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# How can I build a stop/start timer into a 3 volt circuit?

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#### thiscountryside

##### New Member
Hello, How are you?

I am new to this forum and at the moment my knowledge of electronics is basic to say the least. I have built a simple 3 volt circuit that powers a 3 volt motor that bounces around randomly for an installation. I would like the motor to come on for 10 minutes then stop for 10 minutes (this time could vary and doesn't need to be exact).....can anybody help?

I thank you for your time

Dan

Using a lm555 to create a 10 minute astable pulse you probably can. However if you need exactly 600 seconds, then you will need to look for another solution. Gah I couldn't find the voltage range on the datasheet but I remember it going down to 3V and max 18V, somebody please correct me if I am wrong.

Thank you for your reply, the timing does not need to be accurate to the second by any means, the time could in fact be random it's just that the motor needs to stop to rest and cool down and the batteries need to last as long as possible. If it rests for longer and suddenly springs back into life all the better. Apologies for not using technical terms.

Hello Mike,
Thank you, ok, I'm struggling to figure out what resistors and capacitor would be suitable for periods of minutes on(eg 10 minutes) and minutes off (eg 8 minutes off) rather than seconds, the terminology is alien to me. Sorry, this is probably very basic stuff for you!

This should help:

Remember this equation:

T=RC

Where:

T=Time
R=Resistor
C=Capacitor

So let's say I have a 200KΩ with a 200µF capacitor. Well 200,000 x 0.0002 = 40s. So it will take approximately 40 seconds to discharge the cap. Note that when you increase the resistor value the discharge time will decrease and if you increase the capacitor value it will increase the duration of the discharge. Remember to expand all your numbers when you do it; you can't just do 200 x 200.

This equation is especially helpful when designing circuits for the 555 timer.

Using the link put R1 = 50, 000 R2 = 10, 000, 000 and C1 = 100 uF
Let me know if you like the results

Remember this equation:

. Note that when you increase the resistor value the discharge time will increase and if you increase the capacitor value it will increase the duration of the discharge.

V = Vo * e^-t/RC

Brilliant, things are slowly starting to make sense, thank you. Mike, I like the results, thank you

Hello again, can you get 10,000,000 resistors? keep reading stuff and they seem to stop at 1M...1,000,000?

however I can change the capacitor to 1000uF rating and all is well?

Yes you can go either way, 10M resistors exist though
Do you see how to hook up a transistor to your motor?

hello, ok cool. do i need to hook up a transistor to the motor?

Yes, the 555 will not be able to sink/source enough current for your motor

Wow, this is getting complicated. At the moment I don't then. I need to do some more reading I think. Is there any other way of stopping and starting the circuit...a timed switch that cuts out after 10 minutes and has to be pressed again for example?

You mean turns on for 10 mins then stays off until you press a button again?

yes, the motor will be in an installation so the viewer could press the button again and the motor would come to life, an automated system would be better but may be beyond me

The 555 is a pretty basic circuit do not be afraid! The other option you are looking at would be a 555 in astable mode.

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