intermttent motor control

[quacker]

I have an electic motor which runs off of 4 AA batteries. I would like the motor to be intermittent (the motor off for 30-40 seconds, and on for 2-3 seconds). Are there cheap kits or a cheap way to make a controller to get the intermittent on off?

Thanks

 

[ke5frf]

A CMOS 555 timer can do it...drive a MOSFET and figure the trigger cap for the frequency you desire. Basically a modified PWM circuit in slo-motion.

 

[quacker]

ke5frf
could you dummy down your explanation... i do not have much electronics experience

 

[ke5frf]

LOL. OK, well you are kind of tackling a project that requires a LITTLE bit of background, but I have faith you can pull it off.

Start off by Googling "555 PWM" or "555 motor control"

A 555 timer is an ASTABLE OSCILLATOR in a PWM (pulse width modulation) circuit.

PWM means what it says, a chip creates a pulse signal, in this case a "square wave". It is like turning a light switch ON and OFF. Think of a jack-o-lantern smiley face, a row of teeth with a missing tooth inbetween each existing tooth (kind of like this: _-_-_-_-)

The "width" of the pulse, or the DURATION is controlled by the circuit, as is the FREQUENCY of the pulse. Combining these controls allows a pulse of a specific, chosen duration (2-3 seconds in your case) and a on/off frequency that permits spacing of your choosing between pulses (30-40 seconds between pulses in your case)

This chip is not powerful enough to directly drive your motor, so it must be interfaced to the motor via a relay or drive transistor. A circuit that frequently turns on and off is best suited for a transistor instead of a mechanical relay, and in your case I suggest a POWER MOSFET (Metal oxide field effect transistor). These are most commonly used in digital circuits and are very suitable for DC motors.

So, the output of the 555 timer chip turns the transistor on and off, which in turn applies power to the motor. For a MOSFET drive circuit, it is advisable to use a CMOS (complimentary metal oxide) chip because the output impedance is well suited for driving a MOSFET.

The frequency of the timer is set by the trigger capacitor. A large value capacitor in microFarads will slow down the frequency at which the timer creates a square wave. The re will be a variable resistor to control the width of the pulse.

To build this, and understand it, you need to study how capacitors effect frequency in a timer circuit. You should also learn about transistors and oscillators. Knowledge of Ohm's law is important for determining impedance characteristics of the circuit.

Google has a TON of information, but start out Googling the above search terms and come back with questions!

 

[quacker]

more questions

Thanks for your help!
OK,
I did a little research.
I believe I understand the process of the 555PWM
The circuit i would be using I found in the site
**broken link removed**
the values of C1, R1, and R2 were calculated from a 555 calculator
**broken link removed**
I basically plugged in numbers until the high and low were what I wanted. The capacitor is a 0.17 farad, R1=3000 Ohms, and R2=300 Ohms. This gives me a 38.8 high time, and 3.5 low time. I basically threw in some numbers as a trial and error process to achieve this. This may be backwards. Is the high time the "on" to the motor or the "off" to the motor. Can I buy a 0.17 farad capacitor? Does this make sense (is it buildable)?
Where I can I figure out the diode size for a 6 volt application (4 AA)?
Are there different MOSFET's for different voltages. How would I size the MOSFET?
Also,the website/ schematic 555 Timer Tutorials did not have diodes in the schematic??

Thanks for any help!

 

[quacker]

thanks ke5frf
OK,
I have done some homework, and have a few more questions. I believe I understand how a 555 PWM works. I have found a circuit for a pulse control in **broken link removed**
I have calculated the size of the capacitor and resistors to get the timing I need using the calculator found at **broken link removed**
Using a .017 farad capacitor and a 3000 ohm resistor R1 and 300 ohms r2 resistor, I get a 38 second "high" time and 3.5 second "low" time. The first question I have is did I calculate this correctly? Is the high time motor on, or motor off? If I did this correctly, is this "buildable"? I just plugged in values for ohms and farads until I had the timing I wanted. Is there such thing as a 0.017 farad capacitor?
Where can I go to calculate the size of diodes and MOSFET I need?

Thanks