On Tue, 09 Sep 2008 17:50:35 -0400, Bob Engelhardt wrote:


The 4 resistors that are there must be 2 current limiting on the LEDs
and 2 pull ups on the detectors. Wait - this was made in 1976: did they
have LEDs then?


Yes. Commercial red LEDs were avialble in the 60's; I think by the 70's
they were on to other colours and higher output levels.