I am attempting to build a simple tester for my nephew. It is a bit of a gag in that I am building it on a CR2032 battery holder. My electronics theory is rusty and I have been scouring the web for info. The tester is a continuity tester and a 6v/12v battery low-voltage tester that uses an LED as the indicator. My question is this: do I calculate the resistance for the overall expected voltage in the voltage portion of the tester? Please refer to the schematic I have attached. The LED is an NTE30035/2.4V/25mA. The switches are SPDT. I appreciate any and all help. Also, I thank you in advance for the help.

 

You add the zener voltage to the LED voltage to get your losses in the tester. Then size the resistor for the current you want the "leftover" volts to drive through the LED.

A 12V lead acid battery is pretty much dead at 12 volts. You'll be expecing more like 12.5 volts. Subtract (2.4V and 9.1V) = 1 volt across the resistor.

That should get you started.

 

#12 - Thank you for the help. I really appreciate it. My electronics training was back in 1981. I use it about once every 15 years or so.