Beginner, infrared proximity detector working wrong way around

Thread Starter

notquitethere

Joined Mar 23, 2020
14
Hi,

I'm very new to electronics stuff, trying to learn it with my young son. We've been soldering together some basic kits from ebay which has gone well so far.

We've just put together an IR proximity detector, though, and it appears to work backwards to the description. It says it should light up and sound the buzzer when waving a hand near it, but it seems to light up and sound all the time but STOP when I put my hand near it.

The schematic:

Partslist:

Circuit board:

Any suggestions for how I try debugging this?

Ebay posting in case it can help; not trying to advertise it:
https://www.ebay.co.uk/itm/10pcs-DIY-Kits-Infrared-Sensor-Alarm-Electronic-Circuit-Suite-DC-5V/352963796317

(Hope this is the right subforum for it. Wasn't sure if here or 'homework' was better.)
 

jpanhalt

Joined Jan 18, 2008
9,125
D1 is the transmitter; D2 is the receiver. The component list has them the other way around. Are they the same part numbers (emitters can also be receivers)?

Does changing R3 help?
 

djsfantasi

Joined Apr 11, 2010
6,362
This is a typical problem with sensors and the typical solution is switch the connections to pins “2&3” of the op amp (LM358). But this is a kit and that probably isn’t easy. So I suggest where to look without answering your question.
 

AlbertHall

Joined Jun 4, 2014
9,665
Are they the same part numbers (emitters can also be receivers)
In the D2 position the encapsulation is black, in the D1 position it is clear.
The black one is probably intended to be the receiver in which case they are the right way round.
Is D2 fitted the right way round?
Do you have a multimeter?
Does it perform the same in the dark (maybe ambient light is affecting it)?
 

Thread Starter

notquitethere

Joined Mar 23, 2020
14
D1 is the transmitter; D2 is the receiver. The component list has them the other way around. Are they the same part numbers (emitters can also be receivers)?

Does changing R3 help?
On the first one we put together I got D1 and D2 the wrong way around, and I can't get that board to emit a peep any way around.

They're not the same part - the emitter it clear with a square base (and a square outline on the PCB) and the receiver a black "traditional shape" (to my mind) LED.

If I twiddle R3 then I can make it stop responding entirely -- all the way one way it won't stop peeping, all the way other I can't make it peep at all.
 

bertus

Joined Apr 5, 2008
20,465
Hello,

Does the circuit react at all?
If the transmitter and reciever constant see each other, the circuit will not react.
Place a shield (perhaps a piece of cardboard) between the transmitter and receiver.

Bertus
 

djsfantasi

Joined Apr 11, 2010
6,362
D1 is the transmitter; D2 is the receiver. The component list has them the other way around. Are they the same part numbers (emitters can also be receivers)?

Does changing R3 help?
Good catch! But the TS’ picture looks like D2 (transmitter) is wired where it should be. the receiver should be. However, researching similar circuits, I’ve noted two differences.

First, look at this schematic. Note the placement of the IR receiver. It’s on the opposite side of the resistor than this PCB. Nothing much we can do about this easily.

Secondly, I can’t tell how D2 is wired. But the circuits I’ve found has the cathode of the receiver connected to Vcc. The receiver anode is grounded.

If the TS soldered then IR receiver backwards, it could be causing his problem.
Can you double check where the cathode and anode of the receiver are soldered and let us know?
0AB2065E-A0E7-42FF-B1F9-9AE201FD9CF4.jpeg
 

Thread Starter

notquitethere

Joined Mar 23, 2020
14
Is D2 fitted the right way round?
I believe so. The negative flat side matches the flat outline on the board, which matches the shorter leg of the component in the spare kits.

Do you have a multimeter?
I do. I did try reading the product description on ebay to try to debug it but started to flounder. When someone talks about "the voltage at pin X" is that measured between the pin and ground?

Does it perform the same in the dark (maybe ambient light is affecting it)?
It seems to.
 

Audioguru again

Joined Oct 21, 2019
1,304
I could not read the description but the circuit is designed to beep and light the red LED when the black sensor is lighted by the IR beam from the IR LED then stop when the IR beam is blocked.
The list of parts has the black sensor called the transmitter instead of naming it as the receiver.

You can reverse the operation so that the beep and LED work when the beam is blocked by swapping pin 2 and pin 3 of the IC.
 

djsfantasi

Joined Apr 11, 2010
6,362
I could not read the description but the circuit is designed to beep and light the red LED when the black sensor is lighted by the IR beam from the IR LED then stop when the IR beam is blocked.
The list of parts has the black sensor called the transmitter instead of naming it as the receiver.

You can reverse the operation so that the beep and LED work when the beam is blocked by swapping pin 2 and pin 3 of the IC.
I agree. Just like I said in post #4
 

Thread Starter

notquitethere

Joined Mar 23, 2020
14
Secondly, I can’t tell how D2 is wired. But the circuits I’ve found has the cathode of the receiver connected to Vcc. The receiver anode is grounded.

If the TS soldered then IR receiver backwards, it could be causing his problem.
Can you double check where the cathode and anode of the receiver are soldered and let us know?
(For my own benefit, because I still only think in terms of positive and negative
Anode = positive
Cathode = negative
)

My board has the anode connected to ground. The cathode connects to R2.
 

djsfantasi

Joined Apr 11, 2010
6,362
Missed your reply. That is how it should be connected. As @Audioguru and I have stated. You could swap pins 2 & 3 of the IC. Or you might be able to put D2 where R2 is on the PCB and R2 instead of D2 for a test.

Make sure D2 is connected properly, with the cathode/ negative side connected to Vcc.
 

MrChips

Joined Oct 2, 2009
20,914
Swapping pins 2 and 3 would be a bit difficult to do on that PCB.
It might be easier to reconfigure the output driver transistor stage. Are you prepared to do that?
 

Thread Starter

notquitethere

Joined Mar 23, 2020
14
I'll try swapping D2 and R2.

Then I'll... bend pins 2 and 3 upwards, solder some wires on and reverse them. Or is that never going to work?

Good job these come in packs of 10.
 
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