Simple Audio Switch Circuit - Help Needed

Discussion in 'The Projects Forum' started by Aldanor, Apr 19, 2009.

  1. Aldanor

    Thread Starter New Member

    Apr 18, 2009
    Hello guys, I'm currently working on a simple audio switch circuit and I can't quite figure it all out..

    The problem is as follows: there is a walkie-talkie, an iPod and headphones. If there's no signal from the radio, iPod is routed to headphones by default; if there is any signal from the radio - the iPod signal is muted or dimmed and you hear the audio from the radio. The signal from the radio comes pretty loud, looks like it's amplified.

    There are a couple of problems - first, I have to admit that my knowledge in circuits is pretty far from advanced, I'm trying to learn though. Another problem is that there is no Radioshack in Canada - so all that's available to me atm is a bunch of npn/pnp transistors, 4000 series diodes, maybe a relay, and maybe a 741 opamp. And finally, I need this little gadget for the trip and I have one week at most to build it..

    That's what I came up with:

    However, I'm TOTALLY lost trying to place the capacitors and resistors properly.. any advice?

    The diode bridge rectifies the signal, then there is a battery because the audio signal itself doesn't seem to be able to trigger the first transistor (am I wrong?). The first transistor is a not-gate, so it requires a battery (which is a little bit weird - cause this circuit will consume power whenever NOT used..). This not-gate doesn't work as I want it to though - when there is input signal, the output is not zero, it's like (1.5V - epsilon). Should I base the circuit on transistors in the first place? Does it really need two AA's? If it does, how long would the batteries last? Wouldn't the second transistor hurt the iPod sound?

    This circuit kinda-works, whenever a sound signal comes from the radio, the iPod signal is dimmed a little bit and you can hear the radio. However, there are some distortions with the radio signal and also it's very loud (currently, this circuit works only if the radio is set to it's max volume).

    I hope this doesn't look like another "dude" post, I'm really trying to figure things out but having no experience in circuits whatsoever it's getting really hard.
    Last edited: Apr 19, 2009
  2. bertus


    Apr 5, 2008
  3. Aldanor

    Thread Starter New Member

    Apr 18, 2009
    A couple of questions: does this circuit really need +12V? Is there a way to do this with 1.5V / 3V? Could I use 2N2907 (or 2N3906) instead of BC557B and two 2N3904's (or two 2N2222's) instead of BC517 Darlington pair?

    P.S. Is my circuit complete unusable? No way to fix it?
  4. Audioguru

    New Member

    Dec 20, 2007
    Audio is AC. Transistors switch DC, not AC.
    Your full-wave bridge rectifier needs a signal that is at least 1.4V which is very loud.
    Yes, your circuit is unusable.

    A circuit can be made that uses a 74HC4053 3-channels electronic switch to switch line-level (not headphones) and it uses a 3V supply. Then a relay is not needed but a stereo headphones amplifier is needed.
  5. Aldanor

    Thread Starter New Member

    Apr 18, 2009
    Hey guys, so, I assembled a circuit like this (thanks, bertus):

    As a PNP transistor though, I used 2N3907, for Darlington pair I used two 2N3904's, instead of 1N4148 diode I used 1N4001, instead of 47uF bipolar I used two 22uF's in parallel (is any of this essential? I'm not sure I can get other transistors but 4148's I could probably find).

    The relay is nexxtech 5-amp relay (12V DC, 60mA, 200 ohm), it switches on/off at ~6.7V.

    The incoming audio signal peaks at 0.5-0.6V or less. I'm not sure I measured that right, my voltmeter is not precise on AC.. the audio signal is slightly amplified by the walkie-talkie though and it's significantly louder than your iPod line out.

    The way this circuit currently works is somewhy very unpredictable.. it works, then it doesn't, then a relay turns into a loudspeaker, then in 'off' state the voltage drop increases twice, then it works fine again, then it just stops working. I tested all connections and wiring twice - it's pretty solid.

    This circuit is supposed to be powered by 12V DC (in the schematic), however if I give it ~12V via universal adapter, the relay tension is too high even when there's no audio signal. When connected to universal adapter at 9V (somewhy I had to set it to 6V to get 9.10V lead to lead on my voltmeter), this circuit catches the audio peaks (about 8.2V on relay) and very often it starts switching on and off rapidly. Sometimes it keeps locked on, even when the audio signal is very low. Sometimes the relay starts working as a mini loudspeaker (?) - buzzing and reproducing the incoming signal with distortion. Btw should I protect the circuit against relay spike?

    When I connected the circuit to 12V battery (a small A23), it started draining it pretty fast and acted weirdly; when I connected it to 9V battery - sometimes it would give just 3 volts on relay when there is audio signal, sometimes it would only switch on during the loudest audio peaks. If the relay 'catches' the audio peaks correctly, then one way of fixing all this is somehow lowering relay's switch-off threshold? Or kind of latching the 'on' status for a couple of seconds?

    Any help/advice?

    Thank you.
    Last edited: Apr 20, 2009