Design/modification of a circuit to send +/- 4V signals to mic input jack

By maksdesk ยท 10 replies
Apr 23, 2010
  1. i want to set up a circuit which senses electric signals of the body (electro-cardio signals) and sends them to an iphone mic jack.

    I have already downloaded and hooked up a circuit to detect heartbeats (you can see the circuit diagram at the following link, I am excluding 'http' otherwise the website will not accept the message): //, i get a reasonably reliable signal of amplitude approx. +/- 4V ( I have yet to check the exact value with a multi-meter), so what I would like you to comment on is:

    1. can there be an even better circuit (smaller/cheaper to hook-up)
    2. the setup/modification required to input this signal into the mic jack of the iphone

    I may mention that this is in connection with a project i am working on as part of my masters degree (in Sydney), and would be glad to acknowledge your contribution.

    I need to finalise this electronics part of the design ASAP, so if possible then you can please message me directly on my gmail address as well (maksdesk at

    Thnx in advance and regards.
  2. deegeeh

    deegeeh TS Rookie

    Hi Maksdesk, welcome to our forum and I would like to be able to offer a constructive suggestion but I'm sorry but I do not have any knowledge of this field. Hopefully someone else will be able to help, there is lot of knowledgeable people out there.
  3. maksdesk

    maksdesk TS Rookie Topic Starter

    thnx so much

    thanks so much for your reply and welcome message, actually I had posted this query after reading the expertise of 'captaincranky' in an old/closed discussion i have mentioned, so if you could please do an additional favour and somehow convey my request to him then I am hopeful some progress can perhaps be made...
  4. jobeard

    jobeard TS Ambassador Posts: 11,124   +982

    hmm; sensor data is typically analog -- minute changes in voltage or current as the case may be. Our microphone inputs are analog, but in the millivolt range.

    Typically, sensor data is feed to PCs using the COM (RS232) interface.
  5. maksdesk

    maksdesk TS Rookie Topic Starter

    thnx for replying, yes the sensor data is analog, and i have been trying to feed it into the pc using a micro-processor called Arduino, you may be more familiar with it than i am...anyway, firstly the problem is that when i tried the circuit (did you get a chance to see the link in my original message, please do spare a few moments for that), with an oscilloscope, it seemed to be giving a very nice movement, ranging somewhere in +/- 3-4volts, but when i hooked it up via the arduino to the pc, i was trying to visualize the input in the 'Processing' window but the data coming in seems to be erratic, i am today going to check my arduino for consistency using another sensor, but perhaps it may be due to the reason that the voltage is too high for the arduino to handle, so today i will also try to adjust that by using 3x1.5 batteries [i.e. 4.6 volts] instead of 4 batteries [6volts]. Electronics dept people at Sydney uni had helped me set up two sets of power sources to create a +/- input into the circuit...
    now what i want to explore is to input the sensor into the mic jack of an i-phone, that is the real highlight of my project...
    so far i am thinking to first step down the voltage further to 300 millivolts, by using two resistors at the output end (a class-fellow has shown me how to do that...but the biggest question is that captaincranky had informed in another discussion on this website (which i saw about 2-3 days ago and then decided to register to the forum so that perhaps i could get some good advice..) that mic jacks have to pick up an AC current [ref discussion (h t t p) at ://w w], so i am wondering if an analog input will work in that case or not, and what would be the way to go about it...
    looking forward to ur reply...
  6. jobeard

    jobeard TS Ambassador Posts: 11,124   +982

    I built R/F equipment before transisters or 'chip' based circuits and I've not seen a schematic for a PC audio circuit -- but

    1) a uV meter (used to measure audio) is calibrated to 0 dBV = 1 V RMS across 1k load at 1kHz and that is 'massive'

    2) microphone inputs are in millivolts ( 5mV-50mV), and thus +- 4v would swamp the input circuit --
    instead of nice sign wave inputs, the input curcuit would see something very square wave like :(

    3) the input curcuit *MUST* be protected with a capacitor (value depends upon the lowest Fq to be allowed; a 0.1mfd would allow low Fq into the low impedence of transister/chip circuits) in series with the signal lead
  7. maksdesk

    maksdesk TS Rookie Topic Starter

    So if I understand correctly:

    a. we can use the analog signals of the sensor just as they are (no need for AC current, etc.)

    b. we need to bring down the voltage to 5mv-50mv range

    c. we also need a capacitor to protect the circuit
    i am not sure here about what we mean by frequency in this perticular scenario, the sensor is producing signals that correspond to heartbeats, so i guess that would be approximately 50peaks/minute to 150peaks/minute, i guess that would mean about a little less than 1 peak/second to approx. 3peaks/second

    I am sorry for my slow brain power, as i said, i am new to this field, i greatly appreciate your patience and time...
    thnx and all the best...
  8. jobeard

    jobeard TS Ambassador Posts: 11,124   +982

    you got it :) That will produce inputs just like you see on the tv medical shows for the heart monitors.
  9. maksdesk

    maksdesk TS Rookie Topic Starter

    ok, so far so good, next step is I am going to make a circuit diagram and send it to you tomorrow so you can have a final check before i set it up, thnx again for ur time...
    best regards.
  10. maksdesk

    maksdesk TS Rookie Topic Starter

    hi jobeard,
    sorry i got stuck with another assignment, just got through with it so i am back to where i left off my discussion, i have made a rough draft of the circuit diagram and am just waiting for a chance to verify the voltage of the output lead, will send you the diagram as soon as i have that done, i just need to make a power pack for +-6v input, will probably get a chance this weekend so i hope to communicate to u by same day next week. meanwhile take care and please stay tuned...!
  11. CMH

    CMH TechSpot Chancellor Posts: 2,039   +9

    This is interesting. I don't think I've ever seen the mic input ever used this way.

    If it works, posting the working schemetic here would be nice :D
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