I just bought one of these and strangely, I don't get the single-shot action in "AUTO" mode. Instead, it repeats over and over, without me even touching the button. The button doesn't work at all in this mode. In "MANUAL" mode, it performs as expected with a shot that lasts as long as I hold down the button. This is pretty useless as a one-shot, metered dispenser. I've been in touch with the manufacturer and they'll have me send the whole unit back. Frankly, I just want to fix it myself but I can't figure out what is wrong with it.
PIN 3 seems to be held high by a resistor from VCC, and if I put it on my scope, I can see that it is occasionally being triggered by I-dont-know-what. I was probing around, looking for a short to ground that would drive this pin low, but I found nothing. I was hoping to find someone out there who could provide insight rather than me trying to reverse-engineer their circuit.
What I can tell you is, after looking for obvious signs of fault (low voltage - nope 24V), bad components - nope, all values are as labelled, etc., I *did* find one diode that was installed backward, but after desoldering and reinstalling it as indicated on the PCB, nothing seemed to change.
It *is* normal in AUTO mode for the button to work and for it to dispense a SINGLE shot of liquid, is it not?
Mine, incidentally, has two PCBs - a power supply that rectifies and filters (barely) the AC, and a second containing the 555 and interfaces with the DIPs, pots, switches, etc. It carries the part number 982A+ (note the plus, which apparently means plus one more PCB :-)