IR schematic
Cyclone, ground usually is the same as the negative side of your power source - when you have a single supply, as in most digital circuits*. Analog op-amp circuits often run off a dual-supply, where the negative supply is not the same thing as ground. Instead, there are (+) and (-) supply potentials relative to ground, as my previous post illustrates.
In the IR circuit,
https://www.aaroncake.net/circuits/irrec.gif
the schematic is drawn showing a dual-supply arrangement, otherwise IC1 pin 4 would be shown connected to ground, not -9V. It's the use of the two different symbols, -9V and GND, that indicate a dual-supply. This circuit probably works fine with a single-supply, since the output from the op-amp doesn't need to swing all the way to ground. What op-amp are you using by the way? Older ones like the 741 don't have rail-to-rail output swings, many newer op-amps do.
(If the circuit doesn't use a timer or one-shot, what is IC2? I assumed it was a timer chip, used as a one-shot)
Like I said in the earlier post, -9V isn't necessarily ground (for example, in a dual-supply), but the (-) terminal on your battery can be ground (the case when you have a single supply). Is that confusing enough?
When you say you left all the ground pins unconnected to anything, the circuit still worked?! That's one failproof circuit! Can't explain that one...
- CAL
*You'll sometimes see positive and negative signals in digital interface circuits as well, true RS-232 levels, for ex.