Circuit Tutorials

The “Varying Brightness LED”Kit fromProf. ETK brings you a circuit with which to simulate a lighthouse effect. This is achieved by causing a Led to slowly change its light output from “off” through to full brightness, much like the effect created by the light from the rotating light house search light.

The circuit is a relatively simple circuit for those who are familiar with operational amplifier (OP AMP) operation. For those who are unfamiliar with Op Amps, a little introduction is provided here.

Following are the basics to familiarise you with the basic OP AMP.

Prof. ETK has used the popular 741 Op Amp as an example and shown it in its DIL Package as well as the schematic symbol for it and all Op Amps.

                               

Besides the power supply terminals, all Single Package Op Amps have two input terminals and one output terminal. The inputs are named Inverting (Inv , –ve) and Non Inverting (Ninv , +ve). They also have 2 terminals, called “offset null” which is used to adjust the output to zero where this may be required. Our circuit does not require this adjustment.

The OP AMP used in this kit is a dual OP AMP, meaning that the 8 PIN Package has two complete OP AMPS within the one package. The PIN Numbering of the LM358 can be found by referring to the schematic diagram of the circuit.

The – and +signs do not indicate polarity, since it is the relative value of the voltages, when compared to one another. Both terminals may be connected to either polarity at any time.

Op Amps are usually operated from a dual power supply, which means the output voltage can swing above or below ground. They can be powered from a single ended power supply, but then the output voltage will only swing between +ve and ground (0V).

When used as a Comparator, the Op Amp can be thought of as working like a scale. Should the –ve side be slightly more than the +ve side, the scale will tip towards the –ve side and vice versa.

When –ve feedback is employed, the Op Amp tries to keep the differential between its inputs equal to zero, thus providing the means for us to control the gain of the device by inserting appropriate components in the feedback path.

Due to this behaviour, the input impedance of the device now changes to the value of the resistance in its input circuit and its output impedance becomes very low.

The OP AMP, because of its high input impedance, is an ideal electronics-integrating element. A basic INVERTING OP AMP INTEGRATOR circuit powered from a dual ±15v source is shown here. The + input is tied to 0v which is used as the reference. Looking at the behaviour of this circuit will help you to understand the integration process, which is used to provide the ramp voltage for the varying light output from the LED. From the graph you can see how the output voltage ramps around the 0v line

The schematic circuit diagram of the light beacon is shown below.

Circuit Operation

R1 and R2 form a voltage divider, the voltage across R1 being 2.2 times that of R1 and conversely the voltage across R2 being 2.8125V wrt the negative of the battery. This is calculated by VR2 = Vb (R2 / R1 + R2) = 9 ( 100 / 320) = 9 x 3125 = 2.8125V

And VR1 = Vb (R1 / R1 + R2) = 9 ( 220 / 320) = 9 x .6875 = 6.1875V

The potential on R2 is therefor –ve (lower) wrt the potential on R1.

The potential on R2 is connected to Pin 3 of IC 1a as well as Pin 6 of IC 2a.

IC1a, C1, VR1 and R5 form an inverting integrator.

IC1b, R4 and R6 form a comparator with hysteresis.

VR1 adjusts the integrating input current, which in turn affects the speed of the flashing.

 

Referring to the schematic diagram above in conjunction with the output waveforms from the two OP AMP outputs, you can deduce the following.

The output from IC1a is a ramp voltage that rises and falls in the shape of a saw tooth waveform. It is easy to see that as the output from IC1a is rising towards its maximum value, the LED will increase in brightness to its maximum. Thereafter the voltage decreases, causing the LED to fade to minimum brightness.

The saw tooth waveform from IC1a is applied to IC1b via the voltage divider formed by R4 and R6. The output of IC1b will switch from off to on and on to off as the voltage at Pin5 increases beyond the value of Pin6 and below the value of Pin6 respectively.

The rate at which this happens is determined by the time constant set by C1, R5 and VR1.

Construction

Construction of the unit is relatively easy. Insert all the components carefully as per the component overlay shown below. Insert only the IC socket at first. Do not insert the IC at this stage. The LED is usually mounted off the PC board by connecting the two leads of the LED to the hookup wire supplied with your kit. Ensure that your polarity connections are right, otherwise the LED will not light. See figure below.

 

 

Once you have correctly inserted all the components as per the component overlay above, carefully solder all the component leads to the PCB pads.

Use a thin solder gauge, preferably .9mm. Other gauges which are acceptable are .7mm, 1.00 and 1.2mm. Use only Rosin core solder. Do not use plumbers solder or any solder that is acid based. Check that all the solder joints are sharp and shiny and that no solder shorts are waiting to create trouble.

Now, without inserting the LM 358 IC into the IC socket, connect a 9V battery or power supply to the circuit. Watch for any telltale signs of component overheating or other distress signals. Once you are sure that all is well, disconnect the battery. With the battery disconnected, insert the IC with the correct PIN orientation. It may be necessary for you to align the IC Pins gently with a tweezers or long nose pliers. Correct any mis-aligned pins so that the whole IC will fit neatly into the DIL IC socket.

Press down firmly on the IC once it is in the socket to ensure good seating and contact with all the pins of the IC.

You are ready to reconnect the battery.

Once the battery is reconnected, your LED will light up brightly and then slowly reduce its light output until it is dark. From this dark state the LED light output will increase steadily until it is once again bright. The cycle will repeat itself this way on a continuous basis. You can adjust the time of the cycle by turning preset VR1 to suit your liking.

Install the complete kit into your Lighthouse or other model and watch how realistically your finished kit simulates a real lighthouse beacon.

 
   

Have fun and continue to explore the wonderful world of electronics with Prof ETK’s practical electronics training kits.

Prof ETK Electronic Kits provide you with the best in electronics education and hands on training plus the enjoyment of a successful project.

Should you wish to enquire about kits, which are not currently in our range or not shown in our catalogue, please email us.

 
 

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