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Unidirectionnal, Op-Amp-Based Interface
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| 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.
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