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Physics & Astronomy
Project #2 - Flashing LED

Project #2

Flashing LED - Tone Oscillator

Here is a neat and simple two-transisitor oscillator project with a number of applications.


List of Materials


Tip #2

Real expermenters never throw anything away. Salvage some scrap telephone cables. They are great for hookup wire.


Tone Oscillator

Tone Generator


project002A circuit


Let's start with the tone oscillator. Try and follow the schematic and assemble the circuit on your prototyping board. Check the drawing below and make sure you wire the two transistors correctly. Failure to do so can result in blown transistors.



This circuit works from a single 1.5V battery and emits a tone at about 1400Hz. You can use this simple tone generator for morse code practice. I made this into a multipurpose project. With transistor sockets to hold the two transistors, I can use it to test batteries, check NPN and PNP transistors and practice my code sending all in one unit.


Morse code key


Time Constant

Capacitor C1 charges up through resistor R1 and is discharged when the voltage reaches a given threshold after a certain time. At this point Q1 and Q2 both turn on and C1 is discharged to begin a new cycle. The classic formula for time constant (tau) is given as t = R x C. For this example we approximate the operation of the circuit using t = 0.7RC. Hence for the values for R1 and C1 shown, t = 0.7 x 100k x 0.01µF = 700 µs. This is only an approximation and the time constant will be different for different battery voltages.

Now replace the 0.01µF capacitor at C1 with 10µF. Calculate the new time constant and you should arrive at about 0.7s. You will hear a clicking sound coming from the loud speaker every 700ms.


Flashing LED


Flashing LED


The single 1.5V battery is not enough to turn on the LED. Replace the battery with two 1.5V batteries. Replace the loud speaker with an LED and add the 47-ohm resistor as shown in the revised schematic.


project 002B circuit


You now have a simple flashing LED circuit.



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