# COMMUNITY : FORUMS : TECH

## 555 TImer/Chip Help

I need your help people of Insturctables. I am working on an project, and I need to make a circuit whit a 555 timer/chip so that it makes,for example, an LED light up in every 2 second. I mean when it turns on, it needs to be on for 2seconds than off for 2 second...loop. ON(2 seconds)___OFF(2 seconds)___ON(2 seconds)___OFF(2 seconds)___LOOP I need the output to be 12V. Actually I need it for a fan to turn on and off(the fan runs on 12 volts) Can you help me make a circuit for this, what value of the resistors and capacitors do I need... It would be grate if some one could make a schematic of the circuit in Eagle, or so... If I make an Instructables about my project I will surely mention who helped me realize my idea! Thank you all! -Stanislav

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110100101108 years ago
for 2 sec / 2 sec period pick C and X so that C (in uF) * X (in K) = 3000

as transistor allmost any NPN transistor that can take the current of the fan is ok. use a resistor at the base (B) so that 12 V / resistor is less (but not too much less) than max base current of the transistor. its like few K for small transistors and few hundred ohm for larger ones

the capacitor at the left is there for filtering but it may work ok without one
comodore (author)  110100101108 years ago
Thanks a lot! The X resistor and the C capacitor, i see you wrote period pick C and X so that C (in uF) * X (in K) = 3000 for example, what 2 values should that be? Is the X resistor 3000 K? And the capacitor? On the schematic, on the 555 chip, the circle at the right bottom corner, that is pin 1, ground, right?
8 years ago
I wouldn't use that circuit without a protection diode for the inductive load (the fan.)

Transistor switches, scroll down for protection diode...
comodore (author)  gmoon8 years ago
Ok, I see! Thanks!
8 years ago
C and X examples
100 uF 30 K
1 uF 3000 K
33 uF 90 K
4.7 uF 680 K

if C * X is less than 3000 you get shorter time and the other way (not too critical)
use X atleast 1 K and max few thousand K (above that may have stability issues). with higher X the circuit takes less current and is more efficient

circle is pin 1

ground blue
12 V red
comodore (author)  110100101108 years ago
Thank a lot! I sorely mention your great help! :)
guyfrom7up8 years ago
make sure you use an LM555 they can source up to 200mA if your fan is more than that you need a transistor for a 50% duty cycle you will need to use a diode. I havn't used 555's in a while, so yeah...
comodore (author)  guyfrom7up8 years ago
OK, thanks for the advice! :)
westfw8 years ago
You can use the schematic and pcb from my Eagle instructables

For equal on/off times, R1 should be as small as possible (about 1k ohms.) For approximately 2 seconds on and off, you can use R1=1K, R2=300k, C=10uF. There are numerous "555 calculator" apps on the net...
comodore (author)  westfw8 years ago
Yea, I know about the calculators... About the R1, R2 and C, I don't know here to put them, to connect them to what pins of the 555 timer, that why I need some one to make me a Eagle schematic... I am a beginner in electronic, I learned some of the basic parts,I know how to solder, and use a multimeter, to follow a circuit and in some cases to see where the problem is...but sill, my goal is to learn to make my own circuits, and schematics!
I love your Eagle tutorial Instructables, a lot can be learned from it... I want to learn to use Eagle so this will come in handy... But for now, I want to ask you, could you make me a schematic of the circuit I need. If I make an Instructables, I will sorely mention you!

Thank you!

-Stanislav
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