Quote Originally Posted by boldford View Post
Many thanks for sticking with this one. My 'scope is years out of cal' but I'd say the swing on the modified channel is roughly between +1.6v and +4.8v which suggests the ON resistance of the opto is the limitation.
Ah, so the transistor is not saturating. with 100R load, the collector current therefore is (5 - 1.6)/100 = 34mA, whereas it needs to be (5-0.5)/100 = 45mA, so the LED needs to be driven harder as previously discussed. However the 74HC14 clamps at 20mA, so there may be a limiting factor here... try reducing R1 from 1k to around 450R or so, theres no point in going lower... if that doesnt improve things then its not going to get better without significant rework