K8055 Digital input always active

Greetings from Norway!

Iv had a K8055 card in my car and tested it for some time. It has been working great, but suddenly digital input # 2,3,4 shows input all the time, even if no wire is connected to them. Input # 1 and 5 still works. Ive tested by disconnecting all wires except for the usb cable, and when I start the demo program the checkboxes for 2,3 and 4 is always checked. I guess I`ve damaged the board somehow, but:

  1. What may have caused this damage?
  2. What is most likely damaged?
  3. Is there somehow I may apply a “quick-fix” to this problem by remove the test switches or some other components? (Would be great to have it up and running by this wednesday, as I am due to demonstrate a project using this card.)


  • Andy -

One possibility is that the transistor array IC2 is damaged due to some electrostatic discharge or some other reason.
You may replace IC2 with IC4. Both are same type ULN2803A.

Thanks for the reply!

If IC2 is toast and I replace IC 2 with IC 4, may I loose some other functionality? I currently use all digital outputs and the analoge inputs. (Only analog outputs is unused)

Does it matter if the ground connector on the digial connector terminal is connected to ground or not?
How may I protect the card from electrostatic discharge? (Would be a bummer to instantly fry the second ULN…)

More testreults: (Not replaced IC2 with 4 yet)
When I connect the ground cable to the digital OUT ground terminal, (12v) I get 12v (-)on the 2,3 and 4 digital inputs when testing from + on the battery. # 1 and 5 shows 0 at all times. I have 12+ connected to the clamp terminal.

If I understand right your test results, they indicate that the inputs 2, 3 and 4 are defective.

Before replacing the IC2 please make this test:
Disconnect the 12V battery.
Only the USB cable connected to the card measure the voltage between the digital inputs and GND.
Should be about 1.67V from each input to GND.


This is the results:

Input 1: 1,63
Input 2: 0,04
Input 3: 4,52
Input 4: 0,01
Input 5: 1,63

This is how i normally has the card wired: (At least when the digital inputs stopped working)
A out: Nothing on any therminal
A in:
GND: Car ground
A1: Velleman K8067 (0-5v)
A2: Nothing
D out:
GND: Car ground
Clamp: Car battery 12+
D1-8 : Relaycard
D in:
GND: Nothing
1: +12 when not triggered, car ground when I want to trigger it.
2,3 and 4: Nothing when not triggerd, car ground when I want to trigger it.
5: Nothing

Hope this might help to shed some light on what i messed up.

This is one possibility to destroy the inputs of the IC2:

  • Digital inputs 2, 3 and 4 were connected to car GND.
  • The car battery + was connected to CLAMP and via the relay card to the digital outputs.
  • The K8055 card’s GND was momentarily disconnected from car ground.

I see… I`ll check ground connections again, and possibly split them it into several different locations.

Three more things:
1.The 4 GND terminals on the card goes to one commond grund, right?
2.DI 3 is connected to a Bosh SPDT relay that turns a +12v signal into grund “signal”. Might such a relay cause a electrostaticdischarge when deactivated?
3. Would a wrong input or a discharge on one of the terminals be able to destroy all three inputs, or would only the one be destroyed?

[quote]D in:
GND: Nothing
1: +12 when not triggered, car ground when I want to trigger it.
2,3 and 4: Nothing when not triggerd, car ground when I want to trigger it.
5: Nothing[/quote]

Indeed, it seems that input 3 has damaged different way than inputs 2 and 4.
If input 3 was connected to the relay coil this damage may happen.

How the inputs 2 and 4 were connected at that time when this happened?

I think the input protection diodes of inputs 2 and 4 may be damaged due to reverse 12V polarity.

If you look the internal circuit diagram of the ULN2803 you’ll see that there is internal diode from each input to GND. Negative voltage (>0.7V) to the input may destroy the diode and cause damage inside the IC.
datasheetcatalog.org/datashe … s/2801.pdf