![]() It uses an op-amp to boost the level then uses the comparator inside a PIC to trigger a timer. Mine died after a while so I opened it up and traced the schematic. The method I use is based on a "SignaLink USB" box. If you are using half duplex "HELLSCHREIBER" mode you do need fast switching but it would be best done electronically rather than by relay as it may have to switch RX/TX/RX on every character sent. When you know what the source looks like, you can decide whether it needs amplifying or requires an offset. ![]() You probably want loudspeaker simulation so it provides most output without amplification. They pass a small DC current through the load and detect the voltage dropped across it to see if it looks like a loudspeaker, headphones or line (1K) load and set the output level accordingly. ![]() Try with and without a load resistor to simulate a loudspeaker as many PC sound systems adapt their outputs according to the load they see connected. I would first look at exactly what the source signal looks like with an oscilloscope so you can get some idea of it's amplitude. Your problem there is you start with a signal that is probably only a few hundred mV then feed it through a bridge rectifier and expect > 0.6V to come out. If this could, I would point this to a phototransistor and hopefully I could drive the relay with just two components. The attached image is similar to this but it uses a bridge and I would prefer a doubler instead or maybe just a single diode and a capacitor to ground would be enough to switch on the bjtĪnother idea: I have tried a 1.5v IR LED if this can be switched on with just the audio tone. Maybe a small transformer (although more rare) would help in stepping up the audio of the sound card so that no other component other than a diode and the 2n7000 is needed? I was hoping on a simplistic circuit with low components count, not an IC based circuit. I do not care about any "key clicks" for this simple circuit. When the tone stops, then the relay must be switched off quickly. Then it must be kept on for as long as the tone presents there. When audio (a single tone) is present at the output of the sound card the relay must be switched on quickly. I actually do not care about the switch on/off timing. If the threshold voltage varies as the relay draws current you can get some very strange feedback effects! If you use one of those, I strongly advise that you stabilize the threshold voltage, especially if you are using a battery supply. The LM311 can drive the relay but isn't so good when the threshold is very close to ground. The LM339 comparator comes to mind as it's threshold goes down to zero but it would not drive your relay directly, you would still need a transistor to drive it. The other input goes to an adjustable voltage to set the switching threshold. The best way to do that is to rectify the audio and filter it to extract the envelope but not component frequencies then send it to one input of a comparator. That gives good immunity to impulse noise and continuity between words in speech. If my assumption is correct, what you need is a circuit that reacts relatively slowly (tens of mS) to an audio input and holds on for say 500mS in the absence of audio before switching off. You could use a 2N7000, it would be far more efficient in terms of power loss but I'm still not sure it will do what you want. They are really intended to be used as signal switches in situations of low voltage and very low current. You may need a supply several volts higher than the coil rating to have enough to operate it.Īre you intending to drive it from the loudspeaker output or a line level output?Įven at 30mA the J174 would drop 3.75V when fully conducting and it would dissipate at least 112mW so it wouldn't be a good solution. Even when fully conducting, it has a drain-source resistance of up to 125 Ohms so be careful it doesn't get too hot while passing the relay coil current. Make sure you connect the FET with source towards the relay and not the conventional way! You can probably find a trigger point where the relay 'nearly' operates then give it a final push with the audio but at best you will apply audio to the relay coil and make it chatter or squeal.īe careful with the ratings of the J175, it isn't intended to be used as a power switch like that. I seriously doubt your pictured circuit will work as you intend. It should at least include a resistor to pull the gate voltage back up when the switch is opened. Using a JFET with the load in it's source and disconnecting a 'turn off' supply to it's gate is not at all reliable and could even damage the FET.
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