The 555 timer can be used for more than just timing circuits. The 8-pin IC is extremely versatile because it has built in two comparators, a three-resistor voltage divider, and an SR (set/reset) flip-flop. This allows for the 555 timer to be used as a Schmitt trigger in the right wiring configuration. The reason the 555 timer can thus produce a DC square wave from a triggering charging/discharging cycle of a capacitor is why it is also referred to as a Schmitt inverter. The rising and falling input of the saw-tooth waveform produced by the capacitor triggers the output alternate between max voltage and min voltage. This produces a steady “high” and “low” digital sequence that can be seen on the oscilloscope in the video below. The LM556 and LM558 are really just the dual and quad versions of the LM555 respectively and help consolidate your schematic layout when multiple timers are needed.
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Because we change the time constant of the circuits with the potentiometer by varying the resistance, we are able to adjust the frequency of the 555 timer circuits to increase and decrease the frequency of the output. The output is represented by a blinking LED and the digital square wave on the oscilloscope.
555 Timers as Schmitt Triggers for PWM Circuits
Because DC motors (also referred to as digital motors) only have the ability to be either on or off, the matter of variable speed becomes tricky unless a proper pulse width modulation (PWM) circuit is used. The 555 timer can be used in these circuits for this purpose very easily. Because you need to be able to vary your duty cycle to change the speed of a DC motor, the LM555 works well for this because of its built in Schmitt Trigger circuit. The duty cycle is what changes the amount of voltage the DC motor “perceives” and thus alters its angular velocity. Pulse width modulation is really a way to trick the motor into thinking it is receiving a voltage in between the “high” and “low” voltages. If you hook a motor directly to a 12V battery, you can have the motor turned off completely or spin at top speed only. Varying the pulses of 12V and 0V very rapidly (with a certain PWM frequency) is what “tricks” the motor into thinking it has a certain voltage across it in between the 12V and 0V values. The duty cycle is the ratio of “on” vs. “off” or “high” vs. “low” period for each cycle.
In order to use the LM555 timer for PWM circuits, we need to take advantage of its Schmitt trigger capabilities as a function generator. We need the LM555 to output a digital sequence of highs and lows as seen on the oscilloscope, but we need to be able to vary the duty cycle instead of the frequency. This requires different types of 555 timer circuits where we utilize the Schmitt trigger as an oscillator where we can vary the percentage of time the output is high vs. when it is low.
The schematic below illustrates how we can utilize its properties to control the speed of a DC motor with the Schmitt trigger generating a PWM signal.
If we have a 10% duty cycle (or ~10% the speed of the motor) and we have a 1 KHz PWM frequency, the motor will receive a 12V voltage across is for 10% of the time at 1000 times per second. The other 90% of the time, the motor is off and receives no voltage across it, hence the motor is spinning quite slow, but you can begin to see how we can now alter the speed of the motor to whatever works for our application. The ability to vary the speed of DC motors is critically important in the field of robotics, but it has its advantages in a myriad of applications. There are complete motor driver chips, such as the L298N (buy them here), that are much better and more specialized for this purpose than the LM555 chip because of built-in diode over-current and back EMF protection, PWM relay, and driving two motors forwards and backwards per L298N chip. However, using the 555 timer for this circuits application definitely helps with the understanding of how PWM and signal generation works.
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