I thought it best to start a new thread with a different design.

I changed designs to use a mosfet style transistor I found a nice one here The schematic is below. I could not find the BUZ11 mosfet or the schottky diode locally so I made some substitutions. It all works good , I would just like someone to look over my shoulder to see if I did anything really bad.
I first assembled it on the bread board with the only changes being an IRF510 mosfet and a 1N5400 diode. It ran my motor , but the 50K pot only ran the motor between about 75-100% . So I started playing with R1 the pot and C1. What I ended up with is in the schematic below. With the 10K pot I now have about 95% of usable adjustment. The motor starts at 2.2 Volts and I have a max of about 10.3 volts ( Out of 12.1 V ) With this circuit the motor is drawing 2.3amps no load and about 3 loaded up. I did put a heatsink on the mosfet.

Unless I did something really stupid I think I will go with this design.


Original design is on top and the modified design is below.

 

Well after some trials and tribulations I have a working PWM. I went with the authors original design with the only substitution being a IRF510 mosfet. This project only needs about 3 amps max and more likely 1-2.
I would like to thank all who helped in the last thread and for urging me to go with the mosfet design. It works well. It will start my motor at 1.3 volts and has decent torque at 2.5 volts. It looks like my normal operating range will be in the 5-7 volt range for the project.
fellow member jpanhalt lives nearby and wanted to see my mill cut the board and stopped by the other day. It came out pretty good. I did make a few changes to the board layout after cutting it. Mainly wider traces on the output side. I have the schematic and board layout in Eagle format if anyone would like them. I also have the G-code if that would be useful to anyone.
Here are some pics.

 

Scott, That looks great. I am delighted it works. John

 

can i use this circuit to drive a 12volt dc motor with at least a 4amp load?
if not what would i need to do to make it work?
many thanks RCT