RussellMc wrote:
>> Its to go into a reed relay telephone switch from the 1960s.  If
>> something went wrong it would print out
>>     
>
> (? - the circuit you are building 'would' or the original circuit did this?
> Presumably your circuit).
>
>   
>> ...what equipment was at fault by
>> burning upto 90 lines onto electrosensitive paper.  I need to take the
>> 50v from each "pen" and log which ones are live.
>>
>> Im sorry if Im not giving full details but some things you mention I
>> havent a clue what they are. Like a shift register for example?
>>
>>
>>     
> Probably :-).
> ____________
>
> Note that none of the questions were asking about shift registers.
> Rather, some of the answers involved shift registers.
> They still may be a good answer BUT it's looking like the opto-coupler
> solution may be workable and, if so, it saves a lot of extra resistors
> etc.
>
>
>
>                Russell
>
>   
sounds like it is drivers for 90 pin parallel head printer, not voice 
lines directly (but who knows?)
So common ground, no opto isolators needed
use resistor divider on each driver pin/pen to convert to 5V. Since it 
might not be exactly 50V, a resistor divider to reduce volts to 6V out 
from 48V in  and 4.7V zener .

the shift register is a fairly simple very cheap IC, one signal from PIC 
loads 8 pins of 0V or 5V, a clock changed by PIC then allow one pin on 
pic to read all 8 bits in turn. Since the shift register has a shift in, 
you can "daisy chain" 12 of them and with a single pin on PIC connected 
to all 12 of the shift register ICs (about 25c each) you read all 90 
pins of  the "printer driver". then 90 clock pulses reads them to PIC. 
This can be done at about 1MHz, so you can re-read the electrosensitive 
paper pins about 10,000 times a second.

Or you can use 12 x 8 way switches, CD4051. 3 wires select which one of 
8 inputs, and you read all 12 switches at once, change address and read 
again. This method since it is 12 bit parallel, will read at about 
120,000 times a second if using same clock rate as shift register to 
change address and read 12 pins instead of one.




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