Hi! I make this instructables for those persons who are beginning in the electronics world. In this instructable you are going to know the operation of the 555 timer in astable configuration, and the aplication of this circuit to create a very easy semaphore of one way light. Have fun!

Step 1: What Is the 555 Timer?

The 555 timer IC is an integrated circuit used in a variety of timer, pulse generation, and oscillator applications. The most common function is to produce timing pulses with high accuracy, it also can operate as an oscillator.


In the top image, you can see the components of 555 timer.

The comparators are devices whose outputs are at HIGH level when the voltage at the positive input (+) is greater than the voltage at the negative (-), and are at LOW level when the negative input voltage is greater than the positive input voltage. The voltage divider formed by three 5kW resistor provides a trigger level of 1/3 V of DC and a threshold level 2/3 Vof DC. The input control voltage (pin 5) will be used to adjust the levels of threshold and trigger to other values when is necessary. When the trigger input, normally HIGH, momentarily drops below 1/3 VDC, the comparator output (connected to pin 2) in LOW level switch to HIGH SET state and sets the SR latch in HIGHT, causing the output (pin 3) passes and blocking the discharge transistor. The output will remain HIGH until the threshold voltage, normally LOW VCC exceeds 2/3V; then the output of comparator A(pin 6 and 5),in LOW level switch to HIGH. This causes the latch pass to RESET state, when the output is reset to LOW level, the discharge transitor active.The reset input (RESET) can be used outside to set the latch to zero, regardless of the threshold circuit. The threshold and trigger inputs (pins 2 and 6) are controlled by external components to set the mode of operation in monostable or astable.

Step 2: ​*Astable Mode*

In this case we are going to study the astable mode. In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a specified frequency, Oscillates between two states and this state is also called The input threshold (THRESH) is connected to the trigger input (TRIG). The R1, R2 and C1 form the components External timing network determines the oscillation frequency. The 0.01 uF capacitor C2 is connected to the control input (CONT) serves only to disengage and doesn´t affect the operation of the rest of the circuit; in some cases it can be deleted. I recommend to use a ceramic capacitor connected to pin 5.

The oscillation frequency is given by the following formula: f = (1.44) / ((R1+2*R2)*C)

The hight time (positive pulses) is: tH=0.7(R1 + R2)C1

The low time (pulses low) is: tL = 0,7R2C1

The period (T) is= T = tH + tL or T= 1/f

Work cycle= (R1+R2/R1+2*R2)*100% or Work cycle= (tH+tL) / T

Step 3: Building the Semaphore


  • 1 Protoboard
  • Wire
  • 1 capacitor of 470 uF
  • 1 resistance of 1.6 kΩ
  • 1 resistaance of 10KΩ
  • 9V battery
  • 2 alligator wires
  • 2 leds (one green and the other red)
  • 9V battery


  • 2 twizeers to cut and hold wire.

Step 4: Connecting the 555

Well, first of all, we need to put the 555 on our protoboard. then we connect the 555 in astable mode. The function of each pin is:

  • Pin1=Ground
  • Pin2=Trigger NOTE:This pin is connected to pin 6. Remember this practice uses the 555 in his astable mode.
  • Pin3=Out
  • Pin4=Reset NOTE: This pin works with negative logic, therefore is connected to HIGHT level.
  • Pin5=Control
  • Pin6=Thresh
  • Pin7=Trish
  • Pin8=Vcc

When you have all conected like the top photos , is time to question us about: Why we choose those values of resistance an capacitance? Well, the answer is very simple, that values control the operation of our 555. The time of charge in the capacitor and the resistances control the internal NPN transistor current and FlipFlop operation. You can xperiment changuing the capacitance and/or the resistances and look what happens! Come on!

Note:The connection diagram is in the step 2.

Step 5: High Time Calculation:

Step 6: Low Time Calculation:

Step 7: Period Calculation:

Step 8: Frecuency Calculation:

Step 9: Putting De Logic Gate 7404

Now that we have make the calculus of our circuit, is time to put the Not gate in our proto. As you can see in the top image, we connect Vcc and GND and then connect the output of 555 to one input of our logic gate (in this case I use the pin 1), the output(pin 2) will have a red LED. The performance is simple, when the 555 is Low, the NOT inversor will invert the 0 in 1, and his output will turn in on the red LED, the same time as the duration of the 555 timeLow(tH).

Step 10: Does It Work??

Is time to check if we have made all well. We need to connect the green LED in the output pin of the 555 (pin3), and the red LED in the output pin of the inversor logic gate (pin2), and connect this two elements to ground. Then we need to connect our circuit to the 9 V battery using a pair of alligators or simple wire directly to the battery. Finally we have to see the green LED turn in on for 3.8 seg aprox. and the red LED turn in on for 3.2 seg aprox. If you could make it, CONGRATULATIONS!!! GOOD WORK! ;D


The possible reasons could be:

One or both chips(555 and Inversor gate), has a internal trouble in their connections, repalce it.

Check very well your connections, do you read well the diagram?

Some wire probably is broken, check all very well and replace the necessary.

Your battery probably died soon, check the voltaje output of your battery using a multimeter.

Your resistances and/or capacitors probably doesn´t work, replace them.

Check if you connect your LEDs and/or chips correctly.

Step 11: Conclusion:

We have made a basic semaphore using the operating principles of the 555 chip. We demostrate the use of this chip configuring it in his astable mode. The circuit oscillate in two states: HIGHT and LOW. the time was determinated by the resistances and capacitors used and the math formulates. The pulse must be rectangular and has to oscillate in the two states before mentioned. This project is part of the secuencial logic, in this case the semaphore keep two signals for "x" time using the 555 structural elements, like FlipFlop, and capacitor(Basics elements of memory).

<p>Nicely done. Thanks for sharing this!</p>

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