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LM567 circuit troubleshooting

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elsey.jack

New Member
Hello, I'm working on a project that involves transmitting data over a set of walkie-talkies. I am using a microcontroller to generate the signal (703 Hz) and a LM567 tone decoder to detect the signal on the other end.

Problem is, I can't get the LM567 to work! The circuit I have set up should turn off the LED when ever a ≈703 Hz signal hits pin 3, but the LED remains lit. Here's is my schematic:

C1 = .1 uF
C2 = 1 uF
C3 = .1 uF
R1 = 12.93 kΩ
R2 = 1 kΩ
Signal voltage = 110 mV AC (square wave)



I've looked at the datasheet (which is confusing) and pored over all of the other related threads (which are great ;)), but I can't figure out what my problem is. Am I maybe getting something about the logic output at pin 8 confused?

Any help would be appreciated,
Jack
 

MikeMl

Well-Known Member
Most Helpful Member
Is there a DC blocking capacitor between your audio source and pin 3? This pin MUST float for DC.

If you look at the unlocked VCO frequency, is it close to 700 Hz? If not, it will never decode.
 

Hero999

Banned
Why on earth are you using an LM567 in conjunction with a microconrtoller?

The tone decoding can be done entirely in software.
 

elsey.jack

New Member
Is there a DC blocking capacitor between your audio source and pin 3? This pin MUST float for DC.

If you look at the unlocked VCO frequency, is it close to 700 Hz? If not, it will never decode.
Ah! Placing a capacitor there did the trick!

Thanks!!
 

nes

New Member
I have almost the same problam.
I set this circuit but the led do not lit. unles I disconnect leg 7 from GND.
I try i with 2 diffrent lm567.

Any help?

Nes
 

MikeMl

Well-Known Member
Most Helpful Member
Well, since pin 7 is the chip ground, lifting it to get the LED to light proves nothing.

Is the VCO running? Is it running at a frequency close to the audio tone you expect it to decode?
 

audioguru

Well-Known Member
Most Helpful Member
"Leg" 7 (pin 7) is supposed to be connected to ground for the LM567 to work.
The circuit posted here turns off the LED when the signal is at the correct frequency. The LED lights when the signal is absent or is at the wrong frequency.
So your circuit works normally.
 

nes

New Member
So fast answare Thank you!!

Dose it mean the chip is okay?
can I chek the chip by connect only pin 4-vss 7to Gnd and leg 8 will be at vcc?
shoud the led lit when pin 3 is disconnected?
the frequency is 16KHz I use c1=2.32E-08 r1=2400 c2=1uf
 

nes

New Member
audioguru-in my circuit the led do not light when there is no signal at input.
(It dose only when I disconnect pin 7)
guess somthing is wrong here
is it?
 

audioguru

Well-Known Member
Most Helpful Member
The circuit that was recently posted lights the LED when there is no input and when the input is not at the tuned frequency. Yours is wrong.
 

YaronShavit

New Member
Hi guys

I've been trying to detect a off-hook tone using the LM567 and been having some problems.
For some reason, the input is always 2V high even if nothing is connected to it.
I'm using a 5V power and adjusted the R's and C's to match 2600 hz detection with 15 BW.
Any ideas?
 

audioguru

Well-Known Member
Most Helpful Member
A dial tone is 350Hz plus 440Hz in North America and is 425Hz in Europe.
2600Hz is used as a signalling tone.

Since you did not post your schematic and how the LM567 connects to the telephone line then we can't help you.
 

YaronShavit

New Member
Thanks for the quick response.
However, please note that it is not dialing tone I'm trying to detect but rather a off-hook tone consisting of the frequencies 1400, 2060, 2450 and 2600 Hz. My schematics is exactly the same as depicted in the first post on this thread (only I did connect the input line via a capacitor) with proper components values .

I guess my question is this, does it make any sense that I read 2V on my LM567 input pin when it is not connected to anything? Isn't it suppose to be floating? Does it mean I have a problem with the ic itself?

Thanks
 

audioguru

Well-Known Member
Most Helpful Member
Of course the input pin 3 has a voltage on it. It is part of the complicated electronic circuit in the LM567 IC.
Maybe you connected the input capacitor to a telephone line with 90V ringing or 50V battery which destroyed the input of the IC.
 

audioguru

Well-Known Member
Most Helpful Member
I don't have an LM567 to measure. Its input pin 3 should have a bias voltage on it but I don't know how much. You need an input coupling capacitor so that the signal source does not interfere with the internal bias voltage of pin 3.
 
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