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Unidirectionnal, Op-Amp-Based Interface

  Introduction  
This is a very good interface for unidirectional purposes. It works with many computers, but not lap/palmtops. Its simplicity makes it very popular.
  Pictures  
None yet. I have to find my interface atthe bottom of my dive bag and it might take a while...
  The Schematic  

There is no hysteresis with this circuit. The first input of the OA if fed with .7V, the 22K resistor forcing a current in the diod so that the voltage appears. The OA is then used as a comparator with a threshold value of .7V. Since the Aladin outputs a 0-3V signal, this value is OK: somewhat in the middle of the voltage range. One might want to add a diod in series with the first one to get 1.4V as threshold value, nearer the center of the 0-3V signal.

Not much to say about this circuit, but it works!

  The Making-of  
Easy does it... There is no special precautions but the one you usualy find in electronics (don't overheat components, use IC sockets,...)
  Epilogue  
Very good solution for the beginner in electronics. Requires few skills and is reliable. If you want to show-off for your diving girl/boy friend without investing too much, it's pretty cool!

The only problem encountered is when the supply is not symetric, like +7.6/-5.6 . Although this situation might look strange, it happened to me with my new notebook. If the power is asymetric, the voltage V+ + V- is added to the threshold. For the voltages mentionned, this yields a threshold of 2.7V instead of .7V. It appears the circuit is not working properly for these values.

  Enhancements  
Steffen Klupsch sent me two interresting enhancements for this interface:
  • Replace the diod with a LED: the LED lights up when data is transmitted.
  • Use the second op-amp available on the chip for better performance: just add it before the first one, in follower mode (link output with -input and attach Aladin port to +input).

Here's the schematic proposed by Steffen:

Steffen also realized that some versions of the LM358 do not work. Indeed, we work at 20kHz and need about 10V at the output. The op-amp slew-rate must therefor be over 10V/(1/20000/2 s) = 0.4V/us. In order to have a safe margin for this value, do not use an op-amp with a slew-rate under 1V/us.

Last update:
October 17, 2001
  Author: Damien Douxchamps < d douxchamps at tele ucl ac be >
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