# 555 Timer

## Step 3: 555 Timer: Monostable Mode Circuit

As I explained in the last step, a 555 timer in monostable mode will output a high pulse (of voltage ~Vcc) when the trigger pin in pulsed low.  The duration of this output pulse is dependent on the values of R and C in fig 4.  In the last step we calculated the duration of the pulse output from the 555 in monostable mode to be:

t = 1.1*R*C seconds
where R and C are the resistor and capacitor in series in fig 4.

if we choose R = 10Kohms and C = 470uF
t = 1.1*10000*0.00047
t = 5.17 sec

This means that with a 10Kohm resistor and 470uF capacitor, a pulse low to the 555's trigger pin (pin 2) will cause the output to go high for 5.17 seconds.

I built a circuit which connects the output pin of the 555 to an LED, causing the LED to light up for the duration of the pulse.  This way I would have a visual indication that my calculations were correct.  I connected the trigger pin of the 555 to a push button momentary switch, connecting it to ground when pressed.  Photos of the circuit are shown above, and the schematic is shown in fig 5.

Parts List:
555 timer Digikey LM555CNFS-ND
0.01uF capacitor Digikey 445-5297-ND
470uF capacitor Digikey P5185-ND
(x2) 10Kohm resistor Digikey CF14JT10K0CT-ND
470 ohm resistor Digikey CF14JT470RCT-ND
amber led Digikey C503B-ACN-CW0Y0251-ND
momentary switch Digikey CKN9018-ND
22 gauge jumper wire
5-15V power supply- if you don't have a bench power supply, try using a 9V battery and battery snap or use the 5V output from an Arduino

Wiring Info:
The schematic is shown in fig 5.  Connect power and ground to pins 8 and 1 of the 555 timer (red and black wires).  I used a 9V supply and battery snap for my circuit.  As indicated in the schematic in fig 5, connect a 0.01uF capacitor between pins 5 and 1.  Connect a 440uF capacitor between pins 1 and 6, make sure that the negative lead of the capacitor is connected to pin 1.  Connect pins 6 and 7 with a jumper wire (green).  Connect a 10K resistor between pins 7 and 8.  I left the reset pin floating, you could connect it to Vcc as well.

Connect an LED and current limiting resistor in series from the output of the 555 to ground.  The output pin of the 555 will output Vcc-1.2V maximum (the -1.2V comes from some transistors in the circuit that drop the voltage slightly).  My circuit was driven by a 9V supply, so the max output is 9-1.2V = 7.8V.  I used a 470ohm current limiting resistor for my setup, if you use a 5V supply you can use a lower current limiting resistor (like 220ohm), and for higher Vcc use a higher resistance (maybe even up to 1K).

Wire the momentary push button switch in series with a 10K resistor between Vcc and ground.  Connect a wire (yellow) from the junction between the switch and resistor to the trigger pin so that when the switch is not pressed the trigger pin is held high.  When the switch is pressed the trigger pin will drop to low.  See the schematic if this does not make sense.

Operation:
Press the button.  The LED should light up for a time and then turn off.  If you time the LED, you'll find that it lights up for exactly 5.17 seconds, just as I calculated above.

You can experiment with switching out the 10k resistor or the 470uF capacitor (connected to the 555) to see how they affect the duration of the output pulse.  Remember, since t = 1.1*R*C seconds, increasing resistance or capacitance will always increase the duration of the pulse.

I wired up a 10Kohm potentiometer as a variable resistor and put it in my circuit in place of the 10K resistor between 555 pins 7 and 8 (fig 9).  This way by turning the knob all the way to one side, the LED stays on for 5.17 seconds, but when turned to the other extreme the LED turns off immediately after I released the button.  Turning the potentiometer to any position in between will cause a pulse duration anywhere from 0 to 5.17 seconds.
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