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IR emitter at 38khz

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Hey all -

I've built a 555 based oscillator to drive ir led at 38khz. I've got the circuit wired up and I've used my meter on the duty setting to read that it is indeed generated a 38khz signal.

I've verified the IR led is on using a digital camera to "see the light"!

I've also wired up a modulated IR receiver, the one from radio shack set to 38khz. I've verified it's working by pointing my TV remote at it. I've hooked up a red led to the output pin of the receiver and it lights up and flashes when I do this.

Soooo when I point the transmitter at the receiver I get nothing. Well close to nothing. It occasionally flickers.

The red led lights up occasionally especially if I move the receiver back and forth horzontally. I've checked for loose wires [this is all on a couple of bread boards], the led always lights up when I blast it with my remote.

Oh I'm using a 5v source from a wall wart [the radioshack one that you can select the output voltage for] connected to a 7805 with a couple of capacitors. The power source circuit i bought and soldered together, it fits standard breadboards [solar-robotics] so hopefully its okay.

One thing I did notice is on the timing capacitor, if I short the two pins of the capacitor with my finger, the receiving led lights up as if its working fora second or two then goes out. This works if I repeat the process touching the capacitor.

At this point I think I must have

1) a loose wire on my emitter?
2) Capacitor leads are too long and i should trim them
3) ??

I connected a 104 ceramic capacitor from the + pin on the 555 to ground to bypass any noise too.

Anyone have some tip on how to debug this circuit?


You must use a frequency counter to measure the frequency, not a "duty setting???".
You forgot to attach your schematic so we do not know the current of the IR LED.
RadioShack might sell "seconds" (factory rejects) instead of good quality parts.
If your circuit is powered by the output of a 7805, then check the DC voltage going into the 7805 (across the input cap). It should be a minimum of 7V in order to get 5V at the output. I'm questioning this because you said that you only had 5V at the input?
I'm using the following 555 schematic with the parts resistor and capacitor values listed therein:

Pulsed IR Sensor

**broken link removed**

I'm at work and don't have the IR receiver circuit handy but I'll post it when I get home. I don't recal if I'm connecting the red led to ground or to +5V. The IR receiver itself is directly connected to ground and the +5V rails. I have a 1k resistor connected from the output pin to a regular red LED to indicat the presence of a signal.

Do all modulated IR receivers have a auto gain mechanism? I've seen so many of these little pulsed IR led circuits for simple obstacle avoidance that I'm beginning to wonder if I've missed something totally or my receiver just isn't designed for my intended purpose.

The current in your IR LED is very low.
The output high of the 555 is only +3.6V if its power supply is +5.0V. The voltage across the IR LED is about 1.3V so the remaining 2.3V is across the 470 ohm current-limiting resistor which gives a current of only 2.3V/470 ohms= 4.9mA.

TV remote controls use 1000mA pulses in their IR LED.

Most IR rerceiver ICs are designed to reduce their gain when they detect continuous 38kHz from a compact fluorescent light bulb. Their gain is at max when the IR pulses are in bursts of data.

You must make your IR pulses have much more current and in bursts of simulated data.

If you replace the 470 ohm current-limiting resistor in your IR sender with 39 ohms then the current pulses in the IR LED will be about 52mA and the range will be much more.

Is the power supply feeding 7V or more to the input of the 7805? Then is the output of the 7805 at +5.0V?

The datasheet for most IR receivers show a resistor feeding +5V to it then a capacitor to ground. It won't work properly without the resistor and capacitor. Here is an IR receiver circuit from Vishay's datasheet:


  • IR receiver IC power.PNG
    IR receiver IC power.PNG
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Thanks AudioGuru.

I checked when I got home this evening and I happen to be using a 120 ohm resistor instead of the specified 470 ohm resistor. My wall wart is set to 9V, and the 7805 is putting out a steady 5 Volt.

At this point I think its the gain issue that people mentioned earlier in the thread. If I move my hand over both the emitter and receiver [both point up], the led lights up [the led connected to the receiver], connected to the receiver for a second then it goes out. This is repeatable....

Not sure where this leaves me, perhaps the brief signal is all the train guy needs to drive a relay or for a little robot to reverse direction or turn right etc.

The other source i used as a reference is Robot Room - Infrared Emitter from CMOS 555
which accomplishes roughly the same thing but doesn't mention anything about the auto gain.

In general what I'm trying to accomplish was have a prebuilt circuit I could use to drive a robot to do:

1) Turn on/off relay to say reverse a motor or in some other conjunction with an hbridge kit i have waiting to be put together

2) Use it for a simple line follower [what I will probably do].

Thanks again everyone!.
It is easy to simulate "data" with your IR emitter circuit. Simply use another 555 oscillator (or use a dual 555, the 556) to gate the 38kHz oscillator on and off at the rate discussed on the datasheet for the IR receiver IC. Then the receiver's gain will be maximum if it is not in direct sunlight.

The train circuit has a range of only a couple of inches.
What IR receiver that you using now? Some IR receiver will just generate output signal only when it "see" a certain period of IR burst length, say in micro-second range, or in nano-second range like the Vishay TSOP7000 IR receiver.

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Ok so its the AGC. Winson the Ir receiver I am using is from Radio shack, they only sell one type AFAIK.. 38kHz Infrared (IR) Receiver Module -

Audioguru - Thanks for you response and your suggestions are good I will attempt to modify the circuit as you specified. Will make me more familiar with the 555 in any event!

one thing though, I'm not sure what the range comment about the train circuit as in conjunction with the receivers gain?

Does anyone know of any fixed gain receivers? Vishay has one, the TSOP4038 but I cannot find a source for it. Digikey and mouser don't seem to list it.
You should never buy anything from RadioShack because they don't have a clue about electronic parts.
Their technical specs are for the length and weight.
They probably don't even know who made the things.

Digikey sells many IR receiver ICs. Most are made by Vishay. They have detailed technical specs on datasheets.
Are you sure you have enough power to your LED? I tested this **broken link removed** with 5 volts and sucked. With 9 and above works pretty nice...
Simply analyse why the circuit works poorly with only 5V for its supply:
1) The output high voltage of a 555 is 1.5V less than the supply voltage.
2) The emitter of the transistor is 0.7V less than the output of the 555.
3) So the voltage to the three LEDs is only 2.8V.
Three IR LEDs need about 3.9V to be bright but the circuit gave them much less.
The current in the LEDs, the transistor and the 150 ohm current-limiting resistor was almost nothing.

With a 9V supply (it needs to have a 100uF supply bypass capacitor) the voltage to the three IR LEDs is enough to turn them on fairly well. The current is about 13.3mA which is low but might be enough.
What are your thoughts on the 555 sinking the LED current as opposed to sourcing it?

What are your thoughts on the 555 sinking the LED current as opposed to sourcing it?

The datasheet for the 555 shows that its typical output voltage loss sourcing or sinking is typically 1.6V when the output current is 100mA and the supply is only 5V. With a 10v supply the sourcing loss is the same at 1.6V but the sinking voltage loss is less at typically 0.9V.

A medium size power transistor can have an output of 1A when its base current from the 555 is 100mA.

It is a waste of voltage to use the transistor as an emitter-follower.
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