555 Timer with switch and pump

Greetings all! I am very new to electronics but heres what I want to do...

I want a push button (spst) to start a liquid pump for x seconds (enough to fill 1.5oz of fluid) Here is the pump specs:

I have a protoboard circuit that works with a trigger switch and LED that I copied from this site:

I had no problems getting this monstable circuit to work...But since I want to power a pump that is 12-24vdc < 1amp (pump is on order, wont get it for a few weeks ) I am thinking I will need leg 3 of the 555 chip to go to a relay, I will probably need a full 12v to go to the pump to get it to work. I don't know how to integrate the relay and pump in place of the the LED...I picked up a 12VDC relay SPDT rated at 1a part # 275-241 radioShack.. It has

Anyone have any suggestions?

Thanks,

Scott

The issue is only that 0.8A will require something like 20mA of base drive. Thus, you need to make sure the resistor between the output of the 555 will let it pass that much. At 12V, you probably want to use a

You can also use an N-channel mosfet, which won't require any base current. If you do this, you probably want to do it like this:

VCC ------------------------o--------. | | | | | .-o--. - | | Motor 1N4001 ^ | | | '--o-' | | o---------' | ||-+ ||

And here, I just want to say Thanks to Robert for expanding on my reply. I neglected the diode, and was way too lazy to look up the current spec, beta, and all that.

Cheers! Rich

Thank you for your reply Rich. Since the motor will draw .8 amps (I found this out today) Can I still use your method of boosting the output voltage through a PNP amp transistor? I was looking at the max current the 555 can output, it was listed at 200 or 300 ma... can't remember off the top of my head.

Does the transistor amp, boost the current as well?

Thanks in advance!

Scott

Will a IRF511 N mosfet work for this application? I am going to the electronics store to get one now :)

Thanks a bunch!

Yes, it'll work.

-- Regards, Robert Monsen

"Your Highness, I have no need of this hypothesis." - Pierre Laplace (1749-1827), to Napoleon, on why his works on celestial mechanics make no mention of God.

Thank you Robert! I got your design working great! I've been using a 12 V LED in place of the Liquid pump... I can't wait to give it a try when the pump arrives!

I found this schematic just like you suggested:

One more question though... in the schematic they have, they have the Diode across D S of the mosfet, and not across the + - inputs of the motor... does this infact do the same thing by preventing feedback across the D S?

Thanks again!

Scott

Their diode does nothing. There is already a diode there, inside the mosfet! If you look at the mosfet picture, there is a little triangle inside, pointing to the black vertical dashed line. That is a diode, which all power mosfets have, due to their construction.

You should put the diode across the motor, pointing up (meaning the terminal with the line is connected to 12V).