Society of Robots - Robot Forum
Electronics => Electronics => Topic started by: entity on June 27, 2011, 02:36:40 AM
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Gday All,
Okay firstly I know absolutely NOTHING about putting various electronic components together, yet with some recent projects I have now found it imperative that I do learn for the reason listed below.
I have just been running a basic 80Ma vibrating motor off a standard AAA power source which works fine BUT what I know need is to be able to somehow put a timer in the system so that the power is not constant to the motor, IE : So it runs for 1 to 10 secounds, stops for maybe 5 secounds then repeats the cycle. I was told that a 555 timer could be incorporated with some type of either transistor or resistor on a small circuit board ?. I didnt have a clue what they where on about, but I assumed someone here would know how to achieve this ??
I have 3 other projects that I can use the timer function in to alternate the running of some minature gear motors, as an on off swith is not an option. Need to be able to connect the power source when I wish to use the device and have it do its alternating thing for a few hours, then remove power source once finished.
Any help with what most of you would consider a very simple task would be excellent and much appreciated as I have a lot of electronic information to learn and am yet to find a suitable book that explains the ins and outs of robotics and the bits and pieces that go with it. The 555 timer may not be the best option however it is all I have heard about so far.
Steve
PS: Great site as I have seen numerous designs on here that could solve some long term projects of mine.
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Back again,
I forgot to mention that I am trying to fit whatever circuitry I need on a board 10mm wide by 40mm long if that is at all possible. I think my requirements are extremely basic compared to the stuff I have read about on the site, so hopefully someone can help me out.
Steve
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Hi,
[]... a basic 80Ma vibrating motor
Do you perhaps mean "a motor with a current consumption of 80mA", or is it a model number of sorts?
[...] off a standard AAA power source
You'll need a higher voltage to run a 555.
[...] IE : So it runs for 1 to 10 secounds, stops for maybe 5 secounds then repeats the cycle. I was told that a 555 timer could be incorporated with some type of either transistor or resistor on a small circuit board ?.
That is correct. Given that the 555 needs a higher voltage, the output should be limited to keep the motor alive.
I didnt have a clue what they where on about, but I assumed someone here would know how to achieve this ??
Yes, but can you follow a schematic and build from it?
If not, it's gonna be a little hard.
I have 3 other projects that I can use the timer function in to alternate the running of some minature gear motors, as an on off swith is not an option. Need to be able to connect the power source when I wish to use the device and have it do its alternating thing for a few hours, then remove power source once finished.
[...] The 555 timer may not be the best option however it is all I have heard about so far.
The 555 is only good for relative short durations. When you need longer periods or perhaps a sensor or switch modifying the time periods, a microcntroller is the way to go, but then you need to learn programming them.
The 10x40x?? size should be just doable - if you need it smaller, there's surface mounted devices, but they'll be a challenge for you.
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Soeren,
Thank you for taking the time out to reply.
Yes the 80ma was consumption rating.
I have no problem following a schematic as long as I am aware of what part ( name & type ) goes where within the circuit path.
I am currently restricted to AAA for a power source due to size constraints within the component housing. And with this power source I can get in excess of 40Hrs constant run time which is more than ample for its purpose.
If I cannot have a timer due to the power limitations, than is it possible to have some type of electrical component that will allow constant current to the motor but in varying degrees of current. IE : current starts minimal then builds to maximum then cycles back to minimal again on a repeating cycle ????. This would still give me the varying degrees of vibration that I am after without overcomplicating the circuitry and cost of the device. Alternatively I could use a 3v power supply if a resistor could be used to reduce the output to 1.5v, if 3v is enough to run the 555 timer ?.
Once again thank you for responding.
Steve
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Hi,
I am currently restricted to AAA for a power source due to size constraints within the component housing. And with this power source I can get in excess of 40Hrs constant run time which is more than ample for its purpose.
Does that mean a single AAA?
If that's the case, only the (more expensive) CMOS version of the 555 (eg. 7555 or LMC555) will do and it will stop before the battery is fully drained.
Lithium coin cells like the CR2032 might be worth considering, as they're 3V (but only around 1/7 to 1/5 the capacity measured in mAh).
If I cannot have a timer due to the power limitations, than is it possible to have some type of electrical component that will allow constant current to the motor but in varying degrees of current. IE : current starts minimal then builds to maximum then cycles back to minimal again on a repeating cycle ????. This would still give me the varying degrees of vibration that I am after without overcomplicating the circuitry and cost of the device.
That would make a more complex circuit.
If you can get hold of unijunction transistors (and use at least 3V), a simple and small circuit can be made with a pause of your needed length (or even longer), but with just a short(ish) on-time (like 1..2s tops)
Alternatively I could use a 3v power supply if a resistor could be used to reduce the output to 1.5v, if 3v is enough to run the 555 timer ?.
As mentioned, the CMOS variety will run on 1.5V, but 3V is much better, even for that.
A resistor will be fine for reducing the power to the motor in your application, but there's other ways as well. With the CMOS-555 you'll need a transistor to drive the motor (as it hasn't got enough output drive) and using the right Darlington transistor, you're almost there, as some of them drop close to 1.5V
Should the circuit have both on- and off times independently adjustable, or is it set 'n' forget?
I'll be offline for the next 3 days, and with less than 2 hours to sleep, I won't be drawing you a circuit now, but perhaps somebody else can step in, if you're in a rush.
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This circuit should do, as Your motor requires 80mA and LM555CN (http://www.national.com/ds/LM/LMC555.pdf) can output 100mA.
ONtime = 0.7 * R1 * C1
OFFtime = 0.7 * R2 * C1
*resistance in Ohms and capacitance in Farads
By adjusting each pot You adjust time it takes for Your motor to be ON or OFF.
As I have understood You require 0.15Hz frequency (10s ON, 5s OFF). I am not really sure how 555 timer will behave for such low frequency and how stable the performance will be.
P.S. Also, could someone more experienced in circuit design review the diagram, please?
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Hi,
This circuit should do, as Your motor requires 80mA and LM555CN (http://www.national.com/ds/LM/LMC555.pdf) can output 100mA.
ONtime = 0.7 * R1 * C1
OFFtime = 0.7 * R2 * C1
*resistance in Ohms and capacitance in Farads
By adjusting each pot You adjust time it takes for Your motor to be ON or OFF.
As I have understood You require 0.15Hz frequency (10s ON, 5s OFF). I am not really sure how 555 timer will behave for such low frequency and how stable the performance will be.
P.S. Also, could someone more experienced in circuit design review the diagram, please?
Let's see...
Timing:
t=ln2*R*C
0.693*1k*470µ = 326ms
0.693*11k*470µ= 3.58s
Verdict: Design goal not reached
Output Current:
The 100mA output current is mentioned in the section "Absolute Maximum Ratings"
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur.
Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits.
Electrical Characteristics state DC and AC electrical specifications under particular test conditions which guarantee specific performance limits.
This assumes that the device is within the Operating Ratings.
Specifications are not guaranteed for parameters where no limit is given, however, the typical value is a good indication of device performance.
A DC motor taking 80mA average running current will have an inrush current of way beyond 100mA.
Verdict: Design goal not reached
Output Voltage "Trim":
The LMC555 is not able to reach the rails and will drop around 0.5V or more (@ 80mA output current).
That leaves a maximum of 1.0V over the voltage dropper resistor.
1V / 39 Ohm = 25.6mA
Or, to put it another way, the motors equivalent resistance is 18.8 Ohm
There's around 2.5V to drop over the motor plus the resistor
2.5 / (18.8 + 39) = 43.3mA
Verdict: Design goal not reached
Decoupling Capacitor:
Ignoring real inrush currents (which is probbly over 250mA) and assuming that the circuit
will still run from 1.5V (which it won't as shown), the 100nF cap will be worthless for transients of more than 1.88µs
With a more realistic inrush of say 300mA, a start up time of at least 20ms and an allowed voltage drop of 1.0V, a cap of 6,000µF would be needed, so there's really no point in the 100nF, as it won't have any effect.
Verdict: Design goal not reached
Better use larger resistors and smaller caps, as the larger the cap, the more unpredictable the timing. 1M trimmers with 47k pre-resistors goes well with a 22µF cap.
As I mentioned, a transistor must be used, either a BC337 or a 2N2222 will work, but the dropper resistor should be much smaller, about 12..15 Ohm would be a good starting point.
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Back,
Thank you all for the information, was away for awhile hence the delay in replying.
Working on obtaining all the components and shall let you know how I went.
Steve