one for the electronics experts IR led

T

tim_ber

Well-Known Member
Joined
12 May 2009
Messages
1,039
Visit site
So I bought a cheap kit to help secure my Dad's property.

One circuit emits IR and the other circuit receives it (it had a single receiving LED). If the receiver is blocked, a signal is created alerting the house owner.

Firstlly I replaced the rubbish LEDS with decent ones, this increased the range greatly.

But that left me with a device that acted in a straight line (so I could protect a driveway for example)

Then I thought I could perhaps modify the device, add an emmitting LED on a flying lead, add a second receiving LED to the receiver circuit and protect two areas: my theory being that as the receiving LEDS appeared to be in series, if one of them stopped receiving the IR from an emiiting LED, the alarm would sound.

But it doesn't work the way I have explained and I canot figure it out.

I know there are Guru electronic guys on here. Anyone kind enough to show me what I am doing wrong (I love messing about with electronics and learining as I go).

Photos attached.
 
Last edited:
At a guess, I would say the IR is pulsed, so the receiver looks for the modulation on top of the background level.
The modulation gets through on one diode and registers as beam not broken.
 
lw395 said:
At a guess, I would say the IR is pulsed, so the receiver looks for the modulation on top of the background level.
The modulation gets through on one diode and registers as beam not broken.
Click to expand...

OK, a bit over my head some of that.

If I explain my understanding (that may be wrong, perhaps you could explain it in dummy terms for me, please).

The receiving diode is like a closed switch when it receives IR? when IR does not fall upon the receiving diode, does it act as an open switch? That was my thninking and thence as second receiving diode was in series, whole circuit was open and alarm activated. Just like 2 filament bulbs in series, one blows, whole circuit fails.

I figure I have completely mis-understood how these two devices work haven't I?

Is there a way of achieving what I wanted?
 
lw395 said:
At a guess, I would say the IR is pulsed, so the receiver looks for the modulation on top of the background level.
The modulation gets through on one diode and registers as beam not broken.
Click to expand...

I don't know what modulation is.

"The modulation gets through on one diode and registers as beam not broken"

How can the modulation get through if the 2 reciving diodes are in series? May be a dumb question seeing as I don't know what modulation is.

Thanks, by the way
 
Without a circuit diagram its hard to say exactly, but the diodes in series will only be getting half original voltage each at a quess. This kind of 'bodge' rarely works, sorry. In all probability, the design will only work in a straight line anyway and will probably be hard to line up in any case.
 
I doubt if even those who understand the electronics involved would try to achieve what you are asking without access to the circuit details. (sometimes it is possible to work out the circuit visually, but it is extremely tedious and often not even possible because of unidentifiable components).

You say the unit is 'cheap' ... why not buy another? Paralleling the outputs should be possible if that's what you want. Have you considered PIR (passive infra-red) sensors ..... the sort usually used for security lights, but you can use them to switch whatever you want. Cheap (even if you buy a security light and throw away the light part :) ) and generally reliable.
 
VicMallows said:
I doubt if even those who understand the electronics involved would try to achieve what you are asking without access to the circuit details. (sometimes it is possible to work out the circuit visually, but it is extremely tedious and often not even possible because of unidentifiable components).

You say the unit is 'cheap' ... why not buy another? Paralleling the outputs should be possible if that's what you want. Have you considered PIR (passive infra-red) sensors ..... the sort usually used for security lights, but you can use them to switch whatever you want. Cheap (even if you buy a security light and throw away the light part :) ) and generally reliable.
Click to expand...

A broken beam will be more reliable, I would follow the advice to buy another unit.
 
Davegriff said:
Without a circuit diagram its hard to say exactly, but the diodes in series will only be getting half original voltage each at a quess. This kind of 'bodge' rarely works, sorry. In all probability, the design will only work in a straight line anyway and will probably be hard to line up in any case.
Click to expand...

Well, electronics is becoming a hobby for me too, so messing around is good. Didn't factor in the voltage across 2 instead of one, so will measure that too, thanks.

Already, by replacing the stock LEDS with 940nm 100mW ones with 40 degree divergence I have improved the range of the product AND it is MUCH easier to "line up" because of the improved LEDs.

Yes, another unit would be the answer, but I learn nothing from that.

I still can't understand why LEDS in series allow the current to flow when one LED is 'open'. Maybe they are not - I will have to Google the word "modulation".

I may replace them all with narrow beam visible LEDS and get the multimeter out again to investigate.

And I will try to clean my soldering iron more often - did you see that terrible soldering I did?

Cheers
 
Last edited:
The transmitter is a classic astable multivibrator, T1 and T2 alternately switch each other off.
So the IR Led is flashing. I.e 'modulated' at the flash frequency.

The receiver sees a lot of background IR, the diode is never fully off.
The output of the receiver is mostly a steady level, but the modulated signal from the transmitter puts a little wiggle on top of that. Because the receiver knows roughly what frequency the wiggle is at, it can see a small wiggle superimposed on a big steady level.

http://www.nationalstemcentre.org.u...69695a1d1a/22654-22012-99-InfraRed sensor.pdf

Got to go, dinner ready.
 
thanks for that.

will look at the diagrams and try to figure out why I can't put two T1 into the circuit instead of 1.

Maybe if they were about the two T1s (or IR receivers were) 30 feet apart, they may not detedct the "wiggle" etc?

I don't know, but I do have 30 foot of wire and its better than watching tv.

cheers
 
OK, having had a look at the receiver circuit, I can suggest a way for you to try.

First of all, perhaps I can try to explain what is going on (you can skip this bit if you want):

As explained by an earlier poster, the transmitter IR LED is modulated - basically, switched on/off rapidly. This allows the reciever to "ignore" any steady IR, from lights, heaters, etc. and just detect the rapidly changing IR from the transmitter.

The first component in the receiver is a phototransistor - the current that flows through it depends on the level of IR that lands on the sensitive region, which, in turn creates a voltage across the 10k resistor R1 that is proprtional to th elevel of IR. IC1A amplifies this voltage to something more useful - but notice the capacitors on the input and output of this stage C1 and C2 - these make sure it only amplifies the variation in voltage, not any steady value. The next stage IC1B, with the diodes around it is a rectifier, converting the AC output of the first amplifier into a steady DC voltage, proportional to the amount of modulated IR landing on the phototransistor. R8 and C3 provide some filtering so that very brief changes in the IR level are ignored. The clever bit is IC1C which compares the DC level from the rectifier (labelled "COMP" on the diagram) to a reference voltage - if the level is higher, then it turns off T2 and the buzzer doesn't sound, if it's lower (because th ebeam is broken) it turns T2 on and the buzzer sounds.

So, to combine the outputs of two receivers, all you need to do is wire the output ("collector") pin of T2 on both receivers together and if either beam is broken both buzzers will sound. You will need t orun both receivers from the same supply, so that their "GND" pins are joined together, as are "+V"

I hope this helps!
 
You couldn't connect two T1s in series - if they were both looking at different transmitters, it's unlikely that both transmitter LEDs would be on or both off at the same time (synchronised) so you would be unlikely to have both T1s passing current at the same time so the receiver wouldn't work.
 
Right, excellent.

Thank you.

Electronics truly is an amazing topic and you guys that understand it are amazing too.

I have just learned a lot, thank you very much.
 
tim_ber said:
will look at the diagrams and try to figure out why I can't put two T1 into the circuit instead of 1...
Click to expand...
Because the two signals from the transmitters will not be in phase, so the op amp will get mixed voltages. Furthermore, you can't just daisy-chain transistors like this, each one will affect the other.

[Later] just seen Mike's reply #13, I must remember to press refresh before replying.
 
Last edited:
mikefleetwood said:
OK, having had a look at the receiver circuit, I can suggest a way for you to try.

First of all, perhaps I can try to explain what is going on (you can skip this bit if you want):

As explained by an earlier poster, the transmitter IR LED is modulated - basically, switched on/off rapidly. This allows the reciever to "ignore" any steady IR, from lights, heaters, etc. and just detect the rapidly changing IR from the transmitter.

The first component in the receiver is a phototransistor - the current that flows through it depends on the level of IR that lands on the sensitive region, which, in turn creates a voltage across the 10k resistor R1 that is proprtional to th elevel of IR. IC1A amplifies this voltage to something more useful - but notice the capacitors on the input and output of this stage C1 and C2 - these make sure it only amplifies the variation in voltage, not any steady value. The next stage IC1B, with the diodes around it is a rectifier, converting the AC output of the first amplifier into a steady DC voltage, proportional to the amount of modulated IR landing on the phototransistor. R8 and C3 provide some filtering so that very brief changes in the IR level are ignored. The clever bit is IC1C which compares the DC level from the rectifier (labelled "COMP" on the diagram) to a reference voltage - if the level is higher, then it turns off T2 and the buzzer doesn't sound, if it's lower (because th ebeam is broken) it turns T2 on and the buzzer sounds.

So, to combine the outputs of two receivers, all you need to do is wire the output ("collector") pin of T2 on both receivers together and if either beam is broken both buzzers will sound. You will need t orun both receivers from the same supply, so that their "GND" pins are joined together, as are "+V"

I hope this helps!
Click to expand...


What happens if both receivers are receiving the same level but are 180 degrees out of phase?
 
nigelmercier said:
Mike is suggesting combining the outputs, not the inputs.
Click to expand...

Yes, just to clarify - I was talking about combining the outputs of the "T2" transistors. This is a point in the circuit where the transistor is either "on" if the beam is broken, or "off" is it isn't.

Incidentally (in case anyone was wondering), in this setup the "collector" of the transistor acts like a switch to GND, so you can put several of them in parallel and the buzzer will sound if any one of them is "on". The ones that are "off" look like an open circuit and don't affect the output.
 
You must log in or register to reply here.

Share this page

Top