IR demodulator amplifier

Hi all,

A machine I have at home can be controlled with an IR remote. It doesn’t have the IR demodulator integrated, but it should be connected to the device with a stereo minijack connector. On the 2 outer rings there is a 5V power supply, the third one is the code output.

I need to build a few more of these minijack ‘eyes’, for the same type of device where this got lost or broken. I have opened the original thing and there is some circuitry inside which I have to duplicate. The problem is that it’s pretty worn out and hard to read which types of components are used.

There are 5 resistors of different values, 1 electronic capacitor, 1 diode and 2 components (probably FETS but unreadable) that I’m not sure about. The 2 seem to be the same type. Anyway, they have each 3 pins.

I have included the schematic of the thing. The ‘empty’ blocks are the two unknown components. I’m guessing that the circuitry is an amplifier or an inverter or… But I’m not too strong on electronics.

Could somebody help me out? I’m guessing it should be possible for somebody with more electronical knowledge to see what the circuit is doing. I can post pictures of the print later.

The only thing I can read on the missing components is ‘BQ’. The IR demodulator itself reads ‘BY68’.

Thanks a lot!

ir.pdf (22.2 KB)

Allright, I’m a bit closer to figuring out what the circuit does.
I’ve hooked up 2 probes of my scope to both the output of the circuit as the output of the IR demodulator to see the difference. A screenshot of my scope is attached.

A) yellow line: BEHIND the circuit. When there are no IR commands received, the signal idles at 0V and will go to 4V when a signal is received.

B) blue line: BEFORE the circuit. When there are no IR commands received, the signal idles at 2V and will go to 0V when a signal is received.

I’ve also attached a picture of the board. It’s the components that read BQ were I’m not sure about.



The two components are general-purpose NPN transistors (as you would expect, really!).

The function is an inverter and line driver (thus the 100 ohm series resistor on the output) which appears to generate 3.3V logic.

Should be dead easy to replicate.

Allright! I got it working. I used just 1 NPN transistor as the inverter: 47k as base resistor and 680 ohm at the collector. That way I got the 5V inverted.

I then have put a resistor of 1.5k to the GND in parallel with the inverted output to get roughly 3.3V. Could somebody explain to me why this needs to be 1.5k? I got it right through trial-and-error. I would have thought that it would behave as a simple voltage divider with the collector resistor and the 1.5k as R1 and R2, but then I don't seem to come to 1.5k as a result for R2.

I must be misunderstanding something essential. To be honest, I have never actually used even a simple voltage divider before.

Yep, that's a voltage divider all right.

1500 / (1500 + 680) x 5 = about 3.6