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A low-cost electric fence controller Based on an automotive ignition coil, this Electric Fence Controller is ideal for controlling livestock. It’s easy to build and can power fence lines up to 1km long. • • Low cost a nd easy to build Based on an automo ti v e ignition co il • 2mA avera ge c • 12V battery urrent drain operation • Suitable fo r fence run s up to 1km By JOHN CLARKE Electric fences are often used on farms to provide a tempo­rary fenceline or to add security to a fence that is in disre­pair. Their main advantages are: (1) they are relatively cheap (compared to permanent fences); (2) they are easily moved from place to place; and (3) they are very effective when it comes to containing live­stock. Electric fences are also very effective for keeping ani­mals out of restricted areas, such as keeping cattle out of one section in a paddock that has been given over to lucerne. There are many different types of electric fence con­trollers on the market today and each suits a particular purpose. Some can operate fencelines up to 100km long, while others are only suitable for up to 1km lengths. 20  Silicon Chip The main difference between one controller and another is the amount of power which can be delivered to the fence. Of course, the longer the fence the greater the losses incurred along its length. These losses are due to the impedance of the wire, its capacitance to ground and the load on the fence. This load can be provided by a number of factors, including long wet grass, wet insulators and animals contacting the wire. Conse­quently, considerable power is required to overcome these losses and maintain a satisfactory voltage on the fence so that it can still do its job over long distances. On the other hand, the SILICON CHIP Electric Fence Con­ troller is only a low-power unit capable of powering Main Feat ures • • Complies w ith Australia n Standard 3 129.1-199 3 Reverse p olarity prote ction less than 1km of fence. It is designed to operate from a rechargeable 12V battery and this could range from a 1.2Ah (or larger) gel cell battery to a conventional 12V car battery. Ignition coil Because it is only a low-power unit, the circuit is built around an automotive ignition coil. This eliminates the need for complicated inverter circuitry and greatly simplifies the con­struct­ ion. In addition, an ignition coil is cheap compared to a purpose-wound 47  12V D1 1N4004 470 16VW 2.7M 7 1.5k 6 4 B C E B C 8 IC1 7555 2 E L1 IGNITION COIL 6. 8  1W F1 500MA 1 GND Q1 BC327 E 3 2.2k B Q2 MJ10012 C C 100  B 5 0.1 0.68 VIEWED FROM BELOW HT TO FENCE E Fig.1: the circuit uses 7555 timer IC1 to pulse transistors Q1 & Q2 on & off. Q2, in turn, switches the ignition coil which delivers a high-voltage pulse to the fenceline. ZD1 75V 1W ZD2 75V 1W ZD3 75V 1W ELECTRIC FENCE CONTROLLER transformer and this keeps the cost to a minimum. In fact, you don’t even have to purchase a new ignition coil. A second­ hand unit scrounged from a wrecking yard will do the job quite nicely. The circuit has been designed to suit an ignition coil intended for use with a ballast resistor. One problem in building an Electric Fence is coming up with a suitable waterproof enclosure to house the control circuitry. We solved this problem by installing the circuit in a length of 90mm-diameter PVC tubing. End caps were then used to seal the tube from the weather. In addition, one endcap holds the fence terminals while the opposite endcap carries a cordgrip grommet which clamps the twin lead that goes to the battery. The advantages of this type of enclosure are that it is completely weatherproof, is quite cheap compared to other en­closures and can be mounted using standard 90mm clamps. In fact, you may even have some scrap 90mm tubing in your garage which can be pressed into service. All you have to do is purchase a couple of endcaps from your local hardware store and the enclosure is complete. How it works Take a look now at Fig.1 – this shows the complete circuit details for our Electric Fence Controller. Apart from the igni­tion coil, it uses just one IC, a couple of transistors and a handful of other minor components. IC1 is a CMOS 7555 timer wired to operate in astable mode. When power is initially applied, its 0.68µF timing capacitor (on pins 6 and 2) charges via the 1.5kΩ and 2.7MΩ resistors until it reaches 2/3Vcc (ie, 2/3rds the supply voltage). At this point, pin 7 (previously open circuit) goes low and the 0.68µF capacitor discharges via the 1.5kΩ resistor until its reaches 1/3Vcc. Pin 7 now goes open circuit again and so the timing capaci­tor charges once more towards 2/3Vcc. This cycle is repeated indefinitely while ever power is applied to the circuit. IC1’s pin 3 output follows pin 7; ie, it is high while the timing capacitor charges and low while it discharges. As a re­sult, pin 3 alternately goes high for about 1.3 seconds and low for about 0.7ms. This very brief low period is due to the rela­tively low value of the resistor (1.5kΩ) connected between pins 6 and 7 of IC1. Pin 3 of IC1 is used to drive transistors Q1 and Q2. Q2 is an MJ10012 power Darlington transistor, designed specifically as a coil driver in automotive ignition systems. It switches the heavy currents through the coil and so can be regarded as the workhorse of Below: the ignition coil is firmly secured to the PC board using cable ties. Note that you don’t have to buy a new coil – a secondhand coil obtained from a wrecker’s yard will do the job quite nicely. A plastic cap is fitted to Darlington transistor Q2 to help prevent unexpected shocks during testing. July 1995  21 coil. This voltage is about 5kV (across a 1MΩ load) and is applied directly to the fenceline. As a result, a brief (0.7ms) high-tension pulse is applied to the fence approximately every 1.3 seconds. This operation is basically similar to the ignition system in a car, in which the coil primary current is periodically interrupted by a switching transistor or a set of points. In a car, however, the resulting HT voltage is used to fire the selected sparkplug. Despite the fact that Q2 is a very rugged device, it is possible that it could be damaged by excessive backEMF voltages from the coil. To guard against this situation, three 75V 1W zener diodes (D2-D4) have been connected in series across Q2. These limit the collector voltage to 225V which is well within its 500V rating. Note that the circuit is designed to deliver about 5kV by dint of a very brief charging pulse through the coil. In a normal automotive setup the coil would deliver a much higher voltage but this would not be desirable in this case. Electric fences must comply with the Australian Standard (AS 31291981) which sets strict limits on the output voltage, pulse duration and output impedance. PARTS LIST 1 PC board, code 11306951, 171 x 79mm 1 adhesive label, 125 x 50mm (Electric Fence Controller) 1 adhesive plastic label, 85mm diameter (Fence Terminals) 1 adhesive plastic label, 85mm diameter (Input Voltage) 1 230mm length of 90mm diameter PVC tubing 2 90mm diameter end caps 1 12V automotive ignition coil 3 280 x 5mm cable ties 5 PC stakes 2 3AG PC board fuse clips 1 500mA 3AG fuse 2 large binding posts (1 red, 1 black); eg, DSE Cat. P-1731/33 2 5mm ID crimp eyelet terminals 1 TO-3 transistor insulating cap 2 3mm x 6mm-long screws, nuts & star washers 1 red battery clip to suit 1 black battery clip to suit 1 cord grip grommet 1 brass EHT ignition coil connector 1 2-metre length of twin red/ black automotive wire 1 60mm length of red heavy duty hookup wire 1 60mm length of blue heavy the circuit. Q2 also has a high voltage rating (500V) to allow it to withstand the high voltages developed across the ignition coil primary. The circuit works like this. When pin 3 of IC1 is high, PNP transistor Q1 is held off and so Q2 is also held off. Conversely, when pin 3 pulses low, Q1 switches on because base current can now flow via its associated 2.2kΩ resistor. And when duty hookup wire 1 120mm length of green heavy duty hookup wire 1 60mm length of 240VAC insulated wire Semiconductors 1 7555, LMC555CN, TLC555 CMOS timer (IC1) 1 BC327 PNP transistor (Q1) 1 MJ10012 NPN Darlington transistor (Q2) 1 1N4004 silicon diode (D1) 3 75V 1W zener diodes (D2-D4) Capacitors 1 470µF 16VW PC electrolytic 1 0.68µF MKT polyester 1 0.1µF MKT polyester Resistors (0.25W 1%) 1 2.7MΩ 1 100Ω 1 2.2kΩ 1 47Ω 1 1.5kΩ 1 6.8Ω 1W Miscellaneous 1 12V 1.2Ah battery (minimum); 2 x 90mm mounting clamps (to secure the controller to a fence post); 1 x 2-metre long galvanised ground stake; insulators; fence wire (see text). Power supply Power for the circuit is derived from a 12V battery via fuse F1, a 47Ω decoupling resistor and reverse polarity protec­ tion diode D1. The resulting supply line is then filtered using a 470µF electrolytic capacitor to ensure that supply line glitches cannot false-trigger IC1. In addition, a 0.1µF capacitor is connected to pin 5 of IC1 and this filters the trigger point voltage to further guard against false triggering. The primary of the ignition coil is supplied directly from the fuse via a 6.8Ω resistor. This resistor will limit Q1 turns on, Q2 also turns on and current flows through the primary of the ignition coil (L1) via fuse F1 and a 6.8Ω resistor. When pin 3 of IC1 goes high again (ie, after 0.7ms), Q1 and Q2 both turn off and the current through the coil is suddenly interrupted. As a result, the collapsing magnetic field produces a very high voltage across the high tension (HT) secondary wind­ing of the TABLE 1: RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 1 1 1 1 22  Silicon Chip Value 2.7MΩ 2.2kΩ 1.5kΩ 100Ω 47Ω 6.8Ω 4-Band Code (1%) red violet green brown red red red brown brown green red brown brown black brown brown yellow violet black brown blue grey gold brown 5-Band Code (1%) red violet black yellow brown red red black brown brown brown green black brown brown brown black black black brown yellow violet black gold brown blue grey black silver brown EHT TO FENCE GND TO GROUND STAKE FENCE TERMINALS Fig.2 (left): install the parts on the PC board as shown in this wiring diagram, making sure that all polarised parts are correctly oriented. The EHT connection to the coil is made using a brass EHT ignition coil connector. Fig.3 (below): check your PC board for defects by comparing it against this full-size etching pattern before installing any of the parts. CABLE TIE IGNITION COIL CABLE TIE CABLE TIE 6. 8  W 47W 100  Q1 IC1 7555 F1 D1 2.7M 1.5k 12V BATTERY POSITIVE Q2 2.2k 0.1 1 0.68 ZD1-ZD3 470uF 12V BATTERY NEGATIVE July 1995  23 Solder the mounting nuts for Q2 to their surrounding copper pads, as shown here. This is necessary to ensure reliable connections for the collector of this transistor. A brass ignition coil connector (soldered to a length of mains-rated cable) plugs into the ignition coil out­put. The connections to the primary terminals are made by terminating the leads using 5mm eyelet connectors. the coil current until fuse F1 blows if Q2 short-circuits between collec­ tor and emitter. F1 also protects the battery in the event of a circuit fault by limiting the maximum current to 500mA. The overall current drain of the circuit is about 2mA, so a fully charged battery should be able to provide many weeks of operation (depending on its size). Note that the overall current consumption has been kept low by specifying a 7555 CMOS timer for IC1 rather than a standard 555 type. A 555 typically draws 10mA compared to about 150µA for a 7555 and so would increase the current consumption by a factor of 6. Construction The control circuit is built on a PC board coded 11306951 and measuring 171 x 79mm. This board, together with the ignition coil mounted on it, fits Two large binding posts are used to terminate the EHT & ground connections from the control circuit. Note that the two end caps should by sealed with silicone sealant to prevent water damage to the circuitry housed inside the plastic conduit. 24  Silicon Chip neatly inside the 90mm plastic conduit. Fig.2 shows the assembly details for the PC board. Begin the assembly by installing PC stakes at the five external wiring points. This done, solder in all the low profile components such as the IC, diodes and resistors. Table 1 lists the resistor colour codes but it is also a good idea to check the resistor values using a digital multimeter before soldering them into position. Take care to ensure that the semiconductors are correctly oriented. In particular, note that D1 (1N4004) faces in the opposite direction to the three zener diodes (ZD1-ZD3). Pin 1 of the IC is adjacent to a notch in one end of the plastic body – see Fig.2. The battery is connected to the circuit via a length of twin red/black automotive wire. Make sure that the battery lead is firmly secured to the end cap using a cord grip grommet. A 12V battery with a minimum rating of 1.2Ah is required to power the fence controller. Fig.4: the circuit can be used to power either single or multiple stands of fence wire, or you can use metallised tape which is specially designed for the job. This is generally white or orange coloured so that it is easily seen. INSULATOR ELECTRIC FENCE CONTROLLER ELECTRIC FENCE HIGH TENSION CLAMPS GROUND POST GALVANISED GROUND STAKE 12V BATTERY Now solder in the capacitors, taking care to ensure that the 470µF electrolytic is oriented as shown. The two transistors are next – push Q1 down onto the board as far as it will comfort­ably go before soldering its leads. Q2 is secured directly to the board (ie, no insulating washer) using 3mm machine screws and nuts. As well as securing Q2 in place, these mounting screws and nuts also connect Q2’s collector (ie, the case) to FENCE TERMINALS + + GROUND HIGH TENSION (TO EARTH STAKE) (TO FENCE) a track on the PC board. To ensure reliable connections, use star washers under the screw heads and solder the nuts to their surrounding copper pads. This done, fit an insulating cap to Q2 – this will prevent any nasty shocks during the testing procedure. The fuse clips can now be installed. Note that these each have a little lug at one end to retain the fuse after it has been installed. These lugs must go to the outside ends, otherwise you will not be able to fit the fuse. The ignition coil is secured to the PC board using three cable ties, after which the leads can be run to its primary terminals. These leads should be terminated using 5mm eyelet connectors to allow for easy connection to the coil. Don’t just crimp the connectors to these leads – solder them as well to ensure long-term reliability. The ground lead can also be installed at this stage. This can be run using a 150mm-length of medium-duty hookup wire. The end caps will need to be drilled for the fence termi­nals and the cord grip grommet to secure the battery leads. The locations of these holes INPUT VOLTAGE 12VDC <at> 2mA AVERAGE (BATTERY ONLY) RED (+) BLACK (-) Fig.5: here are the full-size artworks for the two end caps. These labels should be made from plastic Dynamark® material. July 1995  25 ELECTRIC FENCE CONTROLLER Fig.6: this full-size artwork can be used to make the main identifying label that’s attached to the side of the conduit. can be determined by fitting the two end­cap labels and then using them as drill­ing templates. Large binding posts are used for the two fence terminals (red for the EHT output, black for ground). Mount these in posi­tion, then install the high tension lead. This must be run using a 90mm-length of mains-rated cable. One end is soldered to the EHT binding post, while the other end is attached to a brass ignition coil connector and plugged into the ignition coil out­put. Similarly, connect the ground lead to the ground (black) binding post, then install the twin battery cable (red to posi­ tive, black to negative). The other end of this cable is fitted with large (30A) battery clips. Testing Now for the smoke test. Apply power and check that there is 12V between pins 1 and 8 of IC1. If all is well, you should hear a short click from the coil at 1.3-second intervals. Stay away from the EHT output from the coil and avoid touching the PC board assembly during this test. This FROM NEW N CHIP O SILIC The connections to the battery can be made using heavy-duty clamps, or suitable screw terminals can be used. circuit can deliver a “bite” which is exactly what it is designed to do. If everything works OK, disconnect the battery leads and carefully slide the assembly into its plastic housing. This done, feed the battery cable through the hole in its end cap, secure it using a cordgrip grommet and reconnect the leads to the PC board. The board assembly will be held in position when the end caps are fitted and, generally, this should be sufficient. Howev­er, if you wish the board to be held even more securely, wrap a small amount of foam rubber around the top of the coil so that the assembly is a tight fit within the conduit. Finally, use a suitable silicone sealant (eg, Silastic®) to waterproof all joints around the end caps, the fence terminals and the power cord entry point. Installation Where possible, the controller should be installed inside a building (eg, a shed) so that it is protected from the weather. If used outdoors, it should be mounted on a fixed structure (eg, a fence post) where it is free from the risk of mechanical damage. Use 90mm clamps to secure the controller in position. The controller should be fitted with a separate earth elec­ trode and this should not be connected to any other earthing device. Fig.4 shows a typical installation. Note that all fence wiring should be kept well away from any electrical or telephone cables and from radio and TV antennas. A bare metal conductor can be used for the fence wire. Alternatively, you can used metallised tape which is specially designed for the job. This is available from farm equipment suppliers and is generally white or orange coloured so that it is easily seen. Do not install the unit in any location where people are likely to come into inadvertent contact with it. In addition, any installation should be clearly identified with warning signs posted at intervals not exceeding 90 metres. These signs should carry the words “ELECTRIC FENCE” in block letters no less than 50mm high. SC 20 Electronic Projects For Cars On sale now at selected newsagents Or order your copy from Silicon Chip. Price: $8.95 (plus $3 for postage). Order by phoning (02) 979 5644 & quoting your credit card number; or fax the details to (02) 979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 26  Silicon Chip