The fundimental problem is the 555 output is not capable of going all the way to the plus rail. Like I said, there are ways around this, I worked OT this weekend, which always wipes me out.
PWM will work, but it will only slow it down from the max you now have. Let me tweak the existing schematic and get back with you. Be back shortly.
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I've eliminated D1, since it wasn't doing anything.
C6 is the important part of making it work, did you view the page I pointed you to? R5 will protect the transistor from surges, but is less important.
You do need R2. See this page.
Common emitter will work, but it has to be off for the motor to be able to generate a signal. The problem with the 555 is it is switched either on or off, hard. There is a transistor connecting the output to the power supply either way, with this scheme there is only a transistor connected when the motor is running.
I tried replacing the output transistor Q1 with a 6 Volts Relay. The out put of 555 goes directly to the relay. The relay acts as a switch between the 6v battery and the motor. When the output of 555 goes high the relay closes and i get the full voltage of the battery for the motor..
however when the out put of the 555 tries to go low, the relay is not opeaning up and there is sputtering inside...
how to overcome that?