555 Timer for Motor Control

Astable or monostable? you have not specified.

Again do you have a Flyback diode across the motor?

A transient generated at the start of motor may be resetting the Timer circuit. or power supply drops significantly due to high current at the start of motor.

As del suggested scope analysis can give you a complete solution, Try to isolate motor from the 555 power supply as much as you can do.

Put A diode and large capacitor on Vcc Of 555, also connect a diode between output of 555 and base of power transistor. add a high frequency capacitor in parallel to Flyback diode.

Hope this will give you some solution.

It is probably either the power supply dropping as the motor loads it (almost certain if the supply is a PP3 battery), or electrical noise from the motor brushes upsetting the 555.

Ideally you need a 'scope to see what is happening.
You could add more capacitance to the power supply or a few ohms (10?--15?) in series with the motor (and 0.1uF across the motor terminals) to try to isolate the problem.
Del

I tried just now putting a capacitor across the motor and the time extended to 3-4 seconds! Thank you very much! You really helped me! And I'll try other methods as well so that it can work better.

Maybe a Super Cap would extend the time long enough. But I like the battery replacement better. So I'll give you a GA.

When does my check arrive?

Use a .01uf on pin 5 to gnd. Noise is almost always a problem with industrial electronics.

your motor is probably too large a load, it works for one second probably because the a charge supply in the capacitor and the power supply are enough to drive the motor when combined but then the reserve in the capacitor (quite a big one to get 20s) is consumed and the motor stops. If as someone suggested you are using a PP3 as a power source then replace it with 4 x AA cells to give you 6V but a LOT more current capacity. PP3 200 mAh compared to AA 2700 mAh. (The PP3 may have quite a high internal resistance which would drop a significant portion of the available output voltage leaving a smaller voltage to drive current through the motor).

If your motor is too large a load for the 555 output transistor you could use the output to switch a Darlington pair transistor (such as a TIP120) as a low-side driver.

http://www.fairchildsemi.com/ds/LM/LM555.pdf

mAh is charge capacity and doesn't necessarily relate to peak current.

One quick check you may want to do is to check the applied voltage with the motor running. The 555 uses comparators and if the Vcc is changing that could play havoc with the timing. The LED would not have the same current draw and would not display the problem. The motor could also be generating considerable noise and that could also cause problems.

Making robots is great fun and a good way to start doing electronics.
Once you bump into the limitations of doing it with timers and logic a small microcontroller will give you a huge amount more fun and flexibility.
It's a bit of a leap in complexity and will hurt your brain at first...but trust this old cat, it is worth the effort. If you get a chance to do a short training course on micro's or if you can get hold of a development kit go for it.
Have fun
Del

I agree, although it is also valuable to kluge things together as you are learning. A simple solution, for example, would be to switch the motor with a small relay, and switch the small relay with the existing transistor, and beef up the battery pack.

RadioShack, incidentally, sells a Basic Stamp development/learning kit (with an instruction manual) that is great for learning about microprocessors.

Once you bump into the limitations of doing it with timers and logic...

When logic fails me, I just resort to emotion and intimidation. When timers fail me, I just stick it back into the microwave until it seems hot enough.

Any chance of a circuit diagram and parts list including power source?

This is a brilliant free screen capture tool. Just save the picture to a jpg on your PC then use the little green camera:-

to insert the picture.

Why not just "Alt-Print Screen" ?

Think simple. Why not use the 555 to switch a low power component, like a relay, which is capable of switching much greater loads rather than having the 555 to switch the load.

Tony

The 555 is switching a transistor not the load. Even with a relay you will still have the problem of induced noise resetting the monostable and/or loading on the power supply as explained above.

Guest,

Is this kind of what you have in mind?

Jon

Two power supplies and an optically isolated switcher. I like it!

What size cap are you using across the motor? What diode, 1N4002?

OK, guys. I'll bite. What IS that device that is bypassed to ground by a cap, in the top center of the diagram? I don't remember seeing THAT one in my Navy Electronics Technician Schools!

Of course, I don't remember much of what I saw 37 years ago, anywhere else, either!

But what is that device?

Micah,

Are you asking about Q1?

If so, it is an an Insulated Gate Field Effect Transistor.

Jon

No. Its the device directly above the 555. This one is paired with a single bypass cap, and the two are connected at their top ends to +12V, and at their lower ends to the line connecting to pin 8 or the IC (along with a whole lot of other devices and pins, obviously). It is represented by one solid black and one solid white (or clear) bar. Probably the bars represent polarity, but it is not a Power Supply/Battery symbol.

In Suresh's version, if it connected differently, but still there, in the same spot.

And it bears no label.

That's an electrolytic capacitor, the solid bar just shows the negative connection.
Del
Gimme a GA for pities sake...I gave one of the first good answers and have been spurned. I shall yowl mercilessly outside you house at 5Am if I don't get my GA.

I shoot cats that yowl with a hypo full of ammonia water. No matter how little penetration I get, it burns enough to make them yowl louder, while going faster, AWAY.

Which one are you screaming for a GA on, anyway? I didn't get into that argument, thinking it beneath YOU, also. But if it'll save the blisters on your sucking thumb, I'll give you a GA.

Sheesh, extorting the commenters for GA. What'll they think of next??

Oh, yeah. Thanks for the explanation. When I was learning to read schematics, the electrolytic cap used one plate curved, or put polarity marks on the plates. Other than either the curve or the polarization, it looked just like a fixed value non-electrolytic.

Gotta go. See you guys all later. I'll probably be back on sometime tonight, after I get home from work.