9 volt battery tester - Electronic playground experiment #23
Battery one hooked up to springs on the playground is represented by LED 1 on the left and is being used to test battery 2 represented by LED2.
Certain resistors are used to set up voltages at the base of the transistors.
“The resistor values were selected so that if the two battery voltages are equal then NPN2’s base will have a higher voltage and only LED 2 will be lit. In fact LED1 will only be on if your Playground’s battery voltage is at least 2V higher than that of the battery you are testing.” Page 37
Electronic Playground and Learning Center by Elenco (Model EP-50)