splitting voltage

Just solder a tap at the halfway part of the battery holder to run the motor. Also why not switch the battery hold to use all C cells unless space is an issue.

thats a good idea, however the battery holder is a weird rectangular prism shape. plus, id like to learn about how to do this.

- - | | resistor 555 | | [pin 3] motor LEDs | |

• +

when I connect the motor, the LEDs will flicker much faster and dont hold a steady brightness. why would this cause the 555 timer to oscillate faster and sporadically? im confused.

-sam

You may want to try using a 100 uH inductor in series with the motor and even a 1n4002 diode reverse biased in parallel to the motor, as the motor may be producing current pulses on the supply voltage. The inductor should help block it. Also try adding an electrolytic cap of about 220 uF to the battery feed to smooth out the varying current the motor is causing. Does the voltage hold steady when the motor runs, or is there a big drop? A 555 timer functions via a voltage divider internally, so if the voltage source varies - so does its timing.

you need to avoid this. The batteries powering the motor will discharge faster than the other batteries and the fresher batteries will continue to push current through them even after they're dead, which can lead to leaks and other nasty behaviour.

vic

Which should I try first, the inductor or the diode? I think you are right about the motor producing pulses, because if I just do:

- |

along with

- |

the 555 timer works normally, whereas when I connect the motor it freaks out.

I'm not sure I understand what you want me to do with a 220uf capacitor.

Thanks for your help,

-sam

It is going to take more information. Regarding the motor - just a resistor and motor will work if you know how much current the motor requires it is easy to calculate the value of the resistor. Half the voltage will be wasted as heat in the resistor.

An alternative to wasting the power (and it is likely the capacity of

Search for: Pulse Width Modulated (or pwm) DC motor speed controller, lot of circuits on the net

You don't show your schematic but I'd expect to see that with a battery supply and a motor load against a 555

The battery voltage changes with the motor load. If the voltage to the 555 power supply changes it is likely to trigger later or earlier in its normal timing cycle - you need to supply the led timer with a seady source of power.

One cheap and easy way to do it would be to add a rectifier diode to the 555 power input pin (pin 8 and 4 on the astable). Its purpose is to only allow current to flow into the 555 not back to the motor. You put a relatively large capacitor on the supply and ground on the 555 side of the diode. A few thousands of microfarads at 6 volts.

Something you should already have is a bypass capacitor on pin 5 to ground (that helps keep the voltage steady on the internal divider string on the 555 and keeps the trigger voltage from wandering around. I think the application notes call for .01 microfarad, but with a slow oscillator and big problems I'd try more - up to a microfarad.(and this may have a deleterious effect on the start up flashing of the LEDs so the timing may be off until the cap charges). 01/.1 won't be noticed and may help some.

The problem is you motor is sucking too much power from the batteries and represents a varying load (with a brush type motor especailly). The 555 really wants a clean steady source of power, so either the power supply has to be stiffer (large caps, large batteries) or the motor load has to be lower, or both.

The inductance in the motor and brushes create electrical noise as each commutator segment swings by the brush. It wouldl be a very good idea to put a ceramic disk capacitor as close to the motor brushes as possible (right at the housing to the motor and across its power leads) .1uf 50 volts ought to be goood. Noise can trigger a 555.

Other tricks - good physical layout. Run a seperate wire from the 555 power and ground all the way back to the battery pack. Do the same thing with the motor - the wire should not carry the current for both the motor and 555, that's asking for trouble.

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Thank you so much for this info.

I've drawn up a schematic...

I've changed the supply to 2x C batteries, and added a diode in parallel to the motor.

Do you think this would work?

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Better would be this:

In other words, keep the 4 cell supply and the existing

On the each 555, be sure to add a .01 uF cap from pin

Ed

I don't quite follow.. what are the recommended capacitors C1 and C2?

Also, where does the inductor connect? (from positive to positive?)

Thanks,

-sam

No

The standard 555 timer requires a minimum of 4.5 volts to work. 3 volts probably won't cut it. One of the Cmos versions might work but some of the parts values would have to change

And if you're shooting for a 50% duty cycle it seems logical to me that the resistor values you show are not going to work - and if you are using 3 volts you don't need the 50% duty cycle anymore (the whole purpose was to reduce the effective voltage) . . . You're trying to trick me!

The 50% isn't going to work because you have a 2.2K and 2K charging the timing cap but only a 2K discharging the cap (it charges from the power supply rail and discharges via pin 7 to ground) For close to

You can't really reduce the 2.2K to achieve the same thing - it is logical all right, but then you're asking pin 7 to sink more current (so keep that at 2.2K or higher)

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Well, I'd make one or more changes to that. You show the motor current in the same line with the LED driver. The inductor should connect back directly to the battery. Ditto the ground wire - just good RFI practice.

And I'd use a forward biased diode to isolate the LED driver from the battery if the motor can pull lots of current (like when it starts or encounters a heavy load) Four AA cells have a relatively high resistance so may not supply all the current the motor wants without allowing the voltage to droop.

Putting a diode and large electrolytic on the 555 will keep the 555 happy (for a time) if the motor try's to pull the voltage down.

I'm running a pair of servo motors on a pan and tilt camera from a supply that is common to the op amps used to drive the motors - the diode isolation worked wonders since the servos were always starting from a stop and would suck down the 12 volt 1 amp power supply to 9 volts briefly. Even though the motors only used 100 ma.

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Two other things you left out the connection between pin four and the positive supply rail, both drawings, and it is good practice to put a cap between pin five and ground to keep the timing stable

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OK everyone, thanks so much for your input. I've put in so many hours to this project and just want to see it finished. You all are a great help.

So as I understand, here's what needs to be modified from the schematics (please let me know if anything is wrong):

- I'm going to keep the power supply at 4x AA batteries, since the 555s require 4.5v.

- I will add .01uF caps on each 555 going from pin 5 to ground (I still don't get what this does.. won't the capacitor just charge up and then sit there charged?)

- The diode parallel to the motor will be an n4001, (is this the correct diode?)

- Change the resistor values on the 555 timer that controls the motor to represent about 40% duty cycle at 100Hz [I'll experiment with this to find correct values]. Read below to see why 40%.

And here are things I'm not sure about:

- Adding a 100uH inductor where? From + rail to the joint just above the motor?

- Adding diodes to the 555 timers. From + rail to pin 1? n4001?

- Connecting pin 4 to +. Whats the advantage?

More about the 3v motor: When connected purely to 2 C batteries (as it was designed to do), it pumps a small stream of water about a foot up. However, this stream of water is too much for what I need it for... my specs call for an even tinier stream of water. Ideally, I'd like to be able to control how much water is flowing, which is why I'm opting for the PWM solution, so I can effectively control the voltage, and as a bonus get to use another potentiometer that controls something. Since

I just purchased an assortment of diodes and a couple .01uF caps, so I'm all set except for the inductor, which I'm hoping won't be needed since I don't understand where to put it and what it would do. I will put together everything tonight and post results.

Again, thanks a lot everyone (especially Default)

-Sam

Yes it just sits there charged. Its function is internal to the 555 and it stabilizes the control voltage to the internal comparators (hopefully preventing them from changing - more useful in electrical noise rejection then wholesale power supply voltage droop).

You should really read up on the 555 so you understand exactly how it achieves oscillation - it is hard to get this stuff to work by just wiring and hoping.

That is good enough for most purposes

Noise rejection components do the most good when physically close to the source of the noise - otherwise the length of wire between the filters and noise source act as antenna and radiate noise.

That will help if used with a large value electrolytic cap to stabilize the voltage to the 555. A cap between pin one and pin eight. That will see that the 555 has a stable power supply to pull from when the rest of the circuit is noisy.

It is just done that way in the typical oscillator application of the

OK

An inductor opposes the rapid change of voltage in this application. When current flows through an inductor it builds up a magnetic field around it, when the current tries to increase or decrease the inductor opposes these changes by taking or adding energy to the magnetic field. Electrical noise, like what may be causing problems in your circuit is rapidly changing voltage/current imposed on a power supply wire where there should just be a steady voltage - the noise plays havoc with timing.

Let us know how you do, and good luck.

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Here's a simple PWM circuit to control your motor:

View in fixed font:

Alright, here's a quick update:

I redid my breadboard and have finished the 555 timer for the LEDs, and just finished the 555 timer for the motor. It's the circuit from this page

Anyway both circuits are working.. I tested the duty cycle of the motor timer by waving the breadboard and comparing the streaks of when the LED is on to when it is off. However, the motor will not start through the TIP31, which could be for a variety of reasons... but I think I narrowed it down.

I'm having a problem with the TIP31, it doesn't seem to allow enough current through to get the motor started. Consider this circuit (wire is =):

The motor will not start, but the TIP31 will get hot.

Do I need a different transistor?

Thanks,

-Sam

This is the same circuit as I linked to in my previous post, minus the voltage protection. Am I correct? If so, I'm glad I'm on the right track :) Thanks again for your help!

-sam

OK, I solved this one.. the resistance to the transistor was too much, so it was only partly on.

-Sam