Testing the circuit on a breadboard

The receiver circuit is based on the TSOP1838 chip. It has an integrated PIN diode and a preamplifier circuit inside the chip. Moreover, the epoxy package is an IR filter by itself, that blocks other light sources beyond IR spectrum. This makes it very reliable, and also the output of the chip is rectangular pulses that can be driven directly to a microcontroller. This chip saves the designer of the circuit (that will be me :D) from a lot of dirty job to design filters and Schmitt Triggers... Here is the (simplified) schematic diagram: (click to enlarge)

The TSOP1838 does the dirty job

The TSOP1838 does the dirty job. It "reads" the modulated IR pulses from the transmitter circuit, and converts it into pulses. So what someone would expect to see directly at the output of this chip, is rectangular pulses with 32 Hz frequency, and 1.5-1.6 mSec positive pulse duration. I pass these pulses through a current amplifier transistor (T1) and then directly to the reset input of the Decade Counter (4017 CMOS). So, whenever a pulse arrives, the counter resets. Keep this in mind for later use.

This was the first part of the receiver. The second part, is the 555 timer. This is connected as an astable multivibrator with variable frequency, controlled with the potentiometer R6. As a matter of fact, the parts around the 555 are carefully selected so that the minimum and maximum frequency will be 3 (minimum) to 8 (maximum) times more than the frequency of the transmission pulses (32 Hz). These pulses are then driven to the clock input of the 4017 counter.

Suppose now that the IR pulses arrives normally at the IR receiver chip, and are normally driven to the reset of 4017. And also, the clock pulses from the 555 normally arrives at the clock of the 4017. What happens? Well, the 4017 increases its count on every 555 pulse, but resets on every IR pulse. And due to the fact that the 555 pulses can be maximum 8 time faster than the IR pulses, this means that the 9^th output (Q9 - Pin 11) of the 4017 will NEVER become 1.

But if something breaks the IR beam, then there will be no more reset pulses to the 4017, and eventually the 9^th output will become 1! At that point, a positive feedback from the 9^th output will be driven to the Clock Enable (ENA Pin 13) of the 4017. This will PREVENT any other pulse from the clock to change the chip's count. So the 9^th output will remain HIGH, as long as the beam is broken.

But if you notice, the 9^th output also drives the T2 transistor, which controls our load (Relay, Siren, Buzzer etc)... Guess what! Upon a beam break, the load will be armed! Easy it is not?

So, here are some screenshots from the oscilloscope:

The yellow channel shows the 555 pulses, and the green comes from the IR receiver. In minimum frequency, the 555 produces approximately 3 pulses per one pulse of the IR transmitter.	In maximum frequency, the 555 produces approximately 8 pulses per one pulse of the IR transmitter.

You could name this potentiometer "sensitivity" or "threshold", or another fancy name. Why? Suppose that you have set the 555 to minimum frequency (pot at maximum resistance). To set output 9, there have to arrive 9 pulses at the clock input. But if the 555 produces 3 pulses per 1 IR pulse, this means that the obstacle has to remain at least for 3 IR pulses. That sounds much, but if you consider that each pulse has about 31 mSec delay, this means that the obstacle has to remain at least 93 mSec... That is 1 tenth of a second

At maximum 555 frequency, the obstacle has to remain for one full IR pulse plus 1/9 of it. That is 31 mSec + 3.4 = about 35 mSec. After 35 mSec, the load will be activated.

The reason i added this pot is simple. In close distances and in environments with not particular ambient noise, high sensitivity can work just fine. But if you operate this circuit in long ranges - almost the maximum range of the IR LED- and in harsh environments, you need to decrease the sensitivity to avoid false triggering. In case that the circuit still gives false triggers, you may need to use a larger potentiometer, 50 K for example, to further reduce the sensitivity. Don't over do it. You don't want the obstacle to remain 1-2 seconds until the load is activated, do you?...

Resistors
R1	Resistor 100 Ohm 1/4 Watt 5% Carbon Film
R2	Resistor 1 KOhm 1/4 Watt 5% Carbon Film
R3	Resistor 330 Ohm 1/4 Watt 5% Carbon Film
R4	Resistor 1 KOhm 1/4 Watt 5% Carbon Film
R5	Resistor 2.2 KOhm 1/4 Watt 5% Carbon Film
R6	10 K potentiometer
R7	Resistor 10 KOhm 1/4 Watt 5% Carbon Film
Capacitors
C1	Electrolytic Capacitor 1 uF 50 Volts
C2	Electrolytic Capacitor 4.7 uF 50 Volts
C3	Electrolytic Capacitor 1 uF 50 Volts
Transistors
T1	BC337 NPN Silicon AF Transistors
T2	BC548 Switching and Applications NPN Epitaxial Transistor
Semiconductors
D1	1N4004 General Purpose Diode Rectifier
Integrated Circuits
IR1	TSOP1838 38 KHz Photo Modules for PCM Remote Control Systems
IC1	555 Timer
IC2	CD 4017 Decade Divider/Counter
Misc
IR1	5V low coil current relay

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