This is just a simple revision which eliminates the zener D9 and replaces it with D13 and D14. I have them listed as 1N4006 but anything you have on hand will do, eg 1N914. You can apply the same change to Bill's simplification. Also added is the recommended 0.1uF bypass capacitor. This can be the same type as C1, or a even a tantalum. Minimum WV ought to be 25v for it.

If you have any questions or comments feel free to email me at paul.moir@gmail.com.