Introduction: Turn Signal Using 555 Timer

A 555 timer is a very popular integrated circuit (chip) that can be used to provide time delays (timer).  The chip contains many transistors, diodes and resistors, including three 5k resistors which is how it got its name.

Programmable microcontrollers like the Arduino and the Propeller are very useful tools, but if all you need is to blink some LEDs then those programmable microcontrollers are excessive.  You can use a 555 time to blink LEDs and at a fraction of the cost.

Our local makerspace the “Rabbit Hole” had a Jameco build night where we were building electronic projects with parts from Jameco.  They included some 555 timers so I decided to make a turn signal that blinks the LEDs in a cascading style.

Here is the list of things you will need to build the turn signal:
  • 1 x half size solderless breadboard
  • 1 x 555 timer 8 pin chip
  • 1 x momentary push button switch
  • 2 x 100 uF 10v capacitors
  • 1 x 9 volt battery adapter
  • 1 x 9 volt battery
  • 1 x length of 22 AWG wire to make connections
  • 1 x wire cutter to cut and strip the wire
  • 5 x 330 Ohm resistors
  • 5 x 10k Ohm resistors
  • 2 x 4.7k Ohm resistors
  • 2 x 1N4148 diodes
  • 1 x 2k Ohm resistor
  • 5 x LEDs
  • 5 x BC547 transistors

I used a half size solderless breadboard from Jameco.  Solderless breadboards allow the ability to prototype circuit designs without soldering the parts to a printed circuit board (PCB).  Breadboards have two columns of holes on each side that are connected to each other (rails) that are often used for positive and ground.  On the interior of the breadboard there are two rows of 5 holes that are connected to each other.  This makes it easy to connect multiple things to one pin of an integrated circuit.

When you are done building you will have a circuit which lights the LEDs in succession until all 5 are illuminated.  Check out the video for more details.

This instructable submitted by the Rabbit-Hole Maker Space as part of the Instructables Sponsorship Program.

Step 1: Make the Basic Connections

Place the 555 timer chip in the middle of the breadboard.  The notch in the chip indicates the top of the chip and the left side is pin 1.  If you place the chip at the top of the breadboard the pins on the left side are numbered the same as the chip.

Pins of the 555 timer:
1 = GND = ground
2 = TRIG = trigger
3 = OUT = output
4 = RESET = reset
5 = CTRL = control
6 = THR = threshold
7 = DIS = discharge
8 = Vcc = positive supply voltage

To learn more about the 555 integrated circuit go to the wiki page on 555 timer IC
There is a good pin out diagram with colored pins that you can use as a reference.  Alternately you can search for the 555 datasheet.  The datasheet (normally in PDF format) contains lots of technical details about the 555 timer.

Make the basic 555 timer connections:

Wire pin 1 (GND) of the chip to ground.
Wire pin 2 (TRIG) of the chip to pin 6 (threshhold).
Wire pin 4 (OUT) of the chip to pin 8 (Vcc).
Wire pin 8 (Vcc) of the chip to power.

Put one 4.7k Ohm resister from pin 8 to pin 7 (DIS) on the chip.
Put one 4.7k Ohm resister from pin 7 to pin 6 (THR) on the chip.
Put one capacitor from pin 1 (GND) to pin 2 (TRIG) on the chip.  Please note capacitors have one electrode that must be connected to ground.  This is indicated by the negative sign on the capacitor. 

The steps up to this point are a basis for many common 555 timer circuits (Astable operating mode).  Only a few more connections and you could make a simple blinky 555 timer circuit.  The 555 chip can be use for many different applications including Pulse Width Modulation (PWM) and tone generation.  As you learn more about the chip be sure to check out the Monostable, Bistable, and Astable modes of the 555 timer.

The next steps are exclusively for the turn signal circuit.

Step 2: Wire Up the Turn Signal Circuit

Now to connect the rest of the components for the turn signal.

Connect the diode from pin 3 of the chip to row 10, the ring around the diode should be closer to the pin 3 row than row 10.  Connect the 2k resistor from pin 3 of the chip to an empty row 10.  The holes in row 10 are starting to fill up, so connect a wire from row 10 to row 13.  This will give you 4 new holes in the breadboard that are connected to row 10.

The transistors must bridge the middle of the breadboard to do this bend the collector leg back, bend the base and emitter legs forward.  Check the specification sheet on the part you have to find the correct legs. 

Put the first transistors in the middle of an empty row on the breadboard, with the collector in a hole on right side, and the base and emitter in holes on the left side.  Insert the next transistor where the emitter is in the same hole as the base from the first transistor.  This may take some work but both legs must be connected for the circuit to work.  Be careful there are no shorts (legs touching where they should not) on the transistors.  Repeat the process for the remaining transistors until they all have the emitter touching the base of the previous transistor.

Connect the LED of the cathode (the short end of the LED) to the same row of each transistor on the right side of the breadboard.  The anode can go one row down or up so they are not connected to each other.  Repeat for each LED until they are all connected.

Connect the 330 resistor from the positive rail to the anode row of the first LED. Repeat for the remainder of the 330 resistors until all the LEDs are connected to the power rail.

Connect the diode from the negative rail to the same row of the emitter of the first transistor (row 15 in the diagram).  The colored band around a diode should be closer to ground than the transistor.

Connect the 10k resistor from row 13 to the base of the first transistor.  Repeat until all five 10k resistors are connected to the base of the transistors. There are only four holes left on row 14 so double up on one hole.

At this point the circuit will work if power is applied, by adding a simple switch to interrupt the power the circuit can be activated only when needed.

Step 3: Wire the Power and Switch

Wire the black wire of the 9 volt adapter to the negative power rail on the breadboard. 

Place a momentary switch on the middle of the breadboard near the bottom.  Place the red wire in a hole on the same row as the bottom of the switch. 

Connect a wire from the top row of the switch to the positive power rail of the breadboard. 

Connect the 9 volt battery to the adapter.  When the switch is activated the power is connected, energizing the circuit!

Your circuit should light up each of the LEDs in succession then start over. 

If your circuit does not work, check for shorts (wires touching that should not be) in the transistors.  Bending the transistor legs to share holes on the breadboard may cause the other legs to touch, make sure they are separated.  If you are still having problems be sure the breadboard is wired up following the diagram.

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