Electronic Basics #26: 555 Timer IC


Nowadays, there exists dozens of integrated circuits a.k.a. ICs in order to make our life easier while building electrical circuits, but there’s one which is most widely used. The so called 555 timer IC and in this video, I will show you what makes this IC tick and what kind of different functions that fills in our circuits by attaching external components. Let’s get started! First off, let’s inspect the insides of the dual inline package of the 555 timer IC. Pin 1 & 8 are connected through three 5 Kiloohm resistors in series. Which does not only give this iconic IC its name, but also creates a voltage divider since pin 8 is connected to the supply voltage and pin 1 the ground. Pin 2 is the Trigger pin which directly connects to the negative input of a comparator. The positive input of this comparator connects to the voltage divider which has a potential of one third of the supply voltage at this point and the output of the comparator is connected to the set pin of the integrated flip-flop. Pin 3 is the output which is connected to an output driver and subsequently to the outputs of the flip-flop pin and force the reset pin which directly connects to the reset pin of the flip-flop and can by connecting it to ground, reset the flip-flop instantly. That is why they are usually tied to the supply voltage. Pin 5 is the control voltage which not only connects to the negative input of the second comparator, but also to the voltage dividers to its potential of the supply voltage. You could either supply voltage to this pin in order to change the reference voltages for the comparators or you could simply hook up a 10 nanofarad capacitor in order to stabilize the reference voltages, which is definitely more common in 555 timer circuits. Pin 6 is the threshold pin which connects to the positive input of the second comparator whose output connects to the reset pin of the flip-flop. Last but not least, we got the discharge pin which directly connects to the collector of a bipolar junction transistor. Its emitter connects to ground and base connects to the output of the flip-flop and That basically makes up a 555 timer, but in order to understand it a bit better, let’s have a look at a practical example circuits to be more specific, a monostable multivibrator configuration. In the beginning, the capacitor is discharged. So there’s a zero volt potential at pin 6 and 7. Pin 6 provides this ground potential for the positive input of the second comparator Since the positive input is lower than the negative one, the output stays low and nothing happens. What we need is a push of a button at pin 2 which does connects it to ground. This way, the positive input of the first comparator has a higher voltage than a negative one and its output becomes high. Because this output directly connects to the set pin of the flip-flop, its negated output turns on as well which simultaneously activates the output of the IC in terms of the bipolar transistor and now that pin 7 no longer connects the capacitor to ground it can charge up through the resistor but once the voltage of the capacitor reaches 2/3 of the supply voltage, the positive input of the second comparator has a higher voltage than a negative one and thus it turns its output on. This resets the flip-flop turns off the output of the IC and activates the bipolar transistor which now discharges the capacitor rapidly and we are back at the beginning. This configuration is called monostable because only the low output is stable. The high output is just temporarily and depends on the dimensions of the used resistor and capacitor. A popular application of the circuits is creating a delay for notification lights or something similar. The next basic circuits is the bistable multivibrator which can as the name already suggests create 2 stable output States high and low. The circuit does not require any RC components and only utilizes the trigger pin 2 as the set pin and 3 set pin as obviously the reset pin. This way by applying a ground potential to either one of them, the integrated flip-flop is ever activated or deactivated and thus creates two stable states. Perfect for turning your led on and off manually with push buttons. The last basic configuration is the a stable multivibrator which is basically an oscillator that can create a rectangle wave variable on and off time and even adjustable frequency. In this example the capacitor charges up through the resistor 1 and 2 and discharges only through the resistor 2. This way, the on time of the output signal is dependent on the resistor value 1 and 2, but the off time is only determined by the resistor 2. To improve this design, you usually see two additional diodes in such a circuits in order to direct the charging current only through R1 and the discharge current only through R2. But unfortunately this design cannot create a decent PWM signal with a constant frequency. To achieve that, we would need to connect the charging and discharging resistance to the output pin Aka pin 3 of the IC by doing so the charging and discharging Should happen in the same time and thus create a 50% duty cycle with adjustable frequency which was definitely not the case. The problem is the NE555 itself. Because it uses bipolar transistors to switch its output on and off while the off voltage is close to ground the on voltage is around 4.6 volts at a supply voltage of 5 volts which creates an unsymmetrical charging behavior. The solution is to use a CMOS 555 timer like the TLC555. This version can achieve an output voltage very close to the supply voltage through the use of MOSFETs as the output stage and thus can create a 50% duty cycle with ease. But not only that, it also consumes less current, can achieve higher frequencies, and work with lower supply voltages. But it cannot output as much current as its TTL counterpart. Now in order to adjust the duty cycle, we simply need to replace the resistor with two diodes and a potentiometer Which separates and adjust the charging and discharging current and thus finally creates a beautiful PWM signal with constant frequency. For controlling your power electronics and book that being said, you already know all the basics of a 555 timer IC I hope you liked this video if so don’t forget to like share and subscribe. Stay creative and I will see you next time.

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