On 2014 Mar 28, at 10:38 PM, John Ball wrote: It's a little hard to say without seeing the original (or rest of) the circuitry, and I'm not familiar with the RA82 internals, particularly the frequency and periods at which this circuit is operating, but a couple of comments in general: While optical sensors are comparitively clean relative to mechanical contacts, the edges can still be a slow transition by electronic standards, due to the 'grey' region as the edges of the optical interruptor take time to pass through the light beam. The 'LS14 inverter is a Schmitt-trigger device, it would have been there to clean up the transitions from the opto-sensor. The Schmitt- trigger function provides hysteresis on the transition thresholds, but it's also providing voltage gain, which sharpens the edge before it's fed on to following circuitry. If the following circuitry is a counter or edge-triggered device, the slow transition directly from the opto/3904 may be producing erratic triggerring. In addition to the missing Schmitt inverter, there is no pull down resistor on the base of the 3904. While the 3904 may still switch as it is, the on-to-off transition of the 3904 may be slow. It also may not be going fully off, as even slight illumination of the opto- transistor will start it conducting and feeding current through the base of the 3904, starting to turn the 3904 on. You might try adding, say, a 10K R to ground on the base of the 3904, or from the opto- transistor emitter (pin-3) to ground. Re the LS14 'leaking a volt' onto the input: a TTL input will tend to 'pull-high' as the conduction path for a TTL input is indeed through to +5, it's normal to see some +V on an (open) TTL input. TTL is current-sinking logic, the 'excited' state of an input is for electron current to be flowing from ground, through the driving output device, into the TTL input and through to +5. The better question might be what was wrong with the original circuit(?)