The LED stick bright when volt of battery model to load new get agree lower 8.3V ( for battery 9V). Which be minimum value that can agree, be a signal know that battery, want to load new already. The circuit can modify to go to apply to transistor radio for is warn change battery the group is new. By this circuit use SCR C106 perform edit the circuit. For drive a tube LED and use Zener diode use number , BZY85C8V2 400mW , perform compare with the level voltage the referable.