Hi Ron,
I'd go along with Torbens idea. It's practical and cheap for the desired effect.
You can increase the effect by using a filament type (or two paralleled bicycle head light lamps) with a resistance of 4Ω for a current flow of assumed 250mA and a desired voltage drop of 1V and doesn't require any cooling.
The lamp will glow dimly when the power supply is OK.
Best bet however is using a low dropout voltage regulator. (LM2940-CT5).
It has the same regulation characteristics as the well known LM7805, but for a low drop out regulator the input voltage can drop to VIN=VOUT+1V (LM7805 - VIN=VOUT+3V).
Boncuk