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Motor Timer| Yo! Several people have written asking for info about variations on this project. I highly recommend first using Google to search for 555 timer. But if that isn't helping you, feel free to write. I'm considering expanding this page to include FAQs or even more step-by-step details on how to build your own 555-based timers. Let me know what you think! My address can be found on my home page. |
The charging path has a 1 megohm resistor in it, which combined with the 22 microfarad capacitor, yields a time constant of about 22 seconds. The formula for the 555 timer has a factor of about 0.5 on the time, so it comes out to around 11 seconds.
The discharge path is through a 10 kilohm resistor, so its time constant is around a fifth of a second. During the discharge portion of the cycle, the 555 output pin is low; this is used to activate the motor. Doing it this way takes advantage of the fact that the 555 can sink quite a bit more current than it can source (but read on, this didn't work as we expected). In addition, it takes fewer components to implement a < 50 percent duty cycle. Use Google to search for 555 timer.
We suggest using a TLC555 - it's a low power version of the classic 555 and it runs reliably at 3 volts. It also doesn't glitch the power supply as badly as other kinds of 555 chips, which can happen when the internal flip-flop triggers. We tried skipping the bypass capacitor, but it didn't work, so you'll still need that.
This circuit can be used with two AA batteries, or a 9 volt battery. It depends on the motor's specifications. The 555 will accept anything from 3 to 18 volts; the timing won't change.
(But don't panic! We found an answer. Read on.)
The weight of the ornament was probably increasing the friction.
The theory is that the low power 555 was limiting the current to the motor, resulting in a loss of torque: completely shorting out the output to ground would let the motor work.
A simple fix might be to skip the diode or add another battery.
Unfortunately we didn't have room in the housing for another battery,
so this began to appear to be a failed project.
The layout can be used to build the timer on a piece of perf board. We used the leads on the big cap and the megohm resistor as a handy way to attach the extra wires needed for the battery and motor connections.
Here's how our version came out:
We solved the problem by removing the diode and adding a resistor and a transistor to drive the motor. Since we already had the 555 set up to make the output go low to make the motor turn, we needed to use a PNP transistor. Drawing current out of the base turns on the transistor. The resistor limits the base current and prevents the voltage between the emitter and base from becoming too large. Current flows from V+ into the emitter, out the collector and through the motor to GND. It seems that the transistor can drive more current than the 555. It works great!
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