Anyone can help me desing a simple circuitry?

Using PWM and PIR heat detecting sensor and also 555timer together with dc motor to turn gear from 1 end to the other at 180degrees motion,the gear will retract back to the 1st position once it hit the limit switch and back to the second position when it hit the other limit switch

Answer:
If you are using a DC motor, and you want it to reverse direction, you will need to construct an H-bridge circuit, using discrete transistors, or look on the web for a suitable chip (there are many special-purpose ICs designed for driving small DC motors without requiring external transistors).
The PWM, heat sensor, or 555 timer would only be important if you have very specific torque/speed specifications. In that case, your job becomes very complicated -- you'll have to look up design solutions online or in a textbook.
You don't need PWM or a heat detector to do this. Since limit switches are used, the 555 timer is not necessary either. Just have the second limit switch reverse the polarity on the motor when the second limit switch is hit or reverse the phase sequence if it is a stepper motor.
As the others said, PWM, PIR and the 555 are not necessary unless you plan to control the speed of the motor.

For a simple DC motor controller circuit, look for the L293D chip, it has an integrated H bridge. Look at this website for the datasheet, it even has an example for connecting the motor to control direction:

http://focus.ti.com/docs/prod/folders/pr...

My suggestion would be to use this IC and a latch to switch directions.

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