I've been following a few websites into making my own homemade optical connector but have found that mine is a bit sporadic if it works or not.

Original Link
http://omapalvelin.homedns.org/electr/fluke-cable/


I've tried using the above circuit on a laptop with a dedicated COM port (which so far vaguely works) and on a much more up to date laptop with a USB to RS232 converter.

I'm about to try the CP2120 USB version but it would good if anyone has any experiences to share on this...

 

Hello,

Did you measure the voltages accross the two capacitors?
The voltage might be to low for a correct working of the circuit.
You could try to use an extern ± 12 Volts powersupply.

Bertus

 

Do you have an oscilloscope so you can observe the waveforms?

Some hysteresis on IC1A may help. Connect a resistor (experimentally test value, perhaps 50kΩ to start) between IC1A-1 and IC1A-3.

You might also try reducing R2 to 5k ohms or so.

 

Have you measured the voltage on pin 2 of IC1? It should be somewhere near 0V, but tweaking it might get better results for you. If you could view pin 3 with a scope, you'd know better where to set pin 2 voltage.

 

thanks guys... Did some measurements this evening:

Between ground and...

Pin 1 : -6.8V
Pin 2 : -3.7v
Pin 3 : -6.7v

When there is a signal (TV remote)...

Pin 1 : -6.8v to 2.2v (a big question mark hangs over whether this is enough to change state to high).
Pin 2 : -3.8v

Pin 8 : 6.6v
Pin 4 : -7v

Probes measuring between pin 4 and 8 : 13volts

I'm wondering if the voltage output is enough to trigger a high state in the computer's port. Shouldn't this be at least 3 volts?

 

Do you have an oscilloscope so you can observe the waveforms?

Some hysteresis on IC1A may help. Connect a resistor (experimentally test value, perhaps 50kΩ to start) between IC1A-1 and IC1A-3.

You might also try reducing R2 to 5k ohms or so.

I've just tried a 4.7 and then dropped to a 2.2k resistor and it seems to be alot better in establishing a connection. Equally, when there is a signal, pin 1 peaks at 4.7v (low at 5v).

 

It kind of works, although connection keeps dropping and won't work with a USB to RS232 adapter (prolific or CM340).

but here is a screen grab when it does work... (one I downloaded earlier in one of the attempts it half worked)

 

i just tried a 50K hysteresis resistor between 1 and 3... sadly just the same results.

 

I managed to get another screen grab, this time of a remote control being fired into the IR photo transistor.

I'm measuring between pin 1 and 4. Any help?

So far i've tried a lower value R2 and a hysteresis resistor between 1 and 3.

 

I built the circuit at http://omapalvelin.homedns.org/electr/fluke-cable/ several years ago and found that it was very unreliable, the main problem being with laptops and USB/Serial adapters, they just dont produce enough voltage on the serial interface.

Although RS232 will work at the 5V level, in practice the voltages produced by the Diodes and Capacitors fall far short of this and not able to power the comparator etc.

So I solved the problem by building this one from YouTube http://youtu.be/9fvNy7Vitys if you read the description there is a link to the circuit diagram.

I tested this on several laptops and the notoriously useless Belkin USB -serial adapter without any issues.

 

Thanks for the reply. I too have solved the problem, although I kinda cheated to some extent. I purchased a CP2102 USB to TTL converter off of ebay and then made this basic converter circuit with it...

It works really well and finally I can use speeds of up to 9600K on the 99B flukemeter.