Posted on Feb 5, 2014

I learned that you could do PWM with the 555 chip. So I tested this idea. I will not attempt to explain the workings of the 555 chip because I found the page by Tony van Roon to be an excellent introduction to the 555 chip. Tony`s page discusses a slightly different setup that what I tested. The key difference is an additional diode.

Click here to download the full size of the above Circuit.

I added this diode so that one resistor is dedicated to the charging the capacitor and the other resistor is dedicated to the discharge process. I added a potentiometer to vary the charging and discharging resistor. Here is my schematic: This circuit uses Pulse Width Modulation to change the LED from dim to bright as the potentiometer is turned. Here is a simulation that closely resembles my circuit. I modified the capacitor so the simulation would run slower. The graphs show the capacitor and the voltage following the LED. When the 555 chip sends a pulse to the transistor, the transistor will allow current to pass from the collector to the emitter. The LED will light and the voltage at the junction of the LED and transistor will drop. When no pulse is sent, the transistor prevents the flow of current through the LED. In this case, we will see a high voltage at the junction of the LED and transistor. In this photo, we see a few more pulses at the bottom. This indicates the transistor is allowing current to pass through the diode in longer durations. The diode appears a little brighter in this photo. In this case, the 555 chip is sending long pulses to the transistor. The transistor is allowing current to pass most of the time. Thus, the voltage at the junction of the transistor and LED is low (most of the time). This results in the near solid line.

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