Controlling a K6714-16 relay board with Arduino

Our light system at home is based on several Velleman kits from the mid-90's. But lately they've been acting strange.

I was doing some experiments in controlling the relay board directly with an Arduino, but I've got some problems that I don't understand.

Here's the schematic of the board:

I'm running the basic arduino sketch which makes pin 13 blink. No magic there.

But when I connect this pin 13 to one of the relay inputs, and the ground of the relay inputs to the ground of the arduino, the relay is constantly on, instead of blinking.

I tried going through a resistor, but no change. If the resistor is too heavy, it doesn't go on at all, and if it's too light, it's still constantly on instead of blinking.

I also tried a voltage divider, thinking there might be some power left in the pin that needs to go to ground when it's off. But no change, the relay stays constantly on.

I found this topic from before:

...but the link referenced at the end gives me a headache. Can anyone explain me in human terms what might be going wrong here? Thanks!

Yeah, Pin 11 does the same. I can notice a very slight light variation in the LED indicator on the relay board though, so maybe if I can find the correct resistance it could be switched from the arduino?

In fact, all pins, whether or not they are active at that moment, make the relay go on. So maybe it's some base voltage trigger?

Alright, will try this next. Thanks!

alright, some extra info. thoroughly confused now.

if everything is disconnected, i measure a voltage of 18 volt between gnd and an input pin. if the k8000 controls the relay board and activates the relay, the voltage i measure is 1 volt. i can activate a relay by connecting an input pin to ground. it’s even encouraged to test the relays by velleman (so apparently it’s not short circuiting?)

i really don’t understand how this circuit works.

we have jumpers instead of the IC’s described in the manual, for “open collector” control. the jumping is also described in the manual.

can anyone help me understand the logic behind the circuit?

by the way, connecting vb to input pins short circuits everything and gives sparks.

Yeah, I'm measuring it on the velleman relay board, but we don't have the ULN2803's, we use it "open collector"? Instead of the ULN2803's there's a jumper bridge.

Looks to me that I'd need the ULN2803 to be able to control them properly with an Arduino. It'll probably make a whole lot more sense then.

But what bugs me still is that I don't understand the logic behind the 18V.

The entire discussion to this point is for nothing.

Hey, at least I'm learning something.

Thanks for your help anyway.