HELP?? Convert to 12V DC

Thread Starter


Joined Aug 27, 2007
Hi people.
Got this circuit all working, but it runs off mains 240V AC. I would like to convert it to run of 12V DC.

It has a 0 Crossing test, to check the 240V so need to fool this into thinking its all good, to run the program in the PIC.

Any ideas? I can power up from 12V DC by supplying 12V to D1. But the self test fails.

HELP please :)



Joined Apr 20, 2004
It is impossible to tell what is going on from your description. Can you post up a schematic?


Joined Jun 13, 2007
Are you sure the zero crossing "test" isn't actually being used as the frequency reference for the clock?


Joined Apr 27, 2007
Why don't you use a transformer to lower the voltage first. It is bad practice to connect a LM78L05 regulator directly to the rectified mains voltage, even if you use a resistive association in series and a zenner to control voltage (that is C1, R1, R2 and R15). Don't forget that you have a variable load, and so, the current biasing the regulator will vary, and the regulator may deep the voltage from the zener. Also, your project without a transformer will drain much more current than with it. More current translates into more power, and since with that extra input power you will have the same output power, expect most of it to be converted to heat.

In other words, you really should consider using a transformer.