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12v on/off timer

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galkaen

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Hi guys I need to build a timer to drive an automotive water pump on for 5 scond and off for 20 minutes. How can i do this?
Thanks in advance for any help.
 

biomedhed

New Member
Hi guys I need to build a timer to drive an automotive water pump on for 5 scond and off for 20 minutes. How can i do this?
Thanks in advance for any help.
I am sure other people on here will shoot me down in flames but a 555 timer chip is suitable for this. You can set the ON and OFF periods using different resistors and capacitor values. The link below should point you in the right direction.

(Your output would be a relay that would switch on or off your pump circuit)

555 Timer Astable Mode

http://home.cogeco.ca/~rpaisley4/LM555.html#3
 
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BrownOut

Banned
I think you can use a 556, dual timer IC, and configure each timer as a one-shot, each feeding the other's trigger input. Program one for the 20min off time, and the other for the 5 second on-time. I've never tried to program a timer for such a long time period, but I think it can be done, just be sure to use low-leakage capacitor types, such as tantalum.

Sorry, I don't have time to draw a schematic, but here is a good primer/turtoral.

555 Timer/Oscillator Tutorial
 

BrownOut

Banned
The problem with the 555 is getting 20 minute delay without some additional counter ICs.
Theoritically, it shoulnd't be that hard. One tantalum I looks at leaks .1ua at room temp. If you pair a 100ufd version of this cap with a 12 MΩ resistor, that gives you the 20 minute delay with only 12*.1 = 1.2 volts drop across the resistor in steady-state. Remember, you only have to get to 2/3vcc to terminate the pulse.

But like I said, I never tried it with real parts. Maybe if I have time one evening...
 
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Hero999

Banned
What about the imput current for the 555?

You need to use the CMOS version 7555.

Also connecting a resistor from pin 5 to +V will also increase the delay because the capacitor has to charge up to a higher voltage before the cycle terminates.
 

BrownOut

Banned
current for the TTL version is .1uA.

http://www.national.com/ds/LM/LM555.pdfInput

Add that to the leakage and you get 12M*.2UA = 2.4V. For a 15V operatoin, you have 12.6 volts at the threshold. You'll have to adjust the series resistor to account for this lowered voltage, but there is still pleanty of headroom.

Again, just from a thoeritical standpoint.

BTW, National sets the max value of the resistor at 20 MΩ, so I think we are in good shape.
 

galkaen

New Member
555 on off timer

Thank you for your time guys. I'm still figuring this out. but now I'm a lot closer to get it to work. Do you know any simulation software besides electronics workbench. EWB is killing me!

Again thank you for your time!

Galkaen
 
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