Read 24V with an Arduino

[jelezarov]

Hi,

like the title says, my aim is to detect on/off states on a 24V input with a microcontroller (arduino). A pretty common problem, for the looks of it, but none of the found solutions meets all of the following criteria:

- the input may vary to some degree (22-30V);
- ideally no heat generation, no 1-2W resistors;
- as small and with little parts as possible;
- and it has to have a LED

So I came up with this:


On a breadboard this seems to work so far, but I wanted to show it here, because most of you are lot more experienced than I am. So, can something be improved in my circuit, considering the above expectations ?


Cheers

 

[barry]

why not just have a voltage divider on an input and drive an LED with a separate output?

 

[KlausST]

Hi,

Add a 1k resistor from base to GND, this ensures higher noise immunity.

Klaus

 

[doddielektronik21]

1k or 10k to gnd not work

 

[jelezarov]

@barry - then if I try to measure more than a couple inputs, the MC will quickly run out of IO pins

@KlausST - thanks for the suggestion

 

[d123]

Hi,

Is that quite usual, to have say 24V into the base and 5V on the collector of an NPN? You can do that, can you, based on Vcb ratings, ignoring Vbe? I thought Vbe was usually 6 to 7V max. I feel a little confused in this thread, I haven't seen that before. Where's the 24V going? Is that schematic wrong? Thanks.

- - - Updated - - -

Hi jelezarov,

If that is not correct because of the BJT Vbe abs. max. ratings, then a 38k or 3.8k into the base, and a 10k or a 1k to ground will divide that 24V down to 5V.

 

[barry]

@barry - then if I try to measure more than a couple inputs, the MC will quickly run out of IO pins.

You said "a 24V input" not multiple inputs.

Maybe you need a multiplexer. I don't think you're giving us the whole story.

 

[CataM]

Is that quite usual, to have say 24V into the base and 5V on the collector of an NPN? You can do that, can you, based on Vcb ratings, ignoring Vbe? I thought Vbe was usually 6 to 7V max. I feel a little confused in this thread, I haven't seen that before. Where's the 24V going? Is that schematic wrong? Thanks.

The Vbe(max) is not exceeded. It is ~0.7V all the time.

If that is not correct because of the BJT Vbe abs. max. ratings, then a 38k or 3.8k into the base, and a 10k or a 1k to ground will divide that 24V down to 5V.

10k-1k-BJT does not make a resistor divider because of the BJT. The BJT will clamp its Vbe to ~0.7V, so there will be no 5 V.

 

[jelezarov]

Thanks all for the replies!

@barry - you are of course right about my description The current experiment I am working on is about watching 6x24V inputs. Anyway I though that in general would be easier to add some more small components than another Arduino if one needs to watch more inputs, so I toke this route. I will check the multiplexer approach too, thanks for the suggestion. It is always nice to learn new stuff.

 

[bobTPH]

Why not this?

input -> resistor -> LED -> GND

If you use white or blue, you will get about 3.0 to 3.3V at the top of the LED when on.

Bob