RS232-TX voltage translator

Question

Would like your help to clear my doubt and confusion for this circuit http://nil.rpc1.org/wordpress/?p=31.

I don't get a sense that how possible the RS232-pin3 TX signal is converted and transmitted out from the IR tx.

As you look into the photo, the RS232 TX is direct connect to anode of Zener and the cathode is connected -> IR transmitter anode -> resistor ->gnd. That's all.

If saying that, it translates from "-12 -> -0."6 and "+12 -> +3.3v" (1n3138 assuming) then transmit out, how would it be possible?

I am thinking ,the connect orientation should be Zener place from ground with anode and cathode -> IR transmitter -anode? Am i right? With this orientation only the voltage translator able to work.

Can some1 explain further?

Answer 1

First, that 1N4148 is just a diode, not a zener.

Second, just draw out a schematic if you can't follow the circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Looking at it, strictly speaking the 1N4148 doesn't seem to be necessary, but it does make the leads line up more evenly to terminate to the DB9.