I use photocells connected to very "consumer" gate automation boards. Those photocells output the infrared beam status via a relay with potential free contacts. The relay is the Hongfa HFD23 (datasheet).
I use the photocells with several types of boards; normally there are no problems but, in rare cases (< 0.1%), there are problems with a particular kind of board - let's call it "board type +24V".
The majority of the boards have the following schematic:
When the infrared beam is detected, the relay closes the contacts and the voltage at the pull-upped input goes down to 0. If the beam is obstructed, the relay opens the contacts and the input is pulled up to +5V.
Using a scope, I see that when the relay closes the contacts have some bounces, for 1-2-3 milliseconds, but that is managed by the firmware of the board and there are no problems.
The "board type +24V" instead, sometimes has problems. The schematic is different:
Basically this schematic is the same as before, but the input works with 24V dc, and the logic is opposed: when the photocell is OK, the input has +24V on it. Again, bounces of the contacts are filtered via firmware. Note: I am not really sure about this schematic, but I measured the input with a multimeter, and it showed precisely 30k; I see in fact two resistors of 10k and 20k on the board.
With this board, using a scope, sometimes I see that, when the relay closes, the voltage rises very slowly: it can take up to a second to reach +24V on the input. I imagine that, with a signal so slow, the digital input goes crazy. This happens rarely, and only with a few photocells; but the defective photocells never give problems with the other boards working with pull-up to +5V.
Reading carefully the relay datasheet, I see this statement:
Minimum applicable load: 1 mA 5V
with the note that the statement is only a reference value.
Doing some calculation, I see that the current for the +5V board is 0.5 mA, while for the +24V is 0.8 mA. Both the boards do not respect the minimum applicable load of the contacts, but the +24 board works worser even if the current is higher. I would understand better what the issue is. I suppose that all has to do with the physics of the contacts, which in this case are gold plated if I well understand.
My question is: may it be that the minimum load current is in relation to the voltage, for what the relay is concerned? I mean: the ratios of the current vs voltage is 0.5mA/5V which is higher than 0.8mA/24V, and so the +5V board is favoured. In other words, its input impedance is lower.
Or would it be better to find a relay able to cope with very low loads, if those relays exist?