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TSOP and 555-timer turning on LED with IR remote control

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May 7, 2022
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I was browsing youtube and stumbeld across this video of a circuit with a TSOP and a 555-timer that turned on a LED with a IR remote control. This is the video:
I was interested to try it out and it worked perfectly! The problem I have is that i don't understand how it works. I would like to know how and why this circuit works.

The video has no parts list and has no schematic.
You need to guess which TSOP part number was used or maybe you can find out which TSOP is available in India.
A TSOP1738 operates from 38kHz pulses of IR, which causes its output to go low.
An LM555 or NE555 datasheet explains how it works.
There are circuits on the internet that show how to make an NE555 do alternating set and reset.

DO NOT carry solder around on the dirty tip of a soldering iron like that guy in India.
I purchased a module at Radio Shack which responds to 38 kHz IR signals. I wired it into my receiver's scan button to enable searching stations when I press my remote control. (Besides Power and Volume the remote has 7 buttons which limited me to a choice of only 7 preset stations.)

The module has 3 wires: power, ground, output. After installation I could lie in bed, press a volume button and start the receiver scanning frequencies upward. To add this function only required a single pulse to come from the module. I had to aim my remote carefully so it didn't trigger unwanted actions.

In the same way this module should be able to trigger a timing event in a 555 circuit.
The Radio Shack module is expensive and old. It is probably obsolete. There is no part number and no datasheet for it.
Digikey is a real electronic parts distributor and sells a new and improved TSOP13538 for almost one quarter the price of the Radio Shack old one. They have its datasheet online and have 993 in stock.

I would like to know how and why this circuit works.


If you haven't found the information you were looking for yet, be a bit more specific about what you are curious about regarding 'how' and 'why'. I suspect that replies #2 and #3 answer the question.

(I have a cheap LED nighlight that works with a single-function/single-button IR LED keyfob transmitter. The receiver battery drains really quickly because the receiver in the lamp is always on and waiting for the signal from the remote.)

If you haven't found the information you were looking for yet, be a bit more specific about what you are curious about regarding 'how' and 'why'.
Sorry if my question was to wide. If i not wrong the 555-timer is used in bistable mode, so that's ok. I think I understad the TSOP also. What about the role of the two transistors and the capasitor?

It is much easier to see how a circuit works when there is a schematic of it. The video shows only a tangle of wires and parts all over the place.

The circuit is shown on a You Tube video that frequently is faked. Then does the circuit not work?

The circuit is shown on a You Tube video that frequently is faked. Then does the circuit not work?
I have tried the circuit and it works.
Thanks for all the help! I think i have the info I need to better understand it.

Hi runjoh,

Sorry for the late reply. I'm glad you have the info you need to understand the circuit because frankly, after watching the video sevral times, feeling like a hostage, and drawing a schematic from it, I have no idea (I have a great liking for the 555 and admiration for its inventor and have done many different things with it for about 8 years, btw).

Perhaps I have misunderstood something in that person's video and there is a mistake in my schematic, but I will say, along with Audioguru, but in different words, the way the 555 is set up reminds me of dodgy 'experts' who share their circuits that as a beginner I couldn't tell the difference between 'good' and 'bad'/'sloppy' and 'BS' circuits by people desperate for attention but lacking scruples, and there are a couple of warning signs in the circuit here: There are no bypassing capacitors on the 555 supply pin; there is no 100nF capacitor from pin 5 to ground, and it is left floating (pin 7 is also left floating, but that is not such a terrible thing, except for the fact that, generally speaking, floating IC pins ARE A BAD THING); IMO there is a serious lack of resistors in a few places - PNP current-limiting base resistor, an NPN pull-down is missing, there are no current-limiting resistors on the LEDs (3.7V - 1.7V = 2V; 2V/0 Ohms = the only current-limiting for the LEDs is the 555 output resistance); a 47 uF capacitor...? - Seems unnecessarily huge capacitance to me (my personal rule-of-thumb is to - where possible - use as small a capacitor as possible with a larger resistor, not a small resistor and a huge capacitor). Also, I hope I have copied the circuit wrong because if I haven't, the NPN emitter connection is just weird and looks like a bad joke.

A quick simulation with a voltage source to replace TSOP1738 didn't tell me anything as what was most memorable was that there was an 8mV output voltage from the 555 pin 3... I assume I missed something in the instructions and my schematic is different to what the person says in the video, but I sat through it some four times to check I had copied it according to the instructions...

By the way, what's that from pin 6 to pin 2 on the timer - an unmarked 0 Ohm resistor, or what? The guy doesn't even say what it is.

Schematic (I am very open to corrections from anyone who can be bothered to watch the video):


I guess you understand that:

1) Press a (/any?) remote control button.
2) TSOP1738 output goes low > BC557 turns on > BC547 turns on.
3) From there, I wouldn't like to guess because of the BC547 emitter connection that is a new concept for me. All my brain sees is ~1.85V at the emitter and ~3.7V at the collector of the BC547, from there my brain melts.

The 555 trigger pin needs to go low (1/3 Vsupply) for the output to go high and the output will go low again when the capacitor charges to 2/3 Vsupply. I don't understand why the timer is biased at Vsupply/2, it's neither one thing nor the other I would have expected.

If that really is a valid bistable circuit, I'll bet I'm not the only person who would use it instead of e.g. a flip-flop if a CD4013 were unavailable. As you say it works well, so be it.

Anyone who understands this circuit, I'm all ears and humility.

Hi again runjoh,

I was curious to understand why my simulation doesn't emulate the video because the sim tool I use is almost always very truthful.

So, I changed the capacitor to 4.7uF and made the TSOP1738 model more realistic this time.

Besides comments about sloppy 555 connections, I just noticed the 555 is biased at mid-supply - that's a new method for me...

As you say you made it and it works, why not upload a short video of your version, and a schematic if you now have one (so I can check I didn't make a mistake in mine) as my simulation doesn't seem to be working, again - the 555 has no output voltage and I am confused, because you say the circuit works.

Here is what the sim tool says:




Didn't watch the video and never used a 555 in my design days (although I did use LS123's)

The IR part demodulates the carrier amplitude modulation into binary codes dircetly to drive the indicate LEDs. This simply uses the 555 as a low impedance CMOS driver with input hysteresis. T

The output resistance of the 555C will might limit the current for the quantity of LEDs driven but only for the battery voltage and LEDs used. That is unregulated.

. Each G,W,B LED at 20 mA will have an incremental resistance around 15 Ohms +/-30% Red only for 5mm LEDs may be around 10 Ohms at 2.1V .

Most of the details are in the AGC Q=10 nice features of the optical part.

Nice datasheet schematic to clarify circuit operation, Tony, and the concise explanation, thanks.

I'll just add a couple of things for our friend, runjoh, to widen their scope of options as they seem to want a latching circuit, and to help them not waste time on beginner-beware circuits.

Hey, runjoh,

This is a useful version of basically the same circuit as you have made, but it doesn't use a 555: ELECTRONIC TOGGLE SWITCH. It uses the CD4013 flip-flop, this circuit is often used as a toggle switch (or to debounce pushbuttons). Same as the NE555, it operates from about 5V but should work on 3V.

Here is the datasheet for the CD4013.

And, last but not least, here is Mosaic Industries lovely page dedicated to
Latch and Toggle Power Circuits.

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