switch and relay question

[beholt]

Hello,

Please forgive me for my ignorance in these matters but I am working on a 12 volt circuit that is closed with a switch (PIR) that then remains closed for 2-5 seconds and then opens again and then locks open for 2-5 minutes and then resets to allow the PIR switch to activate it all over again.

Any advice or help would be greatly appreciated.

[pebe]

How much current flows when the switch is closed? What sort of load is being switched - is it resistive or inductive?

[beholt]

It's going to be running a DC 30A 12V two phase CPU brushless cooling water pump. The motor is listed as having a max rated current of 350mA

[pebe]

What is the connection between the 350mA max motor and the 30A 2ph pump.
How much current do you want to switch?

[beholt]

Hi Pebe,
I'm not sure what you mean. Here is the pump I was looking at using:
http://www.amazon.com/HOSSEN%C2%AE-DC30 ... s=cpu+pump

Again, sorry I'm not more knowledgeable about this; I'm mainly hoping to get pointed in the general direction so I can go read and learn about the components that may be needed for this circuit. It's one of those things that I know the end result I want but I don't even know where to start.

Thanks

[pebe]

Sorry about the misunderstanding. When you wrote 'DC 30A' (with a space between them) I thought you were referring to a Direct Current 30amp device. Which is why I raised the query. Now you have included the Amazon site detail, I can see it is all one word 'DC30A' and that is the model number of the pump.

What you require is easiest done with a couple of 555s for the delays, and a power FET. Do you need a circuit?

[beholt]

Thanks Pebe!

A circuit would be great. I think (maybe) I understand how to use a 555 timer to accomplish the initial 2-3 seconds of the pump running but I haven't figured out how to make the circuit lock open for the time I wanted.

Thanks again

[pebe]

This circuit should be OK for you.

IC1 is triggered by S1 and switches on for a period of about 3secs (T = 1.1 x C(in µF) x R (in megohms) = 1.1 x 10 x 0.27 = 2.97secs. IC1 output goes high to turn on the FET.

At the end of that period IC1 output drops and triggers IC2 on via C3. As its output goes high D1 conducts pulling IC1 pin2 high and preventing it being triggered for a period of 1.1 x 100 x 1.8 = 198secs.

You need low leakage timing caps. Ideally, use the small solid tantalum type.

[beholt]

Wow. This looks great! I'm looking forward to putting it together and trying it out. Thanks so much.

[beholt]

Hey Pebe,
I've about got this circuit built on my breadboard and am waiting on my motor to come in the mail. I have been reading up and feel like I understand how the timing caps work and how the 555 timers generally work.
Just for my education, what do R1, C1 and C3 and R5 do? In other words, why do they need to be where they are in the circuit?

Thanks

[pebe]

Hi beholt,
Here's the explanation.
IC1 is triggered by taking the TR pin 2 below 4V. After triggering, TR has to be taken high again or else it will override the timed o/p pulse and keep O/P high. Hence the arrangement R1,C1, R2.
Normally , both sides of C1 are at 12V. Closing S1 takes the LH side of C1 down to V. Because C1 cannot change its charge instantly, its RH side will also go down to 0V and triggering the TR pin. R2 will start to charge up the RH side of C1 to 12V so the timer will function normally, even if S1 is held closed. When S1 is opened both sides of C1 will again be at 12V ready for the next trigger.

When IC1 o/p is high the motor is turned on, and both sides of C3 are at 12V. At the end of the timed period, IC1 o/p goes low to turn off the motor. The RH side of C3 also goes low and triggers IC2. Its o/p pin goes high for 3minutes. During that period, if S1 was closed again then D1 would conduct and stop TR pin of IC1 from being triggered. That was to satisfy your requirement that once the timed 'on' period for the motor had ended and the motor had switched off it must not be possible to turn it on again for 3 minutes.

I hope that all makes sense.