Switching Audio with DPDT Relay

Discussion in 'The Projects Forum' started by 314159265, Jun 2, 2009.

  1. 314159265

    Thread Starter Member

    May 29, 2009
    I had such good luck with my last topic, I'm going to make a new one.

    Here's a brief rundown of my project:

    I am developing a system to record two-way radio conversations. Both the receive audio and the transmit audio will be routed to a recording device. Normally, audio will be routed from the external speaker output to the recording device. However, when the Push to Talk is activated, audio from the microphone will be routed to the recorder instead of the audio from the speaker. This will allow the recorder to pick up both sides of the conversation on a single channel.

    This will require a DPDT relay to switch the audio when PTT is activated. However, I'm not sure which type would be best. Will an electromechanical relay work in this situation? Should I look at SSRs, or FET gates? I'm looking for a simple, relatively inexpensive solution to automatically switch the audio.

    I really like the idea of a relay because it's easy to connect to. There are 8 pins, and I have 8 wires to be connected: PTT & PTT Ground, Mic (+) & Mic (-), Speaker (+) & Speaker (-), and Output Audio (+) & Output Audio (-). I'd like to avoid mechanical relays if possible, since they would have to be replaced every now and then. However, I don't have the expertise to build some of the circuits I've seen for transistor-based relays online, which have used multiple transistors. SSRs seem pretty expensive.

    I'd appreciate some advice with this.

    Thanks, π
  2. Externet

    AAC Fanatic!

    Nov 29, 2005
    Use one third of a CD4053 integrated circuit.
    The grounds can be connected commonly and do not need to be switched. A single pole double throw set of contacts is enough.
    The PTT can activate the IC and 12 V for the IC supply can be probably taken from the transceiver.
    The microphone level and the speaker level are sustantially different in amplitude. Around a thousand factor. One will have to be pre-amplified, or the other attenuated, or both; to present similar levels to the recorder.
  3. 314159265

    Thread Starter Member

    May 29, 2009
    How would I connect my project to that chip? I looked at the datasheet, but I don't really understand how it would switch audio between two sources.

    I found another chip at Newark (link), but it is way too small to solder anything. However, the logic diagram makes a lot more sense to me. I would connect the two inputs, so that both sides of the switch are triggered by the PTT. Here's what I was thinking for the connections:

    Mic + ------->NO1
    Mic - ------->NO2
    Spkr + -------> NC1
    Spkr - ------->NC2
    PTT -------> IN1 & IN2
    PTT Grnd ------->GND
    Output (to computer) + -------> COM1
    Output (to computer ) - -------> COM2

    When PTT is activated, the outputs will switch from the speaker output on the radio to the microphone inside the hand mic. Is this the right way to do this?

    Can the grounds still be connected (in order to use a single DPST) if the microphone line is carrying somewhere between 5 and 9 volts?

    Also, the PTT line is normally carrying ~5 volts, but when the button is activated, it is shorted to the ground. Does this mean I should switch the microphone and speaker from how they are listed above?

    AAC Fanatic!

    Jul 1, 2008
    Good luck with your project but be aware that the laws regarding recording of telephone communication also apply to wireless. It is illegal the record conversations when the other parties are unaware that they're being recorded.

    By the way, what are you going to use COM1 & COM2 for in this circuit? I saw it referenced on the schematic that you posted.
  5. 314159265

    Thread Starter Member

    May 29, 2009
    That was a concern, but since we're only recording radios within our program, it shouldn't be an issue. Everyone knows that they're being recorded, and is OK with it. The recordings are only for internal review and training--they don't get released or anything like that.

    COM1 and COM2 were the positive and negative output audio to the computer. I guess COM is a bad abbreviation for computer, but I haven't used COM ports in so long, I just forgot about it.

    Anyways, I've made more progress on this project, but hit another road block. I ended up using the ADG436BNZ-ND. However, after everything was finally hooked up, I'm only getting static in the audio output to the computer. I've check continuity between the connections, and some of the more obvious things that I know of, but I'd appreciate any more advise on this. Here's the pinouts:

    Power is run through a 5v voltage regulator, and is 4.98 volts at the regulator, 4.28 volts at the microphone. The microphone setup is as described in my previous topic, with a 2.2 kΩ resistor.

    S1A = External Speaker from Radio +
    S1B = Mic +
    D1 = Output to recorder +
    S2A = External Speaker from Radio -
    S2B - Mic -
    D2 = Output to recorder -
    IN1 = PTT
    IN2 = PTT

    Vss = GND ("Most Negative Power Supply Potential in Dual Supplies. In single-supply applications, it can be connected to ground.")
    Vdd = +5v?

    PTT voltage is 5.2 when receiving, and 0 when transmitting.