How to understand output of TSOP4838 IR receiver?

Question

I am testing the output of the following basic IR transmitter/receiver circuits and not getting the result I thought I would.

^{simulate this circuit – Schematic created using CircuitLab}

I have tested the output of the 555 astable circuit and DMM confirms OK (I have a trim pot to control).

When I close SW1, DMM shows momentary drop in voltage at Vout of receiver (approx 3.3V) but if I hold it closed, DMM returns to reading Vout = 4.5V.

I thought I would see Vout = 0V for the duration that SW1 is closed. What is the expected response and what do I have to change to achieve that?

Answer 1

What is the expected response and what do I have to change to achieve that?

The expected response is one short output pulse from TSOP4838 starting from 4.5V to GND...when the LED is pulsing continuously at 38 kHz.
The problem is that TSOP4838 adapts to a continuous 38 kHz, in a similar way that your eye adapts to dim moonlight at night, and bright sunlight during day. Its adaptation is quite short, so that it quickly gets used to constant 38kHz and returns to its idle state of 4.5V after just one pulse.

What is needed is short bursts of 38kHz, separated by no light output:

Answer 2

The TSOP4838 just does not work like you expect. It is not designed to receive continuous long burst of 38 kHz but to filter it out.

The TSOP4838 will filter out continuous signals at any frequecy. It reads in the data sheet.

So for long enough burst of 38 kHz, it will start ignoring it.

The module has an AGC (automatic gain control) to adjust itself to adapt tp varying lighting conditions. That is why the 38 kHz burst must be short enough to be detected, and that's why there must be a long enough gap between bursts to allow the detection to stay at ambient level. So the part will adapt to long burst being present.

The point is to send bursts of 38 kHz and then a pause to allow the TSOP4838 to stay sensitive enough for the next burst. The suggested minimum and maximum burst lengths and minimum gap between bursts are in the data sheet.

These parts (TSOP4838, TSOP1738, etc) are for remote control receiving, not for detecting a break of continuous 38 kHz beam.