HP 50G - Any way to increase the IR TX/RX range?

[Dave Shaffer]

As I understand Katie’s approach, she is demodulating the calculator signal with the TSOP38238, and then remodulating another LED with the 555 oscillator (gating the 555 with the output from the TSOP chip).

As an alternative, here’s a fairly simple circuit that actually repeats the individual light pulses, with a potentially strong (bright) LED acting as the signal transmitter. This should allow you to extend the range of your HP 50G IR output. Just park your calculator so its output IR signal shines on the detector diode of this circuit and fire away. The output diode blinks in synchronism (at the microsecond level) with the calculator IR signal and if it is bright enough, should allow it to be some moderate distance from the printer. This will not help the mismatch between carrier frequencies which Katie alluded to, so the improvement may actually be marginal.



The circuit uses a QSD2030 photodiode detector sensitive to visible and infrared light, which has an intrinsic very fast response time: 10 nanosec. For the simple circuit here, the response time is rather longer, around a microsecond, set (I think) by the time constant of the diode capacitance and resistor R1. If R1 is bigger, the circuit is more sensitive to light but the response time gets longer, too. The angular cone of sensitivity of the diode is rather small, centered along an axis down through the top of the diode plastic case, so you do have to aim the calculator accurately.

The photocurrent across R1 raises the voltage on the negative input pin of the LM339, which is a fast comparator (response time of around a microsecond here). When the voltage on the negative input exceeds the voltage on the positive input (set by the R2 - R3 voltage divider), the comparator changes output state and turns the LED on. For more sensitivity, the value of R3 can be lowered so that the voltage input on the negative pin needs to rise to a lower value before the LED turns on. At some point, though, a noise threshold will be reached. Also, the QSD2030 may need to be shielded from stray light to achieve the desired sensitivity.

The LM339 is rated to sink up to about 16 mA. If more current is needed to drive your desired IR “transmitter” LED, the LM339 can be used to drive a transistor switch.

I have tried this with only a visible light output LED (so I can see what’s happening!), but it should work with just about any LED. Just make sure your power source can supply the necessary current, and adjust R4 accordingly. R5 is an empirical addition, which assures the LED turns off rapidly. Somebody who understands circuits better than I do can probably suggest a better way to control the LED. Here, the LED (or at least the voltage across it measured with an oscilloscope) turns on and off in a microsecond or so - plenty fast enough to emulate the calculator LED transmitter.

To duplicate this circuit should not be costly - my major expenses were the QSD2030 and LM339, both available for under $1 each (neglecting shipping; see Ebay and/or digikey). The rest of the components I had lying around (as any ham radio operator with a reasonable “junk box” would have!).

I have verified that the circuit responds to a 48SX that I borrowed (whose output pulse frequency is very close to 33 kHz).