Generate a ramp input with an Arduino Uno
A RC filter will create a slow slewrate.







simulate this circuit &ndash; Schematic created using CircuitLab

The circuit creates this signal:


A slow rising signal and a fast falling signal can be created with an extra transistor (the signal will be inverted).







simulate this circuit

For a linear rising and falling edge, a resistor network with a few digital pins is a simple solution. I think it can be created with an opamp circuit as well, but there are also special chips called "slew rate limiter".