hey guys, i have a wharehouse fan that i need to turn on when the door is open and turn off in about 30 seconds after the door is closed.

i've trying for days to produce one using a 555 timer but i havent gotten anywhere, so i'm now asking for help. PLEASE HELP!!!

 

Can you tell us some more about the fan? Is it AC, and what kind of voltage and current?

 

oh my bad. the fan uses a DC source. and its operated by a 30A 12Volt supply

 

Here is a nice tutorial about 555, it will help you make the circuit. Once you make it post it here to check it if you are not sure. To drive the fan you will need a relay as the output of the 555 is not capable of supplying more than 200mA without get destroyed. You can use a car relay because they operate at 12VDC and some of them can control 50Amps load. Choose one appropriate for your application. What is more you will need a transistor to drive the relay from the output of the 555, if the relay draws more than 200mA, and a diode connected anti-parallel with the relay's coil to absorb the back EMF created when the relay is switched off.

http://www.uoguelph.ca/~antoon/gadgets/555/555.html

 

A Potter & Brumfield T92 relay should handle the current, but the 555 would need a transistor to boost the output in order to pull it in.

 

well so far i've came up with the attached circuit. i figured that i would have to use a transistor on the output of the 555 timer and also a relay to start the fan. i was planning on using a automotive relay a 12V 30A (the one's mainly used in the vehicle's alarm system).

I've read the tutorial Mik3, thanx - but i'm kinda new to electronics (just know the basics) but have a better background in electrical installation and power distribution and i didnt learn much from that.

Well so far i cam up with the circuit attached. it switches on, but it does not switch off in a time constant, it does it instantaneously. Any ideas anyone? Please.

(P.S. - i left off the relay and transistor for now - i just placed a 100 ohm resistor to simulate the load)

 

What you want is a switch operated by the door opening that runs the fan and then triggers the 555 timer, set up as a 30 second one shot, as the door closes.

 

It switches off almost instantly because you missed a resistor. You have to place a resistor between the top end of the capacitor and the common point of the DIS and TRH pins.

 

thanks for the tip Mik3, but no success. i think i'm gonna give up on this

 

the thing is - now the circuit does not turn off any at all. i honestly am stumped.

 

Ohhh, you dont need this extra resistor I told you. I was a bit sleepy yesterday night

The problem is that your resistor or capacitor value is less than you think it is. Check them, you might used a 27K resistor or something else.

 

i figured out my problem peeps. its my simulation software - - i went ahead and built the circuit practically and it worked. however i made a few minor adjustments, and some i left off the relay assembly off of schematic itself.

i dug up some old info about transistor switching and put it with the 555 IC and who knows it works!!!

anyway see the attached diagram

 

No the circuit you posted wont work. First, disconnect the collector from the THR pin and connect it only to TRI pin. Also, connect the collector to Vcc via a 10K resistor. Second, connect the DIS pin to the THR pin. Third, connect the RST pin to Vcc.