How to generate constant 38 kHz square wave for IR light source?

Question

• Application: IR light barrier over a distance of 1m

• Receiver: e.g. Vishay TSSP4038 IR sensor (tuned to 38 kHz)

• Ambient temperature range: -10 to 40° C

Here I’ve found a circuit with a Schmidt trigger that should do the trick, if there was a crystal oscillating at 38 kHz. But I can’t find any. Digikey lists 38 kHz crystals, but – looking at the spec sheets – it turns out that they are actually 32,768 Hz crystals.

How do I get 38 kHz?

I’ve also considered an NE555 based circuit, but that’s not temperature stable, and it requires trimming. Another option, I’ve been told, would be using an Arduino to generate the 38 kHz, but that looks like overkill.

Update

Following the recommendation in the application note mentioned in Ali Chen's answer, I finally decided to use an Arduino Nano. For generating the 38 kHz, I used the timer as described in an Arduino forum post by Nick Gammon to the Arduino forum. His code example:

const byte LED = 11;  // Timer 2 "A" output: OC2A

void setup() {
 pinMode (LED, OUTPUT);

 // set up Timer 2
 TCCR2A = _BV (COM2A0) | _BV(WGM21);  // CTC, toggle OC2A on Compare Match
 TCCR2B = _BV (CS20);   // No prescaler
 OCR2A =  209;          // compare A register value (210 * clock speed)
                        //  = 13.125 uS, so frequency is 1/(2 * 13.125) = 38095 Hz

}  // end of setup

void loop() { }

Via an interrupt pin, I hooked the Nano up to a Yún, which runs the main logic and which tells the Nano to turn the signal on or off. Keep in mind that the Vishay application note is about measuring the distance to an object, whereas my application is simply about measuring whether a beam is interrupted or not.

Next steps: Get rid of the Nano; Upgrade from 38 to 56 kHz for slightly faster sensing.

Answer 1

Your effort is misdirected; these receivers are not all that sensitive to precise frequency but they will not maintain output with a constant modulated signal.

Instead, they are engineered to look for "bursts" of modulation and will reject noise that does not look like a train of pulses of modulated carrier; to get them to continue to work you will need not one signal generator but two, one for the modulation and the other to create the bursts in the envelope of modulation.

The first generator produces 38 KHz modulation.

The second generator keys the first generator on and off at a rate within the window of the types of remote signals the receiver is designed to accept.

Typically, you use a microcontroller; use a timer channel divided from the MCU clock to the produce the 38 KHz to closest approximation you can. It does not need to be exact.

Then either have software, or another timer channel key this on and off.

(You could probably use 556 dual timer; but MCUs that can do this are well under a dollar in quantity, require fewer support components, and can do other things as well).

If you were expecting constant output from the detector, you will need to rethink your system design; instead, you probably need to follow the detector with a pulse-stretcher that can fill in the gaps between your transmit pulses; should your transmission cease, the pulse strether will extend the last pulse from the receiver, then stop.

Answer 2

The Vishay receiver specs show the filter characteristic as +-5% at 3dB attenuation relative to nominal frequency (38kHz).

The best (SE555 TI timer) has internal sensitivity of 90ppm/C typical, which over 50 C span results in 4500ppm, or 0.45%.

To get to 38kHz, you will need to use a 10nF capacitor with 2k resistor. You can use NPO 1% tolerant ceramic caps (say, from Murata), and if you get a stable power supply as well, you should be able to get something like 2-3% stability over 50 C range.

However, the above is more like a wishful thinking, since TI itself doesn't guarantee anything, see this blog entry.

In addition, as Chris Stratton mentions, the Vishay receiver part does not guarantee a steady output if a steady optical signal at carrier frequency is applied, the receiver needs some "breaks" between burst of carrier pulses, see specifications for a similar part as TSOP6238.

UPDATE: I just did a quick experiment with a similar IR receiver, GP1UV700QS (36kHz, by Sharp). With an IR LED driven from a signal generator as source, the behavior is as follows: if no IR signal, the receiver output is HIGH; if signal is started and continues, the receiver goes LOW, and then goes HIGH by itself. The LOW pulse duration depends on proximity to emitter. At close distance (20cm) the pulse can be as long as 300 ms, while at a longer distance (1m) it shrinks to 1-2 ms, down to toggling at 250 us. Since the carrier frequency response characteristic is rather broad (5% at -3dB), and the dynamic range is huge, it really does not matter much if the carrier gets deviated by +-5kHz, results look the same.

BOTTOM LINE: If the IR emitter signal is unmodulated, just a constant-amplitude 38 kHz carrier, the class of IR receivers as Vishay TSSP40xx or Sharp GP1UV70xx will not sustain the logic detector output. To get these ICs to function as "light barrier", the carrier amplitude must be modulated into distinctive bursts of about 1 ms ON, and ~1 ms carrier OFF. Unfortunately, the output will toggle accordingly, so to use the receiver as "go-nogo" detector the processing circuit must detect the toggling.

So it might be easier to use a crystal with a microprocessor, and use internal programmable timers to get the right frequency and right modulation, as the original Vishay appnote suggests.

Answer 3

Grab an ATMega328p and put a 7.6MHz quartz crystal on it. Then with Phase Frequency Correct mode with ICR1 as 100 you will output an exact 38kHz signal.

To do this:

Set the MCU fuses when uploading the code, to these values:

low_fuses= 0x7D = 1111 1101
bit 7 = 1 = CKDIV8  = Divide clock by 8
bit 6 = 1 = CKOUT = clock output
bit 5 = 1 = SUT1 = Select start-up time
bit 4 = 1 = SUT0
bit 3 = 1 = CKSEL3 = Select clock source
bit 2 = 1 = CKSEL2
bit 2 = 0 = CKSEL1 
bit 0 = 1 = CKSEL0

The code to produce the 38kHz on pins PB1/OSC1A and PB2/OSC1B pins will be:

// Set Timer1 to phase and frequency correct mode. NON-inverted mode
TCCR1A = _BV(COM1A1) | _BV(COM1B1); 

// Set prescaler to clk/1
TCCR1B = _BV(WGM13) | _BV(CS10);

//ICR Register, which controls the total pulse length
ICR1 = 100; // Divides clock by 100/2, so 7.6MHz/100/2 = 38kHz
//OCR Registers, which control the duty cycle.
// OCR1A + OCR1B must be = IRC1.
OCR1A = 50; // 50% of the pulse will be LOW state
OCR1B = 50; // 50% of the pulse will be HIGH state

Answer 4

If the frequency is all that critical you may want to consider a "Phase Locked Loop" (PLL). A crystal oscillator having a "Smallest Common Denominator" with the frequency you are trying to produce should produce less jitter (if that is even a factor). Just as an example 1.9 MHz divided by 38 kHz = 50 times. So a 1.9 MHz divided by 50 would be your 38 kHz. I hope this helps.

Answer 5

The Vishay TSSP4038 IR sensor spec shows responsivity vs relative frequency. A 555 with temperature stable external components should keep you within a reasonable range of the responsivity, but you need to figure out what percentage responsivity you can suffer over the full range (a spec of 0% is never realistic, but the actual percent is very application specific). You just have to do some math to figure out worst case drift vs temperature on the RC components and figure out if you can take the responsivity hit.

Alternatively, a microcontroller with a crystal for timing utilizing a timer output to toggle a pin at the correct time would work and would no doubt be more stable.

Answer 6

38kHz crystal on Ebay Just use the selection box for the right frequency.

BUT! considering these exact sensors are used with Arduinos, that usually have 16MHz clocks, I would consider doing some research on Arduino code for these sensor kits, as there might be some finished code for your project!

Answer 7

Two simple options for a 38kHz square wave:

1. Use the Voltage Controlled Oscillator built into this PLL chip: http://www.ti.com/lit/an/scha002a/scha002a.pdf (See p. 16 of the PDF for how to choose components to set the output frequency)

2. Build a relaxation oscillator with an op-amp and temperature-stable resistors: http://www.falstad.com/circuit/e-relaxosc.html

A 555 would likely be the easiest solution, and it might be stable enough across a large temperature range.