screening / glitchy operation - sanity check

[Jonathan]

Your code looks like it should be fine.

I've switched to a 9VDC battery, not much difference, after which I've earthed the driver chassis, which dropped the noise levels significantly.

So did it solve the problem? Since using a battery didn't make a difference, that implies the noise is not emanating from the supply. The most likely source is the 'mess of breadboard wires'. You should aim to keep the wires short to minimize stray inductance, keep power wires close together for the same reason and use decoupling capacitors.

For example, you can see in this photo how the breadboard on the right uses much longer wires than necessary and the capacitor is a long way from the IC. My one on the left is neater...



The circuit above still worked as it operated at quite low frequencies (where stray inductance is less of an issue). If it hadn't then it would be worth rearranging it.