Step 6: 555 Timer: Astable Mode Circuit

As I described in the last step, setting the 555 timer up in astable mode causes it to output a continuous series of pulses.  In this circuit, I'll set up the 555 timer to output a pulse wave with a frequency inside the audible range, this way I can connect the output to a speaker and hear the results.

Parts List:
555 timer Digikey LM555CNFS-ND
0.01uF capacitor Digikey 445-5297-ND
100kOhm linear taper potentiometer Digikey 987-1300-ND
10kOhm 1/4watt resistor Digikey CF14JT10K0CT-ND
0.47uF capacitor (or anything between 10uF and 0.1uF should be fine) Digikey P5173-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 6.  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 6, connect a 0.01uF capacitor between pins 5 and 1.  Connect a 0.47uF capacitor between pins 1 and 6, make sure that the negative lead of the capacitor is connected to pin 1.  Wire a 10kohm resistor between pin 6 and 7.  Wire a 100K potentiometer wired as a variable resistor between pins 7 and 8.  Use jumper wire to connect pins 4 and 8 to each other (red) and pins 2 and 6 to each other (yellow).

Attach the positive lead of a speaker to pin 3 of the 555 and connect the negative lead to ground (pin 1).

When you power this circuit you should begin to hear the pulse waveform coming from the 555.  Turn the potentiometer to change the frequency of this pulse wave.  If you want to generate a particular frequency, try changing the values of RA, RB, and C according to the following equation (derived in the last step):

Frequency of Output = 1/[0.7*(RA+2*RB)*C]
where RA, RB, and C are shown in fig 7

Based on the components I used in this sample circuit, we can calculate the range of possible output frequencies as follows:

assuming the potentiometer is turned all the way to one side and the resistance = 100kohms
Frequency of Output = 1/[0.7*(100,000+2*10,000)*0.00000047]
Frequency of Output =~ 25Hz

this output is shown on an oscilloscope in fig 8

Low values of RA should be avoided because they prevent the 555 timer from discharging the capacitor C normally.  When I turned the pot all the way to the other side (for a resistance of 0ohms) the 555 timer stopped working as expected (fig 10).  So let's calculate the output frequency from the timer when the pot is turned to its halfway point, for a resistance of 50Kohms.
Frequency of Output = 1/[0.7*(50,000+2*10,000)*0.00000047]
Frequency of Output =~ 43Hz

this output is shown in fig 9 (note- there is some error bc I was guessing at where halfway was)

Also notice that although the frequency of the output changes between figs 8 and 9, the duration of the low output phase does not change significantly.  This is because the duration of the low output phase is not dependent on RA (the variable resistor).  As calculated in the last step:
t = 0.7*RB*C
substituting RB = 10kOhms and C = 0.47uF you get:
t =~ 3ms
which can be verified in figs 8 and 9.
RickyO11 month ago

In the diagram pin 7 is connected directly to the pot and then to Vcc, but in your breadboard the pot goes to pin 6. Can you explain this discrepancy?

nezarsayegh2 years ago
plz, if i want to find the water level by using 2 plates capacitor, then i will connect the output of the timer to a f/v converter and display the voltage
which capacitor i should change with the 2 plates??
do you have any idea how to install into a XBOX360 Controller? can it be a two or three wire hookup?
amandaghassaei (author)  blaze.ninja42.02 years ago
no idea, can you give me any more info? what are you trying to do?
trying to hook it up so i have a rapid fire controller using a momentary on button, but no idea where to wire in the 555 timer at :/ since my controller doesn't have a crystal oscillator.
amandaghassaei (author)  blaze.ninja42.02 years ago
I've never tried modding a controller like that, but I know it's possible, do you have a schematic?
no lol i don't, that sure would be helpful though.