Calculate the voltage, Electrical Engineering

1. The circuit is:

A circuit which lights a LED when a voltage is above 14 volts and flashes the LED (at about 1 Hz) below 10 Volts. The circuit should also continuously sounds a buzzer when it goes out of this range in either direction. This circuit could tell you when your car battery was flat or when the regulator was causing overcharging

Note that specific values for temperature etc. have not been given so you should choose your own approximate values depending on the circumstances. You should also implement all functions using standard circuits which allow you to control the parameters. That is, don't use a ready-built 'flashing LED' or a buzzer where a speaker and frequency source is specified.

2. Use two 9 volt batteries or two 6 volt batteries to power your circuit unless otherwise specified. The 'small' voltage regulator ICs type 78L05 and 79L05, might be very useful in some cases. If you have one, a power supply with a voltage between 5 and 15 volts will also do the job. The special components specified in the introductory book are suitable for these circuits.Most of the functions can be achieved using integrated circuits. Transistors may be needed to provide current to motors controlled from low current ICs.

3. Choose values for all components needed, purchase or otherwise physically acquire the components.

4. Construct the circuit and get it to work. For this, you will need the equipment listed earlier.

5. Make some specific measurements of its performance and relate these to your intended performance. The purpose of this is to evaluate how well your circuit worked and it is up to you to choose what you measure in order to do this. A digital multimeter should be all you need. See the ideas under 'Evaluation of Circuit Performance'.

6. Do a theoretical calculation of the voltages at the important points in your circuit which can be so calculated, and for all conditions of inputs. Also determine from theory if the currents needed by inputs can be safely supplied by the drive circuits.

7. Prepare a report for assessment which must include (for each case) the following details:

  • The title of your chosen function.
  • A neat sketch of the circuit showing all components and values and approximately following the layout conventions used in the study material.
  • A timing diagram (where appropriate).
  • A table which might look like this:

Position in Circuit

(test point)

Theoretical Value

(volts or mA)

Measured Value










  • Results of any other performance measurements.
  • A photo (.jpg) of your circuit under test, with enough detail to show some component values and measured readings. Photo(s) should not exceed 1 Mbyte.
  • A concluding statement (less than a page) that summarizes your experiences while building the project, plus your conclusions about the usefulness and performance of the final circuit.
  • Include an appendix showing calculations to justify choice of component values.

Evaluation of circuit performance

It is not enough to simply get a circuit to work. We need to be sure that it is a good quality circuit, likely to keep working for years, able to work even if we replace a component etc. There are lots of details which need to be checked.

Some examples are:

  • Measure the voltages at certain key points, for a variety of conditions, high and low logic, peak or trough of a waveform, etc.Such measurements may indicate that voltages are outside the normal ranges allowed. An example is if a low voltage from a TTL gate is 0.6 volts when it should be <0.4 V.
  • Vary the current being drawn from a circuit where varying current is expected. This test may show that the circuit only works for a limited range of loads. This test would apply to a voltage regulator.
  • Calculate the current needed to drive a following circuit and check that it does not exceed that specified for the output of the previous circuit. For example, an op amp is not normally designed to put out 10's of mA of current and so is not suitable to operate LED at (say) 15 mA.
  • Change the way you operate the circuit to see if the sequence of events changes the way the circuit performs. For example, a monostable may operate correctly provided you trigger it once and return the trigger voltage to what it was before triggering. If the voltage on the trigger is not returned quickly, the circuit may not work as expected.


Posted Date: 2/21/2013 4:38:39 AM | Location : United States

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