Cheap Voltage Detector circuit

[thomasn101]

Hello everyone,

I am running the microcontroller on 5V power supply (using the LM7805 regulator). I want to be able to detect the input voltage of the LM7805 when it falls down to around 6V. I don't have any ADC port left on the microcontroller. All I have is a digial input. My goal is to build the simpliest and cheapest circuit possible. Maybe with one transistor and few resistors. Do you have any idea? Thank you in advance!

Best regards,
Thomas

 

[Lewish]

Opamp comparator @ $0.27, reference diode @ $0.20, 4 resistors @ $0.04

That is as cheap as I can do it accurately.

 

[nbucska]

What is the nominal Vin ? <nbucska@pcperipherals.com>

 

[msprayberry]

Lewish is absolutely correct that is about the cheapest that can be done. You say you are using a LM7805 but you only want to be alerted after the voltage goes below 6V, to error on the side of caution when using regulators (LM7805) if should be 2V above regulated voltage I suggest a 7V warning. I would suggest using the comparator circuit with a 7V reference.