making PT reader, was re:weird find: papertape "patch" puncher

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sat Mar 6 11:28:33 2004

> One further catch if you are building a tape reader ... or doing a "radical
> rebuild"* on a very old design: make sure that the power to the light source
> is reasonably well smoothed ... an AC ripple here can lead to mysterious
> inconsistencies when trying to read tapes :-)

Indeed. Better designed readers are relatively insensitive to light
source variations [1] but I suspect a 50Hz/60Hz/100Hz/120Hz ripple would
cause problems.

Most readers seem to use 12V (car) or 24V (lorry) bulbs. They have the
advantage of being easy to obtain, and can be run off easy-to-make
regulated supplies.

[1[ The Trneds I mentioned have a total of 10 photodiodes in the read
head. One on each data track, 2 on the sprocket track, spaced 0.25" (2.5
characters) apart. A cheacter is in the right position to be read when
the signal from the feed hole sensor exceeds that from the other sensor
by a certain threshold. A weighted average of the signals from these 2
sensors is then used as the threshold for the data tracks.

This system is relativelty insensitive to light source variations,
transparanecy of the tape, etc. There are treakers on the PCB to set
various thresholds, etc, but I have never found their settings to be
critical!

-tony
Received on Sat Mar 06 2004 - 11:28:33 GMT