MT8870 Decoder circuit review

Discussion in 'General Electronics Chat' started by Razor Concepts, Nov 28, 2009.

  1. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
    212
    1
    Here is a schematic I have made, it takes an audio input from a cell phone, puts it through the MT8870 that outputs binary, and put the binary output through a CD4028 for a BCD output. I think the guts of it are correct (audio to binary works fine on breadboard).

    http://i45.tinypic.com/2d6hxd.jpg

    JP1 and JP2 are generic pin headers for external connections, no specific application, just a digital out. Then there is a resistor array and LED indicators.

    The pot R4 is (I think) used to change the sensitivity of the MT8870, so signals with low peak voltage will be best read with a lower resistance, and signals with a higher peak voltage is best with a higher resistance. The 1K resistor is there in case the pot is set to 0 ohms.

    With the leftover in/out on the darlington, I may just put the binary outputs on those just in case it is needed for something.

    First, is the 1k resistor to "protect" necessary?
    Second, the incoming audio signal is stereo. Should I combine the two signals into one, or only just use one as shown in my schematic? If I were to combine the signals, can I just connect both the left and right signals together?
     
  2. rogs

    Active Member

    Aug 28, 2009
    279
    37
    Connect pins 5 and 6 of the 8870 to gnd, (pin 9). (Correction -this is not strictly necessary if you're using an MT8870D, which has internal pull downs. If you're using an earlier version, you may need to connect them to ground, as I suggested. Check with the relevant data sheet if it's not a 'D' version - or just connect them to ground anyway!)

    As you've drawn it, the 8870 binary outputs will only be active whilst it's actually decoding. If you want the data from the 8870 to stay latched, connect pin 10 to 5V, not to pin 15. (Same comment as above. If you're using a 'D' version, then you can leave pin 10 floating (it's pulled high internally) but you need to connect to 5V for earlier versions.
    If you do want the outputs to go 'off ' when there is no code, you will need to pull each output low ( to gnd) via a resistor - say 100k. As you've drawn it, they are left floating with no audio input, and no code output, when pin 15 goes low.
    You could use a 5 pin 4 resistor network as the pins are all next to each other.

    One thing about the 8870 that's an absolute pain -it has no power on reset, so if you do leave the outputs enabled (pin 10 high) then you have no idea of the output latch states, until the device has received at least one valid code after power up!

    With R1 at 1K, you have an adjustable input audio gain range of some 34dB - which is greater than the dynamic input range of the 8870! (That is about 30dB max).

    Make R1 more like 10k - and make sure the signal at pin 3 of the 8870 never exceeds about 4V p-p. If it clips the 5V supply, you can get false decoding!

    Aim an audio input range of 500mV to 2V p-p at pin 3.

    Just take a mono audio input. If you do join the stereo input it probably wont matter, but some stereo head phone outputs don't actually like being joined together without series resistors. Rare these days, but why take the chance when amono input should provide all the information you need.
    Remember the 8870 has quaite a large input detect range (up to almost 30dB, as I mentioned above).
     
    Last edited: Nov 28, 2009
  3. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
    212
    1
  4. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
    212
    1
  5. sushanth_pai

    New Member

    Dec 3, 2009
    1
    0
    dude..i wantd to noe..can i gnd pin 15 std of mt8870D...
     
  6. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
    212
    1
    I'm not sure of this, but pin 15 I think just pulls low when a signal is detected. So you could leave it unconnected, or ground it but grounding it shouldn't do anything.
     
  7. rogs

    Active Member

    Aug 28, 2009
    279
    37
    No - pin 15 is an output which goes high (to 5V) when a valid tone pair is detected.

    If you don't need to use it, leave it disconnected.
     
  8. Eng.Obidah

    New Member

    Jun 23, 2012
    1
    0
    Dear Razor Concepts .. thanks 4 u .. could u please take us the attachment of this board (not containing SMD items )
    Regards
     
Loading...