Forums » CircuitLab Feature Requests

I am writing up some instructions for improved interfacing of a rotary encoder to a microcontroller (Arduino); and want to demonstrate a simple way to deglitch the noisy switching using an RC and a comparator. The standard choice for a cheap comparator is the quad comparator LM339 (or dual LM393) or its CMOS equivalent, which have open collector or open drain outputs.

Unlike some odd requests I see for obscure devices (vacuum tubes etc.) , I think this falls squarely into typical use for a tool like this. A comparator with a few parameters would be nice. Something with parameters even simpler than your opamps:

• output impedance

• Open collector/drain or rail-rail output (or maybe this is two devices, the latter specifying supply rails as with the opamp devices)

• delay

• input bias current

I think gain would be a confusion factor, though allowing for a hysteresis might be nice from a generic system level comparator block.

I implemented my comparator with your logic output comparator block and a MOSFET, but this adds an extra inversion such that the nominal implementation using an OC comparator would need to switch the input polarity. (Obvious, but a confusion factor for many of the sorts who will be using this tool).

By the way, my congratulations on the high quality of this initial release; the bugs are minimal, I use an unsupported browser (Safari on OSX) with no problem. Not suitable for professional use, but quite functional and VERY NICE LOOKING schematics. (Better than most expensive CAD systems!)

The recently revised voltage controlled switch element (the threshold and hysteresis are now parameters) allows an even simpler way to emulate an OC comparator:

You can fake an open-collector comparator by using a fast op-amp with a diode, cathode to the output. Only about 300mV away from reality.