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Let’s start building our circuit by plugging our IC into the breadboard. First, place the breadboard vertically (with the divider strip running up and down). Look at the breadboard and note that the strip separates the breadboard into two halves. There will be pinholes to the left of the divider and pinholes to the right of the dividing strip. The 555 timer IC will straddle the divider. Place the IC so that four pins will be on either side of the center of the board. Orient the 555 timer chip so that the little groove or notch on the surface of the chip faces upwards. Make sure that the pins line up with the holes on the board. You may have to bend the pins slightly to make them line up correctly. Press the IC into the board, being careful not to bend any pins or to stick pins into your fingers. A slight side-to-side rolling motion sometimes helps in plugging DIP chips.
We are now ready to start wiring up the circuit. Remember, the IC chip should be oriented so that the notch is at the top of the chip. Pins are numbered counterclockwise, starting with pin 1 at the top left position of the 555 chip and ending with pin 8 at the top right position. After the IC has been plugged onto the breadboard, connect a red wire from the Positive rail along the edge of the breadboard to pin 8 (+VCC) of the chip, and a black wire from the Negative rail on the edge of the breadboard to pin 1 (GND). Connect the red lead of the battery clip to the rail where you connected the red wire coming from pin 8, and the black lead into the rail where you plugged the black wire coming from pin 1. DO NOT CONNECT A BATTERY TO THE BATTERY CLIP yet!
Connect a 0.1uF capacitor (if you have one) between pins 5 and 1.
Connect one of the capacitors between pins 6 and 1, or, between pins 2 and 1. Make sure that the negative lead (short) of the capacitor is connected to pin 1.
Connect a 10k Ohm resistor between pin 6 and 7 or between pins 2 and 7. Resistors are not polarized (+ or -), so either end can be connected.
Connect a 10k Ohm resistor between pins 7 and 8.
Use a wire to connect pins 4 and 8 to each other (red) and pins 2 and 6 to each other (yellow or other available color).
Attach the positive lead (long) of an LED to pin 3 of the 555 and connect the negative lead (short end) to a third 10KOhm resistor.
Connect the other end of the resistor to pin 1 or ground (GND).
In some cases, you may need to use wires to extend a connection to the appropriate locations. Remember that pin 1 is connected to the negative rail, or ground, or 0V, or –V, and are electrically the same point. Pin 8 and the positive rail are also electrically the same point, and sometimes referred to as Vcc or +V.
Re-examine the circuit to ensure that all components are connected correctly. Failure to connect the circuit correctly may cause damage to some of the components. Ask the instructor or helper to check that the circuit is connected correctly before connecting the battery.
Now, connect the 9V battery to the battery clips and check that the LED begins to blink continuously. Touch the components to make sure nothing is overheating.
Disconnect the battery from the battery clip.
Now, remove the capacitor from the circuit and replace it with the other capacitor. Be sure to check that it is plugged in with the correct polarity.
Re-connect the battery and check that the LED blinks continuously.
Does it blink faster or slower? Why?
Disconnect the battery and remove the capacitor.
Connect the first capacitor back into the circuit.
Re-connect the battery and check that the LED blinks. Disconnect the battery.
Do not remove any other components from the circuit.
Optional Exercise:
By adding another resistor, LED, and Red wire, you can create a circuit that blinks two LEDs alternately, such as located at a railroad crossing.
Do the following:
Disconnect the battery from the battery snaps.
Connect one end of a fourth 10K ohm resistor to Pin 3 of the 555 timer IC.
Connect the other end of the 10K ohm resistor to an unused location on the breadboard.
Connect the negative end (short lead) of the LED to the free end of the resistor.
Connect the positive lead of the LED to the positive 9 volt rail. You may need to use a Red wire to make the connection
Reconnect the 9 volt battery. The two LEDs should blink alternately.
Disconnect the battery.
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