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Power-on delay circuit

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mbeaver

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I am looking for a way to delay a small electric motor (3V DC) from starting, so that instead of activating the "on" switch once to start the motor, the "on" switch must cycle twice to start the motor. Kind of like if you had to flip a light switch on, then off, then on again, to get it to light up.

I have no idea what kind of circuit I need or if I need to have one made custom. Any help you can provide will be very much appreciated.
 
Why would you want to flip a switch twice before something worked?


Or do you mean you just want to delay the motor starting for a predetermined amount of time after you flick the switch. If so, how much time and what is the reason you need to delay it?
 
I also am looking for a very simple light weight circuit to delay a 1.5 volt motor to start 10 to 30 seconds,,it does not have to be the same every time.This is put on a streetcar that runs down a track and is stopped by a micro switch and stops the car for a short period of time....Simple and light weight and no relay if possible..


Thanks
 
RC time delay

If you just want a small delay, you could use a RC time delay circuit. This is basically a resistor in series with the input voltage and a larger cap in parallel with the output voltage. You would then put a relay or something similar across the output. As the cap charges(in a logarithmic fashion), the output voltage will increase until the relay clicks on, switching 3V to your motor. The bigger the cap, the longer the delay. The time delay(time it takes to charge the cap to 63%) is: t=RC, with R in ohms and C in farads (google RC time delay). Choose these values according to what you have on hand, e.g. large caps = expensive, large resistors = cheap.
 
Thank you Slosjo. Will I only need a resistor and cap plus the relay? No transistor? Could you please draw a picture of this circuit?


I am eternally greatful for your help.
 
Check out this pdf. They have a graph of a RC circuit so that you can see what I'm talking about and also some equations that you can use.

**broken link removed**

Also look at this page for a nice description of RC circuits

https://everything2.com/index.pl?node_id=745343

I have attached a picture of what I'm talknig about.

Keep in mind that this delay is not very precise because of the variability of the capacitor. It won't be very long unless you use unusually large components. Look at the equations on the above pages and plug in some values of resistors and caps and you'll see what I'm talking about. Remember that t=RC and R is in ohms and C is in farads. Most caps you work with are in units of micro or pico farads so take that into account when doing your equations.

To make sense of the time delay, consider this:

t=RC, with time in seconds, so, breaking down the units you get this:

ohms * Farads = (Volt/Amp) * (Coulomb/Volt). If you look, the volts cancel each other and you end up with:
(Coulomb/Amp). An amp is a Coulomb per second, so if you substitute that you get this:
(Coulomb/(Coulomb/second))

Coulombs cancel and you are left with seconds.

Hope this helps.
 

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Depending on how heavy the load is(a small motor, in this case, which can easily be driven by a transistor- i hope!), i would suggest you use a more precise switch as the relay contacts may experience "clattering" if its coil voltage depends on the capacitor voltage which builds up over time. You can use the LM393 comparator with the resistor and capacitor on its non-inverting input and a reference voltage on its inverting input. Its output can then drive a transistor with the motor as its load. This should give a more precise switching.

Hope that helps!
 
Al_Pacino said:
Depending on how heavy the load is(a small motor, in this case, which can easily be driven by a transistor- i hope!), i would suggest you use a more precise switch as the relay contacts may experience "clattering" if its coil voltage depends on the capacitor voltage which builds up over time. You can use the LM393 comparator with the resistor and capacitor on its non-inverting input and a reference voltage on its inverting input. Its output can then drive a transistor with the motor as its load. This should give a more precise switching.

Hope that helps!


I agree, this would probably be a better method of doing it. If you want to keep it a little bit simpler, a solid state relay would functionally accomplish the same thing and also be bounce free like the method above.
 
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