Split power supply and Battery power difference

[april]

I have a circuit running on 24V which is supplied from a split rail power supply. -12V one side and +12V the other . Zero in between but not used here. This is one of those kit power supplies popular 20 odd years ago in magazines.

It runs my project which is a LED spreader light with a constant current source attached inside and fed 24V from the above .
The light is switched on and off by an ATTiny85 chip through two transistors which turn on a MOSFet.
and the ATTiny is powered by the same supply dropped down with a LM78L05 voltage regulator. A PIR movement sensor takes 5V also and tells the chip when movement is detected and an LDR tells the chip when it is dark .

So there it is and it all works particularly well, however...

When I try to connect it to a 24 V battery bank it wont work , the symptoms it displays are strange and changeable

Can anyone suggest anything obvious that might be going on ?

 

[cowboybob]

Makes no sense at first blush.

Exactly what are the symptoms?

Do any of the internal voltages/currents change with the switch from PS to battery (can't imagine that they would)?

 

[april]

Thats just the point . It does not show cosistent symptoms / The monitor LED flashes differently , the MOSfet switches on randomly and sometimes stays on sometimes there is no activity at all , I have not done a full check in the field , it is rather difficult. Its 20 ft up the mast.I doubt that internal voltages would change , why would they

I'm looking for suggestions that might indicate what a different souce voltage might affect/change?

I'm going to try putting caps on either side of the voltage regulator in case its oscillating and interfering with the ATTiny85.

 

[april]

Yep that fixed it . Well thats surprising.. Battery power makes the 78L05 occillate. I would have thought the lab power supply fed by 240 A/C would be the more likely.
Live and learn

 

[cowboybob]

Interesting result. Although it's been my experience that it's always a good idea to put in the caps on the IO pins of the LM78xx regulators.

Maybe some tiny ripple from the split rail supply was suppressing oscillations??