How to set a threshold for lm339 when used as a comparator between two signals.

[Polly86]

I want to make sure that one of my signals is 5V . To do this I was recommended to use a comparator like an lm339 and compare my signal to the Vcc of the circuit, that only deals with TTL levels. Now I wonder if there is any threshold or how I could set this. For example, I could live with a signal that is 4.8V but not 4.0V…any suggestions?
Thanks!
/Polly

 

[bychon]

Use resistors from Vcc to ground to form a voltage divider set to your preferred voltage and connect that to the negative input of the comparator. When your "signal" on the positive input excedes the set point, the comparator will flop.

 

[Mikebits]

I want to make sure that one of my signals is 5V . To do this I was recommended to use a comparator like an lm339 and compare my signal to the Vcc of the circuit, that only deals with TTL levels. Now I wonder if there is any threshold or how I could set this. For example, I could live with a signal that is 4.8V but not 4.0V…any suggestions?
Thanks!
/Polly

For TTL devices a voltage level greater than 2.2v should suffice as a logic one. Four volts should not be required for TTL levels.

 

[MikeMl]

You also have to check the "common mode range" of your comparator inputs relative to the VCC that it is operating from. AFAICR, the common-mode range of an LM339 stops at about Vcc-1.5V, so to really compare something to ~4.8V, the supply voltage to the 339 would have to be 6.3V or higher.