Aims and Objectives Design a digital stopwatch that displays the time to a resolution of 1 sec on two 7 segment displays showing seconds (00 Þ 59). The unit is to be controlled by three push buttons/switches: START, STOP and TIMER RESET. Approach/Guidelines • Generally, digital clock circuits derive pulses (frequency of 32768 Hz) from a crystal oscillator, which produce timing waveforms of very high stability with minimal drift (tolerance) due to temperature change. However, due to the expense of crystal oscillators a function generator is to be used, which can be set to give a square wave output equivalent to a crystal oscillator.
• A list of available and compatible ICs is supplied. Limrose F S Switch 1Hz Square wave Oscillator 32768Hz 15-stage counter Start/Stop Counter 0 to 9 Counter 0 to5 Figure 1 Block Diagram of Digital Stopwatch • There isn’t a 15-stage counter IC available so you need to build it up from 2 or more separate ICs. • Draw the schematic circuit diagram in block form showing all IC connections. • Explain the operation of the overall circuit Issues to Discuss in Your Logbook • How accurate is the circuit and are any inaccuracies introduced, if so, where are they introduced. Assume the function generator frequency is accurate. • How can the resolution of the stopwatch be increased to display either 1/16th and 1/128th OR 1/10th and 1/100th of a second. • Calculate the cost to produce your design, taking account of the IC costs, power supplies and crystal oscillator etc. Deliverable: Logbook • The log of work should be in an appropriate laboratory exercise book and should be hand written.
• It should include o schematic/circuit diagrams of the final design and subsections where appropriate. o explanation of each IC/subsection operation/functions and thus why each is required. o suitable test results for each system block, which demonstrates the operation. o Livewire or MultiSim schematic, plus simulations. o indicate what problems were encountered and how they were overcome or, if not overcome, what attempts were made to solve them. o any suggestions for improvements that could be made. • Although designs that meet all requirements will receive the highest marks, it is not necessary to achieve this to reach a ‘pass’ level. Marks will be awarded for a systematic methodical approach, sensible investigation into design alternatives, the demonstration of an understanding of the behaviour of circuit elements used, clarity of log book records kept for the design work. • You may work in small groups to discuss ideas, designs and build a prototype but detailed copying of logbook reports and background technical information is forbidden and will be penalised in accordance with the University’s regulations on plagiarism. Guidance: Electronics 1 Logbook Structure 1. Aims and Objectives: Set out in full the aims and objectives of the assignment (usually listed in the assignment brief). Expand if possible 2. Experimental Procedure Discuss how each block/section of the circuit was developed and how it functions, detailing all the relevant circuit diagrams and ICs used (pin-outs and appropriate datasheets). 3. Result Detail the results for each circuit section (this could include typical waveforms) Detailed costing information Relevant data sheet information for all the ICs used in the circuit design. 4. Discussion This is a major section of any logbook/report – Discuss your findings – How is the displayed accuracy increased, as specified in the assignment specification? – How accurate is the circuit (if a crystal oscillator was used i.e. accurate signal source were other inaccuracies are present)? – CLUE: look at the gating of the start signal and the clock source before the 15 stage counter. – Possible circuit refinements – PCB, surface mount etc. – Increased device resolution (1/2 sec, 1/10th etc.) – Possible circuit integration. Briefly discuss the process of integrating all the circuitry on to silicon. – Overall cost, in terms of ICs, to produce the circuit (give full cost breakdown). 5. Conclusions In this section you discuss your findings and conclude the report (did the circuit work and were there any possible improvements that could be performed. – Was the final circuit design efficient? – Could it have been improved (refer to the discussion)? – How could it be adapted for a commercial product? – Did you learn anything, if so, what. Note. DO NOT use personal references in a logbook/report, such as ‘I’, ‘we’ etc. The report should also be written in past tense, such as, ‘ result were taken’ etc. Typical Student Logbook/Report: Discussion Section Version 1 We measured the output voltage with an oscilloscope and after a while we managed to measure the voltage at 3 V. The tutor said that this was not correct and that it should have been 3.5 V. We are not sure why this is the case. Version 2 The output voltage Vout was 3 V for and input voltage Vin of 0.5 V, giving a gain of 6 (Vout/Vin). However the expected value of Vout was 3.5 V to give a gain of 7. The approx. 17 % difference between the expected and achieve results is due to several non-idealities, which are …. Version 3 Results Measured Expected Vin (V) Vout(V) Gain Vout(V) Gain Difference (%) 0.5 3 6 3.5 7 17 Discussion of Results A 17 % difference between the achieved and expected Vout ….. Laboratory Work and Logbooks: General Guidance The competent professional engineer records practical work as it is carried out. Occasionally the results of this work must be presented for the benefit of others. These two activities are expressed by two quite different types of document. The first activity involves the use of a laboratory ‘Log-book’. This is a concise record, made in the laboratory, while work is in progress.