Thread Starter

Alberto Cestari

Joined Jun 21, 2022
Hi guys,
my customer ask me to realize one digital input (the pin source the voltage to make reading) with large hysteresis and some 18V pulse for contact cleaning.
For make digital input with hysteresis is not a problem (LM2903 comparator with positive feedback) but for contact cleaning at 18V I'am not sure wich is the best way for implement it.
Now, in my design, with external trigger we change the voltage sourcing from 5V to 18V (small pulse of 0.5mS) and I filter it with RC filter befor the comparator.
Below my actual schematic of circuit.

Does someone has already implement it?
Any suggestions?

Thanks for any suggestion.



Thread Starter

Alberto Cestari

Joined Jun 21, 2022
The input contact is connected between pin 3 or 5 to GND.

My contact is normally open and the wetting current circuit is provided by 18V pulse signal (see Q14 and U24).
My dubt on my circuit is: how I can read right when voltage change accross the connector pin?
I have put one RC (R131/132 and C187/188) for limit the voltage on comparator input, there is any other solution?

Normally the wetting current is only applied into closed loops circuits?


Joined Sep 17, 2013
I don't see any need for raising the voltage to 18V. Wouldn't a fat capacitor across connector pin 3 (5) and gnd suffice?


Joined Aug 7, 2020
Normally, the wetting current is applied constantly, just by using a smaller-than-usual value of pull-up resistor.
If you have a 24V supply, I don‘t see the point of deriving an 18V supply for the wetting current - just use 1k instead of 750Ω.
Here’s a dead simple way of doing it using a 555 (but does need a supply that is 15V or lower)


Joined Jan 23, 2018
How about just having the open voltage across the switch be 18 volts, very much current limited. And any cpacitance value set to provide the energy that the customer requires. A voltage divider with values to procide the specified current. Simple and cheap, and reliable.