Infra Red Alarm System HELP.......

pt202c example

Hey there Guys,

A while ago i managed to get my hands on a couple of old electronics magazines, "Electronics Africa", in it i found an on going article on how to build an infra red alarm system.(Picture attached)

Although it may be an old design, I have decided to try build this project, but have come into a few problems, and was wondering if any one could assist me? When the project is switched on, the relay is automatically triggered, therefore switching my siren on permanently(Broken beam).

I managed to get hold of most of the components listed, except for the BPW14(Photo-transistor) used in the receiver unit. My electronic outlets dont stock this item any more as it is an old component. With some digging, ive come across the newer components such as the TSOP1736, which reads a 36kHz signal... where as my circuit runs on a 1kHz signal. Now i've tried out a transmitter that sends a 36kHz signal, but this does not work? Would i need to re-design my control board to allow for the higher signal?

I've also tried using the PT202C(Photo LED), using the 1kHz signal. When using my multi-meter, i can see the transmitter communicating well with the PT202C, but still have no success in the actual circuit working. Ive checked and RE-checked all my wiring, and everything appears correct.

I dont get much of a signal out of my receiver, after passing through C3(470pF), which could be part of my problem? But also D2(IN4148) on the control board is suppose to rectify a negative voltage from C11(100nF)... which it does not do? Ive tested the diode, and it IS working.

How do i get T3 to switch on, in turn switching T4 off.... meaning an unbroken beam?
I have also made the necessary adjustments with my power supplies, to get the recommended voltages, and currents...

Please if any one can help me id be eternally grateful!!!!

thanks very much.

Running at 36KHZ is not a good idea. All of the circuit would have to be retuned. Plus, any IR remote control equipment in the room would stop working because of the contain flood of IR light energy at 36KHZ.

The BPW14 is just a standard IR photo-transistor. What phototransistors are available to you? You want just a plain transistor, not an IR module.

Thanks for the quick reply....

I cant seem to get a hold of any other photo-transistors, besides the TSOP1736.
When you say i need a plain transistor, are you meaning i can use for example a BC547B? or a BC107?

Would you say my problem lies with the receiver unit alone?

Dude just design a astable multivibrator of required frequency at transmitter
and PLL tuned to same frequency at the receiver its simple and secure without interferance. just google on this topic.

and PLL tuned to same frequency at the receiver

Click to expand...

Sorry for asking , but what is meant by PLL?

Id prefer not to go and build another system, as i've already spent alot of time, and my money on all these components.

With my current set up, when the 1kHz signal is received, i measure 0 V at the PT202C. With lack of signal (Broken Beam), i measure +- 9V according to the distance between transmitter and receiver. What sort of reading am i suppose to get at the source of T2, or after C4?

I know that C3 is supposed to 'get rid' of any potential constant DC... but seems to cancel out all voltages.?

When I said "plain transistor", I was referring to a plain phototransistor without and internal filtering. The signal at C4 is still a 1KHZ signal.
The problem is that you are using an IR module which is tuned to 36KHZ. By design, it has little to no response to 1KHZ.
Here is a trick to test your transmitter. Many Camcorders and digital cameras are sensitive to IR light. Take a digital camera or camcorder and look through the viewfinder while someone points a TV remote at you and presses buttons. If the TV remote causes flashes in the viewfinder, then the camera is sensitive to IR. Now repeat the experiment with the IR transmitter you built. This will give you a visual indication of whether this transmitter is working.

I really think your problem is the lack of a phototransistor. Sure, you can redesign the entire circuit, but what is the point? Seems like alot of effort because of one component. What mail order sources are available to you?

WOW.... the digital camera trick is amazing!! so i tested my tranmitter.... and it seems to be working great.

So if i use the PT202C for receiving my 1kHz signal, should it be working too?

Im not sure of any international mail order sources, but i use a local company, **broken link removed**

thanks again for all the help

Added after 4 hours 59 minutes:

i was just thinking....

if im using a pt202c instead of the bpw14.... would it work if i managed to hook up a seperate 1kHz oscillator to my receiver, so that when a signal is being received from the pt202c, the oscillator ir running, and sends the 1kHz signal to my control board... but as soon as there is no signal, the seperate oscillator switches off?

Ok, the PT202C is a phototransistor, not a photoLED. At least on the datasheet that I looked at :

**broken link removed**

This phototransistor is sensitive to visible and near IR light. The visible light you will have to filter out. But, first you must compare the wavelength sensitivity curve of the phototransistor with the wavelength emitted by your transmitter. Looks like the PT202C is most sensitive around 900 nm.

If the wavelengths are close, then you must block out the visible light to keep the phototransistor from saturating. Color slide film that has been developed but never exposed to light works well. Search the internet or see

**broken link removed**

Often at least two pieces are required. The filter will appear opaque to the naked eye. However, put it in front of your digital camera and do the transmitter test, you will see that the IR goes right through.

To check if the photo transistor is saturating, measure the voltage at the collector. Bottom side of R5. If the voltage here is close to ground or only a volt or two, with the transmitter off, then the transistor is saturating. Use the IR filter or for testing use a dark room at night with most or all lights off. If it starts to work, then you know your problem. Some phototransistors have a black or smoke gray body. These have an IR filter built into the plastic. The original transistor may have been of that type, which could explain no mention of a light filter.

Since, the transmitted frequency is 1KHZ, you are in luck as this is in the audio range. You could take a series capacitor to block the DC and a voltage divider attenuate the signal and then feed the result into an audio amplifier. If the signal is present, you will hear the constant 1KHZ tone. (The voltage divider is either a variable resistor or two resistor in series. Try a series capacitor of around 1uF and a variable resistor of about 20K to 47K.)

Also, since this is an audio range signal, you could feed it into a PC sound card. There is some software that will sample the audio input and display it like an oscilloscope. I have never used it, but you can search the web.

Whether you use the audio ampilifier or the PC sound card, always use a blocking capacitor and some series resistance to limit the potential harm to your equipment. There is some risk to the audio amplifier or the PC. Many, many people have done this successfully without problems. However, a wrong connection could cause damage, usually people buy a cheap sound card for this method rather than using the feature built into an expensive laptop or motherboard. If an accident happens and the cheap sound card is damaged, they just throw it away and buy another one.

Finally, if you have several issues of the magazine, I would study the issues for the months following this article. If the article had any corrections, they would show up in the following months. Also, read the letters to the editor. If others built the circuit with great success, they probably wrote a letter about it. Also, if people could not get it to work, the letters column may mention this as well.

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