Hello everyone,
Is it possible to use a step down transformer for both voltage sensing and power for the Arduino?
My idea is to have a 240V step down to 9V. The secondary will feed two parallel circuits. The first one will have a bridge rectifier to a 7805 circuit and be used to power the Arduino. The second circuit will be used for voltage sensing as outlined on the site.
Thank you for your assistance and comments.
I.
Oh dear, Bill. It’s impossible with a full-wave bridge. I explained this in the old forum - you need to look at how the voltage on the transformer windings jumps about with respect to the d.c. side as the current switches from one pair of diodes to the other.
You can do it easily with two isolated secondary windings, but not with a common secondary winding. You need to use another isolating transformer - which largely defeats the objective, or an op.amp with differential inputs, to achieve the necessary isolation.
Thanks for your comment. I shall proceed with separate circuits.
Nuts… Scratch one erroneous post. 
But you can do it with a centre-tapped transformer and a full wave rectifier - but it’s not a bridge configuration. That obviously can’t be done with a standard a.c. adapter, as it has only a two-wire connector.