EGR214 Laboratory Activities 1School of EngineeringGrand Valley State UniversityEGR 214 – Laboratory #8The One-Shot Door LockSWS Technical Paper #2Objectives• To design and implement a timed-pulse solenoid actuatorPre-Lab Assignment1. Read through the entire laboratory proce dure.Pre-Lab DeliverablesThese deliverables are due at the beginning of your laboratory period. Your instructor will verifythese deliverables as you enter the labor atory and will use them to construct your laborato ry grade.1. None for this lab.IntroductionAs suggested in a previous labo ratory, a one-shot pulse generator can be used for disengaging a lo ck on a doorwhen an access car d is held up to a card reader. The actual lock mechanism is a solenoid, an electromagnetthat can pull on a movable rod (also called an armature or slug) when it is engaged. The basic principle isas follows:1. An electromagnet is formed by wrapping several turns of wire around a core. The core can be ironfor greatest magnetic field strength, or just left e mpty (i.e., an air core) if the electro magnet is to beformed around another object. An example of an air-core electromagnet is shown in Figure 1.Figure 1: Air-core electromagnet [1]Copyrightc 2010 Padnos College of Engineering & ComputingEGR214 Laboratory Activities 22. When current is passed through the coil, a magnet is created. The magnetic lines of flux circulate fromthe north pole to the south pole and pass throug h the core of the coil. Figure 2 s hows a cross sectionof an energized co il and the resulting magnetic lines of flux.lFigure 2: Wire wound in a coil around a core creates magnetic lines of flux inside the coil (shown withcirculating lines with arrows) [2]. The figure shows the cross-section of a 7-turn coil. Circles with dots inthe middle represent a cross-section of the coil wire in w hich the current is flowing out of the page, towardsthe reader. Circle s with crosses in the middle represent a cross-section of the coil wire in which the currentis flowing into the page, away from the reader. The dimension l represents the total length of the coil.3. The magnet now created by the coil is capable of mag netizing the solenoid’s movable armature, whichmust be made of ferromagnetic material such as iron, and turning it into a temporary magnet. Thismagnetized armatur e is now drawn towards the center of the coil as shown in Figure 3. If the curr e ntin the coil is left on indefinitely, the armature will come to rest in the center of the coil. In this way,the s olenoid is actuated.Figure 3: The magnetic field created by an energized coil can tempor arily magnetize a ferromagnetic objectso that it behaves as a magnet a nd is attracted towards the center of the coil [3].Copyrightc 2010 Padnos College of Engineering & ComputingEGR214 Laboratory Activities 3Rather than rely ing on a tempora ry magnetization of the armature, an even stronger solenoid can beconstructed using a permanent-magnet armature.4. When the current in the coil is turned off, a spring mechanically pulls the armature back to its originalposition.In this lab you will construct an electro magnet that is timed to generate a 3-second pulse to simulate thedoor lock mechanism described above.AdviceAs you work through this (and subsequent) labo ratories, SHOW YOUR RESULTS TO YOUR INSTRUC-TOR AS YOU OBTAIN THEM. Your instructor will be able to guide you towards the “correct” (or expected)results, thus saving you from having to re-do large portions of the laboratory.Part I – One-Shot Pulse Generator (estimated time: 45 minutes)The first task is to construct a one-shot pulse generator capable of generating a 3-sec ond 10V pulse. Thecircuit of Laboratory #5 can be used with modifications for a 3-second pulse duration and to allow the pulseoutput to reach 10V.Build this cir c uit and ensure that it works prior to proceeding to the next step. Keep good notes on yourdesign pr oc e dure to help you in writing your pap e r.• Include supporting derivations, schematic, procedural steps, and o scilloscope traces to justify yourdesign.Part II – Coil Characterization (estimated time: 45 minutes)Obtain an electromagnet from your laboratory instructor. Your task is to characterize this electr omagnet’sinductance and resistance. Resistance can be measured directly with a DMM. Inductance should be measuredaccording to the technique of Laborato ry #6.Ensure you document your procedure and observations carefuly to help you in writing your paper.• Include supporting schematics, procedural steps, and oscilloscope traces to justify your estimate o f theelectromagnet’s inductance and resistance.Part III – Coil Driver (estimated time: 90 minutes)The output current of the op-a mp will not be nearly high enough to e nergize an electromagnet. While theop-amp is c apable of so urcing 1-2mA comfortably, your electromagnet will demand approximately 100 timesas much current.We can use a MOSFET as a current amplifier by allowing it to switch the electromagnet current on andoff under control of the one-shot circuit. Consider the circuit shown in Figure 4. A pulse from the one-shotcircuit is used to turn a MOSFET on, and this MOSFET allows current to flow through the electromagnetrepresented by the inductance L.Some additional design notes:• The MOSFET should not be a 2N7000 as it is not designed for the 100mA-200mA of current we’regoing to need. Use an IRF540 or comparable power MOSFET (e.g., IRF520, IRL2703, NDP4060L ,NDP6060L). All of these devices have the same pinout, shown below for reference:Copyrightc 2010 Padnos College of Engineering & ComputingEGR214 Laboratory Activities 410V+-+-IRF540or similarGSDL RlRg10V0V1N4002igilvlFigure 4: This circuit can be us ed to amplify the low-current output pulses of the one-shot generator tohigh-current pulses in a n electromagnet (represented by the inductor L).• The 1N4002 diode is not critical and you can use a 1N4001, 1N4004, etc. Can you explain why thediode needs to be there?• The resistor Rlis intended to limit the curr e nt in the coil to the 100mA-200mA rang e . This resistoracts in s e ries with the resistance of the electromagnet coil itself so the total resistance in steady state isRcoil+Rland the current in steady state will be I = 10V/ (Rcoil+ Rl). Also take the powe r dissipationof res istor Rlinto acc ount. In steady state, w ill a standard 1/4W resistor be acceptable?• The resistor Rgis designed to limit the current leaving the one-shot