Purpose of 1 microfarad capacitor in IR receiver circuit

Question

What is the purpose of the 1 microfarad capacitor in this circuit? The source webpage says:

Output of TSOP1738 oscillates at the rate of 38KHz, which is applied to clock pulse of 4017. So we have connected a 1uF capacitor across the output of the TSOP so that this 38KHz pulse train is counted as one clock pulse to the IC 4017.

However, I could not understand what this means:

So we have connected a 1uF capacitor across the output of the TSOP so that this 38KHz pulse train is counted as one clock pulse to the IC 4017.

The circuit does not state the voltage of the capacitor, but I bought and used a 1 microfarad 16 volts capacitor. How 1 microfarad capacitance and 16 volts rating of the capacitor affect this circuit? Is the capacitor really needed?

Source webpage (remote-controlled light switch): https://circuitdigest.com/electronic-circuits/remote-controlled-light-switch

UPDATE:

If I understand correctly, each pulse from a TV remote control for example produces a maximum of 6 volts at the output pin of TSOP1738 and charges are stored in the capacitor. But how does the capacitor know when to release the charges it holds to IC's pin 13? Does the capacitor wait for the burst of pulses to finish before releasing the charges to the IC's pin 13?

Answer 1

... the datasheet is too advanced for me. Is there a beginner way to understand ...

Sure. Let's do it in stages.

Modulated waveform

Figure 1. Infrared modulated and demodulated signals.

• Remote control signals are transmitted as a series of pulses. If you look at the LED on your TV remote using your camera phone you can see the irregular flicker as you press the remote buttons. The remote control codes are sent at a relatively low speed of up to 1200 bits/s according to the datasheet.
• Each pulse for one bit will, in turn, be made of a burst of high frequency pulses. The top of the datasheet shows that the chips are made in versions from 30 kHz to 40 kHz. Having rapidly changing levels like this helps the receiver discriminate between background infrared from other sources. Effectively the receiver is looking for a fast AC signal and ignores anything else.

Figure 2. The innards of the TSOP demodulator.

• The TSOP chips make decoding the signals easy because they demodulate the output and restore the low frequency signal. See the output signal of Figure 1.
• The demodulated signal is used to drive the output transistor. Note that this has a pull-up resistor which normally holds the output high. When the transistor turns on the output is pulled low.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 3. The output of the sensor.

• If the transistor, Q0, is off (no signal received) the output is pulled high by R-pull-up. This charges up C1 to V+.
• When the receiver turns on Q0 it discharges the capacitor to 0 V.
• When Q0 turns off C1 starts to charge up via R-pull-up. The time taken to charge is given by \$ \tau = RC = 100k \times 1 \mu = 0.1 \ \mathrm s \$. As a result the output will stay low for about 0.1 s after the last received pulse.

But how does the capacitor know when to release the charges it holds to IC's pin 13? Does the capacitor wait for the burst of pulses to finish before releasing the charges to the IC's pin 13?

Yes. As explained above the pull-up resistor is always trying to charge up C1 but every pulse from Q0 discharges it again. The time constant is 0.1 s so it will charge again about 0.1 s after the last pulse detected.

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I think it is getting clearer especially the 0.1 s. But how does the Pin 14 of IC count all the pulses of a series as 1 pulse if the capacitor discharges and charges many times if I press one button in a TV remote control?

That's easy.

• Every time the TSOP receives a pulse the capacitor is discharged and will take 100 ms to recharge.
• The TSOP chip is demodulating the received signal into nice square pulses. We don't know, but let's say they're coming in at half the maximum rate the TSOP can handle = 1200/2 = 600 pulses/s. So, one pulse = 1/600 s = 1.7 ms.
• So, from the above we can see that while the button is pressed on the remote control the capacitor will be discharged to zero again every 1.7 ms. It's only when the button has been released for > 100 ms that the capacitor can recharge.
• The result is that the counter gets one pulse for each press of the button provided there is at least 100 ms between presses.

Answer 2

I see you already have a very good answer from Transistor (upvoted), but I want to clear up a specific confusion in your question.

Output of TSOP1738 oscillates at the rate of 38KHz,

This is wrong. The IR input to the TSOP oscillates at 38 kHz. This is the carrier frequency the TSOP is looking for. The purpose is to be able to detect the TV remote's IR signal from the IR background. There can be significant ambient IR, but that isn't going to be modulated at 38 kHz. To be able to "tune out" the ambient IR, the TSOP is looking for modulation at a specific frequency that is very unlikely to occur by natural causes or without deliberate action.

The particular carrier frequency is built into the TSOP. Variants are available in a range of fixed frequencies they will detect, somewhere from 30 to 45 kHz if I remember right.

Reacting to only a narrow frequency is the same as what a typical radio does. If you tune your radio to 1.03 MHz, then you only hear that station. It doesn't matter what a radio station broadcasting a 980 kHz or 1.08 MHz is doing.

The TSOP demodulates the IR carrier, which in this case is 38 kHz. It essentially puts out a signal telling you whether it sees this carrier or not. Of course it takes a few cycles to make sure the carrier frequency is really there. The tighter the frequency band around 38 kHz, the longer it takes to make sure that's really what it is receiving.

In the case of the TSOP parts, I vaguely remember that they promise to assert the output after a maximum of 8 carrier cycles are received. I used these parts in a IR remote system once, and I remember making the minimum carrier pulse length 10 cycles.

So the TSOP output does NOT tell you when individual carrier cycles are received. It tells you when enough continuous carrier is received to be sure it really is the carrier. That signal won't toggle anywhere near as fast as 38 kHz.

For example, let's say that you are transmitting bursts of 38 kHz carrier of 10 cycles long, with gaps in between also 10 cycles long. The data part of the signal you are sending is really a (38 kHz)/20 = 1.9 kHz square wave. The TSOP output will therefore be a 1.9 kHz signal, NOT bursts of 38 kHz.

Added in response to comment

Don't think of it at the level of individual electrons. Look at the macro features of voltage and current.

When the TSOP detects the IR carrier, it pulls its output low. This is done with fairly low impedance, so discharges the capacitor quickly. When it stops pulling low, the capacitor gets charge up again thru R1 and the internal pullup in the TSOP. Those together are much higher impedance, so the capacitor charging up takes longer. Transistor estimated that Q1 will stay on for about 100 ms after the TSOP releases its output.