I am a little stuck on weiring the digital outputs of my K8055 board. I am currently trying to interface my rowing machine to my laptop. The rowing machine is a fairly basic rowing machine although it does have a fan like air resistance system. The the its really lacking is a decent rowing computer. The one it comes with simply measures time, distance, speed and estimated calories burn. Since I am software engineer by trade I figured I would have a go at writing my own by interfacing it with my laptop.
The rowing computer simply measures the number of rotations of the fan and the number of strokes (movements of the seat up and down the rail). Both of these are detected by a reed switch and a magnet attached to the moving part.
My plan was to run both my interface and the original rowing computer at the same time, so I could attmept to experimentally work out the relationship to the input and the outputs of the rowing computer. That way I can work out the formula for each of the outputs.
So far I have the reed switch on the fan hooked up to the input on my K8055. Then I have my software setting one of the digital outputs to match the status of the input. My idea was to connect the rowing computers input (what was originally connected to the reed switch) to that, the problem is it doesn’t work. I see the output LED flash but never see any speed values on the rowing computer. According to my multi meter the rowing computer has a 2.5 vault between the two input wires.
I think my understanding of how the digital outputs work and therefor is wrong. So my question is how do I wire the digital output to effectively work as a switch completing circuit of the rowing computers input.
If I understood right you like to replace the reed switch of your rowing machine with the digital output of the K8055.
If you connected the more positive input pole of the rowing computer to the digital output of the K8055 and the more negative pole to the GND of the K8055 then the connection is OK.
The problem may be that there at the digital output of the K8055 remains some voltage even when the output is activated. The remaining voltage is about 0.7V.
Your rowing computer may not detect this as logical low.
If this is the problem then the solution is to use a reed relay between the K8055 output and the rowing computer input.
If you use a reed relay with 5V coil and the coil resistance 500 ohms (or more) you can use the internal +5V supply of the K8055 as driving voltage for the relay coil. You’ll find the +5V from the upper pin of SK2 on the K8055 board.
[quote=“VEL255”]If I understood right you like to replace the reed switch of your rowing machine with the digital output of the K8055.
If you connected the more positive input pole of the rowing computer to the digital output of the K8055 and the more negative pole to the GND of the K8055 then the connection is OK.
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Thats correct. However the rowing computer has its own power supply (2x1.5v AA batteries). So if the k8055 is outputting its own voltage won’t that cause problems. I really only need it to complete the circuit between the two rowing computer input wires (like the reed switch does).
I agree a relay would be an ideal solution because it provides separation between the two devices however won’t there be a potential problem with the speed at which the relay will need to operate. When rowing quickly the fan completes several rotations a second. Since there isnt a large load involved (2.5v when I tested it) is there some sort of semiconductor device I could use instead.
The K8055’s digital outputs are passive open collector transistor outputs. They are not outputting any voltage. There must be in the receiving system a voltage supply and a pull up resistor.
When the K8055 output is activated (pulled down) there is still some remaining voltage due to the darlington transistor array used at the K8055 digital output.
About the relay speed: Reed relays are quite fast. I think the speed is no problem in this case.
A semiconductor solution may be to use an N-type MOSFET as a switching component. But in this case you must do some more wiring: Add pull up resistor between the K8055 digital output and +5V of the board. Connect the digital output to the gate of the MOSFET, connect GND to source.
Connect the drain of the MOSFET to the more positive input of the rowing computer and GND to the negative input.