stop before forward’ state is reset to be 100% sure the state machine is in the correct condition according to the now-static switch data. On the trailing edge of the pulse, the state machine is then clocked to the ‘forward’ state, and the gate begins to close. The closed switch is triggered when it is shut, and the machine is set to the ‘stop before reverse’ state. If the button is pressed again, the state machine is reset to this condition on the leading edge of the pulse, then clocked to the ‘reverse’ state on the trailing edge, and the gate starts to open. The stopped states are applied on the leading edge of the control pulse to ensure that, whatever state the This is the type of universal motor typically used to drive a sliding or swinging gate. They are typically powered from 24-48V DC or rectified AC although some run from as little as 12V. controller was in before, the motor stops before it starts moving. This way, the gate always starts in the correct direction and doesn’t attempt to run itself past the stops set by the two limit switches. Circuit details The circuit is shown in Fig.3. Either power-cycling or gate over-current is designed to set the gate into the ‘stop before reverse’ condition. This does not cause a problem even if the gate is power cycled in the fully reversed condition, as with the next activation of the remote control, the state machine is forced into the correct condition (ie, ‘stop before forward’) before the gate starts its motion. One important feature of the design is that the limit switches are debounced. The cross-coupled Fig.2: more detail on how the state machine is implemented using digital logic chips. When either the remote button is pressed or a limit switch is activated, a pulse is generated. These pulses are ORed to create a pulse that advances the state machine to the next state. The pulses are also ANDed with the limit switch signals to force the machine into either the fully closed or fully opened states when needed. 78 Silicon Chip Australia's electronics magazine siliconchip.com.au inverter gates (IC1a, IC1b, IC1e & IC1f) very effectively debounce a changeover switch, unlike other methods using RC networks, Schmitt triggers, delay timers etc. This method is mainly time-domain independent, and the 7404 logic ICs are not harmed because their outputs are only forced low for the very brief propagation time of the inverter gate. 74-series ICs, while good at sinking current, only weakly source it. One interesting consideration is whether to regard the two limit switches as independent items, or two items acting together. The two limit switches are entirely isolated from the mechanical perspective, and it is essentially impossible to activate them simultaneously. After all, the gate cannot physically be in two places at once (open and closed), and the spring arm that activates the switch can only be pushed in one direction at a time. However, the switches are mounted close together, and the cables to them are in one bunch. So very heavy RFI (eg, from a nearby lightning strike) could possibly fool the electronics that both switches are activated at once. Therefore, I concluded it was best to XOR the signals from the two gate microswitches using gate IC2d as a form of ‘digital common-mode noise pulse immunity’ because an XOR only responds if its inputs are complimentary. In other words, if both switches are seen as closed at once, it is treated as if neither is closed. The debounced and XORed limit switch outputs are then strobed into the state machine’s clear & preset terminals, with approx 100ms pulses from 555 timers IC7 & IC8. These are triggered by a command from the remote control (or pushbutton) or a state change when a limit switch is activated. This arrangement ensures that the limit switch states set the correct state machine state (via the CLR and preset inputs of the two 7474 flip flops, IC6a & IC6b), while the remote control can also cycle through the sequence by clocking the first flip-flop, which in turn clocks the second flip-flop. The outputs of the state machine (labelled A & B) are uniquely decoded into two simple control signals, forward and reverse by another XOR gate (IC2a) and a pair of NAND gates (IC4c siliconchip.com.au Parts List – Remote Gate Controller 1 double-sided PCB coded 11009121, 209.5 x 134.5mm 1 sealed ABS enclosure, 222 x 146 x 75mm [Jaycar HB6132 ➊] 1 24V AC power supply (plugpack or mains transformer, sufficient to handle the full motor current) 1 radio receiver board with relay output, plus one or more matching keyfobs 2 3-way terminal blocks (CON1, CON2) 1 2-way terminal block (CON3) 1 6-way PCB-mount barrier terminal (CON4) [Altronics P2106] 1 3-way pin header with jumper shunt (JP1) 2 24V DC coil 24V/30A SPDT relays (RLY1, RLY2) [Jaycar SY4047] 2 M205 PCB fuse clips (F1) 1 M205 4A slow-blow fuse (F1) 1 5kW mini horizontal trimpot (VR1) 2 6073B-type 19x19mm TO-220 mini flag heatsinks (for REG1 & D8) [Jaycar HH8502, Altronics H0630] 4 M3 x 8-10mm panhead machine screws 4 M3 flat washers 4 M3 star washers 4 M3 hex nuts 4 M3 x 6mm self-tapping screws 1 or more cable glands (to suit installation) ➊ it will fit in Altronics H0312A or H0313 boxes, but the mounting holes will not line up with the plastic posts in the base Semiconductors 1 7404 or 74LS04 hex inverter, DIP-14 (IC1) 1 7486 or 74LS86 quad 2-input XOR gate, DIP-14 (IC2) 1 7408 or 74LS08 quad 2-input AND gate, DIP-14 (IC3) 1 7400 or 74LS00 quad 2-input NAND gate, DIP-14 (IC4) 1 7402 or 74LS02 quad 2-input NOR gate, DIP-14 (IC5) 1 7474 or 74LS74 dual D-type flip-flop (IC6) 3 555 timer ICs, DIP-8 (IC7-9) 1 7805 5V 1A linear regulator (REG1) 2 BC639 60V 1A NPN transistors (Q1, Q2) 2 BC548 30V 100mA NPN transistors (Q3, Q4) 1 BS270 P-channel small signal Mosfet (Q5) [Digi-Key, Mouser element14] 3 1N4148 signal diodes (D1-D3) 4 1N4004 400V 1A diodes (D4-D6, D8) 1 30A rectifier diode, TO-220-2 (D7) [eg, SDUR30Q60 or STTH30R04W] Capacitors 1 4700μF 63V snap-in radial electrolytic (optional) 1 1000μF 63V radial electrolytic 2 100μF 50V radial electrolytic 4 10μF 50V radial electrolytic 1 2.2μF 50V multi-layer ceramic 15 100nF 63V MKT 5 10nF 63V MKT Resistors (all 1/4W 1% metal film unless otherwise stated) 1 1MW 1 120kW 3 47kW 2 9.1kW 1 4.7kW 6 1.5kW 1 1kW 2 620W 2 430W 2 100W 3 68W 5W 10% wirewound 1 0.68W 50W 10% wirewound [element14 Cat 2478215 or 2946343] Australia's electronics magazine February 2022  79 & IC4d). These signals are inverted by two 7404 gates (IC1c & IC1d) and used to drive two BC639 transistors (Q1 & Q2) that switch the two 24V relays, driving the gate motor forward or in reverse. Current sensing resistor (R1), in series with the motor, develops a voltage proportional to the motor current. 80 Silicon Chip The commutator noise is filtered out by an RC-low pass filter comprising a 1kW series resistor and a 100μF capacitor to ground. If the gate collides with an obstacle, the output voltage of the filter increases and this forward-biases the base-emitter junction of transistor Q4, generating the OVR signal. Australia's electronics magazine This stops the gate and sets the state machine to ‘stop before reverse’. However, when the gate starts up and accelerates from a stopped position, there is a motor current surge. To ensure the current detector is deactivated when the motor starts in either the forward or reverse direction, timer IC9 generates a pulse of around 1.3s siliconchip.com.au Fig.3: the full circuit for the Gate Controller is somewhat complex but you can compare it to Fig.2 to get an idea of which section does what. The three timers, IC7-IC9, each act as pulse stretchers to ensure that brief events such as a short button press are not missed. duration, which causes Q3 to inhibit the charging of the 100μF filter capacitor. The motor can be powered by halfwave pulsed DC using just the power rectifier, but you can speed it up with the addition of the 4700μF capacitor. I used an IXYS 30A rectifier to ensure that it would not fail. siliconchip.com.au Pull-up resistors One subtlety of the design that isn’t immediately obvious is the need for the 1.5kW pull-up resistor at the output of IC5a. The 74xx TTL logic device outputs only go up to about +3V when high, despite running from a 5V supply. That isn’t a problem when they feed the inputs of other 74xx devices, Australia's electronics magazine as the inputs are designed to handle this. Note that 3V is above the ~1.7V trigger threshold of a 555 with a 5V supply. But given the weak pull-up current from a 74xx device (around 0.4mA), it’s much better to have an external pull-up resistor so that the 555 is reliably triggered, especially February 2022  81 since the trigger signal is capacitively coupled. Construction The Gate Controller is built on a double-sided PCB coded 11009121, which measures 209.5 x 134.5mm. Refer to the PCB overlay diagram, Fig.4, as a guide during construction. There is nothing particularly difficult about assembling this board, so the usual technique of starting with the lowest profile components and working your way up should work well. Start with the small resistors, checking the value of each lot with a DMM before fitting them. Then mount the diodes, ensuring that the striped cathode ends are orientated as shown in Fig.4. Next, install the ICs, taking care that their pin 1 ends are located as shown. I don’t recommend using sockets as they are a potential failure point, and as mentioned earlier, all the ICs used in this design are very reliable. We only fitted them to the board shown for development reasons. Follow with the sole trimpot. Then fit the smaller transistors, being careful not to get the different types mixed up, followed by the smaller MKT and ceramic capacitors, which are not polarised. Next, mount the larger resistors, spacing them off the PCB by a few millimetres to allow cooling air to circulate. Follow with the fuse clips, ensuring the retaining tabs are towards the outside so you can insert the fuse later. Bend the leads of REG1 and D8 to fit their respective pads, with the device tab holes located over the matching mounting holes on the PCB. Slip the heatsinks between the PCB and the device’s tabs, then attach the tabs securely using M3 machine screws, nuts and washers on either side. Ensure they are secure and the bodies and heatsinks are straight before soldering and trimming the leads. The large 50W resistor is held to the board using two M3 screws, nuts and washers on either side. Once you’ve mounted it in place, bend a lead offcut from one of the 5W resistors so that it reaches from the pad towards the centre of the PCB to the 50W resistor lead, then solder it in place. The tabs of the relays should drop right into the slots provided on the PCB. Make sure they’re pressed all the way down, and use a generous amount of solder on each pin to hold them securely to the PCB. Now mount the terminal blocks (wire entries towards the outer edge of the PCB), barrier terminal strip and the larger electrolytic capacitors, ensuring the latter are orientated with the longer positive leads to the pads marked + on the PCB. Bend another off-cut to go from the other lead to the AC terminal as shown in Fig.4 and the photo, then solder it to the other end of the resistor and clamp it down in the screw terminal. Wiring it up Before mounting the PCB in the case, you will need to figure out where the radio receiver module will be mounted (it might be possible to fit it to the inside of the enclosure lid), which wires need to enter the box and where the best place is for them to enter. The wire entry will need to be waterproof if the unit will live outside, which can be done either using one or more cable glands (as mentioned in the parts list) or seal the holes with neutral cure silicone sealant after running the wires through. Most likely, you will have ten wires to run in two twin leads and two multicore cables: two for the low-voltage AC power input, two wires going to the motor and five or six wires going to the limit switches. Ideally, use cables with a round profile and run each through its own cable gland. You could use a four-core screened cable for the limit switches and twocore round cable for the others, meaning you need three glands and thus three holes in the case. If you can’t fit the radio receiver in The finished Driveway Gate Controller is located in a plastic enclosure near my gate with a liberal amount of waterproof tape applied (shown on the lead image). This means I can still open it up to access the board (however unlikely that is now) while still keeping the water out. I certainly wouldn’t want water getting in and corroding away all my hard work! 82 Silicon Chip Australia's electronics magazine siliconchip.com.au the case, you will need to run some additional wires to the outside. These are two wires to power the receiver board (assuming you aren’t supplying it with power externally) and two which run from the receiver’s relay contacts to input connector CON3. They could be run together using three- or fourcore screened cable. Note that, as there is no room in the box for a mains transformer, you will either need to use an AC plugpack or (more likely) mount a mains transformer, mains input socket (or captive cord), fuseholder and wiring in a separate insulated box. We won’t give any instructions on how to do this, except to say that you need to use correctly-coloured mainsrated wire where appropriate (Active = brown, Neutral = light blue and Earth = green/yellow striped). You will also need to ensure that all exposed mains conductors are insulated (eg, with heatshrink tubing) and tied up neatly with cable ties so they can’t float around in the box if they break loose. If you aren’t experienced with building mains-powered equipment, you will be better off finding a suitable plugpack instead. Drill holes for these glands (or the bare wires, if using silicone) near where the relevant connectors will be once the PCB is mounted in the case. Mount the glands securely, then install the PCB, insert the wires, attach them to the relevant terminals (as shown in Fig.4), pull out most of the slack and tighten the gland nuts. If you have room to fit the receiver in the box, you could attach it to the inside of the lid using neutral cure silicone sealant – make sure it isn’t going to foul any components on the main PCB when the cover is in place. Another option is to use tapped spacers and screws (assuming it has mounting holes), but if you do that, make sure you seal the screw holes through the lid so moisture can’t get in. If mounting it on the lid, that also siliconchip.com.au Fig.4: assembling the PCB is straightforward. Fit the parts in the locations and orientations shown here. Note how the large resistor is attached to the PCB using machine screws, then two wires are soldered to its exposed terminals. One goes straight down to a pad on the PCB, while the other end connects to one of the low-voltage AC input terminals on CON4. Australia's electronics magazine February 2022  83 The electrolytic capacitor sandwiched between Q3 and VR1 should be 100μF as shown in the circuit and overlay not 10μF as shown in silkscreen of the photos. Our first batch of PCB that we are selling have this listed incorrectly, so keep an eye out when assembling! Subsequent PCB batches will have this problem fixed. allows you to run the receiver antenna around the inside of the lid, assuming it is using a length of wire as a whip. Testing, setup & use There isn’t much to go wrong, but since the motor will not be running initially, you could connect a safety resistor (say 10W 5W) in series with the AC supply the first time you set it up. Check the AC voltage across that resistor; it should be well under 1V. If it’s more, switch off and check the board and wiring for faults. Assuming it’s OK, measure the voltage between pins 1 and 14 of IC6 (or just about any of the 74xx ICs). You should get a reading close to 5V. Next, check the voltage at the 68W 5W resistor leads right near the edge of the PCB relative to the tab of REG1. This reading should be between about 22V and 28V if a radio receiver board is connected, but it could be somewhat higher than that (up to about 35V) if there is no radio receiver board drawing power from the unit. If that all checks out, remove the safety resistor and connect the low-voltage AC supply directly to the board. Now is also a good time to fit the onboard fuse, which protects the motor. The remainder of testing assumes 84 Silicon Chip you have the unit wired up to your gate. Double-check that the connections to the limit switches and motor are correct before proceeding. We’ll assume the gate is initially closed, although it would be best if you could manually open it slightly. It is ideal if you are near the gate and can manually activate the limit switches easily. Set VR1 to its midpoint, then power the controller up. It should reset in a state where it’s ready to open. Press the button on the remote or short the terminals of CON3. The gate should start to open. If it tries to close instead, remove the power and swap the wires to the motor terminals. If it simply doesn’t budge, or move a tiny amount then stops, you might need to wind VR1 up to allow more motor current. Assuming it starts to open, actuate the fully open limit switch and verify that it stops. Then press the remote button again and check that it starts to close. Actuate the fully closed limit switch and verify that it stops, and that if you press the button again, it begins to open. Assuming it does that, check that it opens and closes all the way. If it stops partway, turn VR1 slightly clockwise and try again. Australia's electronics magazine If it opens and closes all the way the first time, try winding VR1 anti-clockwise a bit and repeat. Continue until it stops working reliably, then turn VR1 clockwise slightly and verify that it works reliably again. The idea is to set VR1 just far enough clockwise that it opens and closes every time, but not too much further than the minimum setting to achieve this. That way, it will stop quickly if something gets in its way. All that’s left is to seal it up and tuck it away. Your Gate Controller should work reliably for many years to come! Conclusion One great advantage of this gate controller is that it uses standard garden-­ variety 74 or 74LS series TTL digital logic ICs. These are rugged and generally very reliable. Many commercial gate controller manufacturers will not release their firmware or schematics; even if they did, it would require the specific programming hardware and utilities to re-program a new microcontroller if needed. On the other hand, this design can be repaired easily and at minimal cost if it goes wrong. Mine has been running for over 15 years now and has proven to be very reliable and trouble-free. SC siliconchip.com.au