Optical compressor, Led won't light up

[seadi123]

So i build the Dod 280, and it doesn't work. Only when i max both the pots i hear just a tiny bit of signal. Bypass works so offboard wiring is ok. Also replaced the chip. I have used 3904 for the transistors, which i dont think is a problem. Ran a knife through the rails to ensure they're separated, also checked the cuts. Checked the voltages on the led, chip and transistors collector, looks fine

One thing i have noticed is that the led won't light up at all when i strum the strings. I have used 5mm green led and the ldr from tayda. Chip is also lm358 from tayda. The led lights up when i touch the ldr leg with my finger, which also makes a buzzing noise. Also one strange thing i have noticed is that when i measure the resistance actoss the ldr it says 26k, which is impossible since the ldr is in parallel with the 1M resistor. Is there anything else i can try before replacing the circuit?

http://www.taydaelectronics.com/photo-conductive-cell-resistor-ldr-650nm-radial-ke-10720.html

[Kipper4]

go here


http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

report back.

Do you have an audio probe?

schematic link

http://www.generalguitargadgets.com/pdf/ggg_opcomp280a.pdf

[duck_arse]

eliminate the impossible, and you are left w/ the merely improbable. in this case:

.... i have noticed is that when i measure the resistance across the ldr it says 26k, which is impossible since the ldr is in parallel with the 1M resistor ....

we can eliminate this sentence. the ldr resistance is in parallel with the 1M, so the total MUST be less than 1M, and less than the ldr resistance. if'n you seperate the 1M from the ldr, you should see a resistance of ~30k on the ldr. this indicates you have it too lit, it is probably compressing to the max from ambient light. you need to keep the ldr in the dark, so it is only influenced by the action of the led.

what kipper said, plus can we see photos of your built, please?

[seadi123]

When i hover the ldr with my hand the resistance increases, however i thought that since the ldr is in parallel with 1m resistor, the combined resistor cant be lower than 500k. Here are the measurements:

Schematic: http://www.tonepad.com/getFile.asp?id=40

Input : 9.2V
After D1 : 8.8V
After Led : 7V
Ldr legs : 4.3V
Q1 and Q2 Collectors : 7v
Q1 and Q2 bases and emittors : 0V

IC:
1 4.3v
2 4.3v
3 3v
4 0v
5 4.3v
6 4.3v
7 4.3v
8 8.8v

Photos (I took the heatshrink tube off of the ldr/led)

[seadi123]

Wait a minute!!! The 100n Cap!!

[seadi123]

It lives! I had forgotted to solder in the input cap, there was actually no signal at all going through the circuit. Sorry for bothering you all

[Kipper4]

Easy fix well done.

Sent from my SM-G920F using Tapatalk

[duck_arse]

excellent.

[PRR]

> When i hover the ldr with my hand

That may not be enough. When I developed a complex LDR system, I mostly worked in a dark room AND with tight covers on the LDRs.

> i thought that since the ldr is in parallel with 1m resistor, the combined resistor cant be lower than 500k.

Don't think. Experiment!! Put 1Meg and 27K in parallel. What does it read?

[seadi123]

I had the formula wrong. I thought resistance in paralel was just R1+R2/2