Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

[televizora]

Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

Use MC and relay with NC and NO contacts. The MC keeps the relay open, until the voltage threshold is reached. Not very elegant but you can set any threshold voltage and it will work. You need NEGATIVE logic.

[eccerr0r]

Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

Not exactly sure the TL431 can be perfectly used by itself, think that an op amp using the TL431 is still needed. TL431s don't drop their voltage to anode voltage, they work like zener diodes where the voltage will reach a plateau and stay there. This works fine for series connected LEDs which will only turn on when its voltage and the TL431 voltage is met - and the total drop will be the sum of the two.

sounds like what's wanted is the voltage drop to be 0 or close to it when voltage reaches 4V, which would require another amplification device. A TL431 plus a transistor or op amp will do.

Actually the 2.5V of the TL431 + 0.7V of a transistor = 3.2 V which may very well be close enough to do without any programming resistors. Current limit is still needed else 614 teraamps will flow...

[stj]

Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

6 months from now when you actually make something, you will probably find that the sensor output is digital like i'v been saying!!

[stj]

Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

Use MC and relay with NC and NO contacts. The MC keeps the relay open, until the voltage threshold is reached. Not very elegant but you can set any threshold voltage and it will work. You need NEGATIVE logic.

that will make a great buzzer/RF jammer when amplifying a digital stream from a remote control - read the thread fully

[televizora]

Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

that will make a great buzzer/RF jammer when amplifying a digital stream from a remote control - read the thread fully

You can always use MC directly if you work with low power signals. Yes, I haven't read the whole topic. The relay was only if you need high current switching capabilities, for example use MC indirectly to switch a high power circuit, for which you cannot use the micro controller directly.
There is a difference between using a low power signal to switch a high power circuit and to amplify a low power signal. If you have 5V source that can drop to 4 or less volts for example 2.5V, then you have a point that has 2.5V voltage difference from both GND and 5V. It's interesting if you can use it as a reference point, measure and use this difference to switch the circuit.
If we had the schematic, I think that we can do much better.

[stj]

Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

he has an IR reciever and thinks he can boost the range by boosting it's output - it's that simple.

[televizora]

Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

he has an IR reciever and thinks he can boost the range by boosting it's output - it's that simple.

I think that in this case the output of the receiver should be digital and I dont think that it would be easy to boost the range this way

[budm]

Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

So far OP refuses to provide the spec of the IR receiver, the actual schematic, the scope screenshot of the IR receiver output, etc.
See original thread here: https://www.badcaps.net/forum/showthread.php?t=81784