The RPM control signal is technically the same as in the first generation tester. A 12 V rectangle signal needs to be created and send to the cluster. Instead of a dedicated transistor circuit the 12 V output drivers are going to be used. Required frequencies are comparably low, therefore no issues were expected.
As often as this sentence is uttered, the result isn't as expected. When trying it for the first time, the needle moved to a maximum of ~4000 RPM and fell back to 0 if a higher value was selected. Sounds like an issue with the signal quality. To further analyse it, my trusted Philips oscilloscope came to the rescue. Its doesn't offer any fancy calculations but it ideal to have a look at the overall wave form.
Looking the the input signal from the Arduino to the output driver, everything looked fine. It's 2 V per division on the display grid making it ~4.6 V peak-to-peak. As the Arduino's logic level is 5 V, that's fine.
Input Signal
The output on the other hand didn't look very much the same. Output voltage is at an OK 12.6 V peak-to-peak but the signal doesn't look very "square". It seems the output driver has issues dropping the voltage back to 0 V after being told to do so. The higher the frequency the bigger the problem until the RPM's detection circuit calls it quits.
insufficient Output Signal
A little research in the data sheet of the output driver showed that it's a so-called Open-Emitter driver and therefore can't actively bring down voltages at its output. As the cluster's detection circuit isn't good at either above waveform is the disappointing result. The good thing: It's easy to solve. Simply adding a 1,5 kΩ resistor between ground and output of the driver. Even if the driver is at 12 V the current at the resistor is very small (8 mA or 100 mW) but helps dramatically dropping the voltage to 0 V:
much better
Integrating this resistor into the tester would be possible without a new PCB design but as it's probably not the last change required, we are going to put on the list of upcoming changes for the next PCB order. For now let's enjoy the quick response of the RPM needle when changing the corresponding value on the tester:
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