I have decided that even though the relay will work as isolation for the printer signals, I see that it would take a lot of time to perfect and build the number that I need. Sooooo, I am rolling my own opto-couplers. @Morning.Star provided the seed to go this way and it is so simple. The CdS cells are from old style night lights found in area thrift stores and the Leds I already have so take a look at the construction images and smile a little. BTW I have seen this method else where on the internet and it works quite well. With no light the cell has a resistance of 15k ohms and full led light it goes down to 400 ohms. IO and the printer work well with it.
The SCR? and 1meg resistor might find a use later on.
Good swing in resistance.
Here is a very short video of two couplers connected between IO and the printer. No timing or signal level issues :-)
Discussions
Become a Hackaday.io Member
Create an account to leave a comment. Already have an account? Log In.
CdS cells don't have a specific resistance for every cell and level/color of illumination, so much as a range that they almost all tend to fall into some part of. (...not to mention ambient temperature variances!) They're not exactly, er, precise instruments... if you're depending on a very specific resistance for light and dark detection, instead of the previously-mentioned set of ranges, you're going to have a lot of trouble.
If I were doing this, I'd find a way to rig up a comparator circuit and have a CdS on one side and one of those eBay issue blue-box precision-turn pots on the other. Once the two are /roughly/ equal, a bit of hotsnot aka hot glue (or superglue or...) will keep the pot in place for a remarkably long time. Any significant variance in illumination, at that point, will be enough to upset the comparator and make it squawk...
That said, I tend to think in chips, not discrete componentry... I don't know if you can make a comparator easily out of the sort of transistors you're using. I'd use an LM324, probably, as the combined output control for four optocouplers... but, again, that's a level /above/ you right now, and I simply don't know enough to advise on the level at which you're working. Maybe look at old triode circuits and tinker with adapting them to RTL?
Are you sure? yes | no
Using discrete transistors, a comparator is quite easy to do, it's called a differential amplifier :-) (that requires 2 or 3 transistors and some resistors)
Are you sure? yes | no
Time will tell. Have a couple of these now and both work well and the requirements I need to meet are not at all narrow. Just switching the 2 volt signal at the printer and it's working very nicely :-)
Much easier to wire up than the relay ;-)
Are you sure? yes | no