Saturday, December 1, 2012

555 Timer Setup

Recently I set up a 555 timer and three AAA batteries to experiment with. The first thing I wanted to do was flash an LED. I wrote a program that will let me figure out the frequency of the flash, size of the capacitor, or the values of the resistors. I decided on a 10uF capacitor and a 100k resistor to give me a blink slow enough to see. This was interesting but then I decided to put a potientiometer in place of R2 so that I could control the rate of the blink. I also put a potentiometer in place of the resistor to the LED so that I could control the brightness. Tuning the potentiometer so that the LED wasn't very dim or very bright was difficult. After doing some reading I saw that it is possible to adjust the on and off times independently of each other if you use a diode with R2. I replaced R1 with a potentiometer so I could control both the length of the flash and the pause between flashes.

Next I connected a piezo speaker to the timer. It ticks every time the current to it is switched on or off. According to the datasheet, the current doesn't have to be turned off but should actually be reversed. I am not sure how to do this with less than four transistors. By using the potentiometers to make the timer pulse quickly, the ticks of the speaker come fast enough to blend into a tone. I tried using smaller capacitors from the TV I took apart with the timer and the tone was much higher. I put another potentiometer in place of the resistor to the speaker to control the volume. One thing I noticed is that the speaker made a growling sound when I tried to turn the volume up too high. It seems that the timer was failing and restarting many times a second because the circuit was drawing more than the 200mA the timer can source. To remedy this I put a resistor in series with the volume potentiometer. This is also a good idea for the other potentiometers so that the timer and LED don't get damaged either.

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