 Hello students. I am Mr. Nagesh Arthambake, Assistant Professor, Department of Mechanical Engineering, Valkchan Institute of Technology, Sulapur. Today we are going to discuss automobile electric horn and fuel gauges. At the end of learning outcome, at the end of this session, students will be able to explain automobile electric horn and fuel gauges. In this particular session, we are going to discuss what is relay that is used in electric horn, then electric horn circuit diagram, then introduction of fuel gauges and different types of fuel gauges. Now, before learning the electric horn, we must know what is relay. Can you think regarding these question students? What is relay? That is the answer. This was touched on before, but it's worth special attention. Remember that certain devices require considerable current and that in turn requires thicker wire. High current devices require big, heavy switches to handle the current. Unfortunately, these would be ugly and expensive. So engineers use relay. A relay consists of a small coil of wire around a central iron cover. When the actuating switch energizes the coil, this core moves heavy duty contacts together, thus allowing high current to be passed to the device. That's how a small switch can control a high current device. We already know the starter solenoid is a high current relay. Other devices that typically utilize the relays are the hogs, power antenna, air conditioning compressor, power seats, power windows, engine cooling fans, and power tops. These are the applications of the relay. This is what the electric horn circuit diagram, in which the horn is there, A and B, these are the circuit breakers. There is a horn relay in between that consists of the circuit breaker. The horn button is there, the current is coming from the battery and the ignition switch is there. When you start your ignition switch, that time the current flows from battery towards the horn relay and when you push the button that time with the help of circuit breaker, the horn is going to ring. So likewise, the horn circuit is designed. Now how it works? The battery voltage travels through a high current wire that is the red wire that you have seen in the circuit diagram, through the relay to the horn and also through a smaller wire that is a blue one through the ignition switch to the relays, low current coil. The first thing you should be aware of is that the horn circuit is always hot or live when the ignition switch is turned on and all that is needed is a path to ground. That path is completed when you push in the horn button. When the button is pushed, the ground connection is made. Energizing the relay coil, A, the coil iron curve in this particular design pulls down arm, connecting high current contacts B. High current then flows from the battery to the horn. The horn is connected to the ground because it is mounted to the chassis of the car. See how it works? Actually one thing is missing from this circuit, there has to be a fuse somewhere in this circuit. The high current wire from the battery might go through an appropriate fuse on the fuse panel or there might be an inline fuse near the horns. It depends upon the production engineering decisions as to the most economical placement. But your schematic drawing will show its location. So what is this particular fuse, automotive fuses? Almost everything in the car is wired through a fuse. Fuses are designed to fail when too much current is drawn through the device. This prevents heating of the wires and subsequent melting of the insulation followed usually by fire. Fuses are simple in design. Inside a fuse, a short wire with a specific cross-sectional thickness is there. This dimension dictates how many ampere can be carried before the wire melts. Too many amperes, current, the fuse fails, serving the rest of the circuit from the damage. Most of any car's fuses are located in fuse panel but some are in line. Inline fuses are found under the dash and in the engine compartment. Next thing is fuel gauges. Gages are usually more expensive than indicating lights. Gages indicate the values of important parameters required to be known by the driver for smooth, trouble-free and safe operation of the vehicle. Such as a remaining fuel exit. The gauges have the different designs and are mostly electrically operated. In one type, a coil-off resistance wire is wound around a bimetal arm. The movable end of the arm is connected by a linkage to an indicator needle which is shown the value against a scale panel placed behind the needle. Here what? The schematic diagram of thermostatic coil type of fuel gauge is there. This is one of the type of the fuel gauge in which there is a bimetal arm and the hydro coil is there which will turn the needle. Ignition switch is there and there is a fuse. Whenever high current flows through it then definitely the fuse is going to melt and the current will not be passed toward the coil. And there is a sending unit is there so which sends extra currents to the earth. So how it works? A series coil carries full instrument current. The current then splits part going to a variable resistance in the sending unit and the rest going to the shunt coil. Higher resistance in the sending unit increases the shunt magnet strength and a low resistance weakens the magnetic strength. Current flowing through the sending unit changes as its resistance changes. The indicator needle is deflected by the difference in the magnetic strength of the sensing and shunt coils. Next type of fuel gauge is the balanced coil type. Here in this particular central figure that you can see there is a magnet between the two coils and the magnet is connected to the needle. So here also there is a sending unit and ignition switch terminal is there and in between there is a fuse. So at left upper figure if you see there is a fuel tank and there is a float. So whenever the level goes down that time you will see in your fuel indicator that it is showing the fuel is at the lower level with the help of this particular balanced coil. And at the right side if you see there is a variable resistor is there with the help of that variable resistor what it does it increases the resistance and it sends the current towards an indicator and indicator shows a full reading when the fuel tank is filled with the fuel. So whenever the resistance decreases that time current increases and wherever the resistance increases that time the current decreases. So with this phenomena the indicator is showing the level of the fuel. So we will see again how it works exactly. The sender unit in these cases is the fuel tank which connects the instrument electrical system to ground. Thereby directly connects to the negative side of the battery to complete the circuit. In the fuel gauge sending unit a variable resistance reward stat is connected to the hinge tank float. When the fuel level is high the gauge circuit connects directly to ground allowing fuel current to flow through the instrument coil which moves the instrument needle to the top of the scale. As the fuel is used the float drops this causes the instrument circuit current to flow through part of the variable resistance adding resistance reducing the current flow on thermal instrument. This reduces the instrument coil temperature so the gauge reading lowers. In both the cases gauge reading is proportional to the tank fuel level. The instrument gauge mechanism is sensitive to the current flow therefore it is important to have a constant voltage source for attaining accuracy. In these instruments generally supply is made from the vibrating point voltage regulator which may be a separate unit or part of fuel gauge level that keeps the voltage at a constant value usually that is 5 volt. These are the references. Thank you.