You surely can.why can't I make it turned on by the audio of the sound card after rectifying it with a germanium diode?
You surely can.
Sounds like you are thinking about audio voltage level and MOSFET Vgs threshold. What did you find out?
The 2N7000 can need up to 3V on it's gate (relative to source) before it starts to conduct. Allowing for the diode drop, you probably need 3.5V audio peaks to drive it into conduction. That equates to about 1.5W of audio if you are driving it from a loudspeaker output. Line level output will be too low to use.
Also check that the PC still produces audio when the output is unloaded, some have sense circuits that drop the audio level considerably when it sees a high impedance, you might have to wire a resistor (~22 Ohms) across the output to simulate a loudspeaker being connected.
Warning: even though the 2N7000 is an enhancement mode FET, you can easily damage it by excessive gate voltage, disconnecting the ground or injecting body static into it is likely to kill it.
You might find a small BJT works better in this application as base current is readily available and the turn-on voltage is much lower.
Brian.
That's a very odd tutorial. 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. It should at least include a resistor to pull the gate voltage back up when the switch is opened.
I seriously doubt your pictured circuit will work as you intend. 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.
Be careful with the ratings of the J175, it isn't intended to be used as a power switch like that. 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. You may need a supply several volts higher than the coil rating to have enough to operate it.
Are you intending to drive it from the loudspeaker output or a line level output?
Brian.
No I intend to drive it directly from the laptop's sound card phones output, the green plug. He connects the drain to the ground.
I was trying to find a way to do it reliably in a single devise. I thought that the "already on" feature of the JFETs would allow me to set the bias in a critical point that the FET does not conduct. Then the extra audio would make it conduct. This would give max sensitivity.
Maybe I should better use the J174 which can pass more current? My relay draws 30mA at 9v.
And maybe I should first rectify audio and feed the DC as an extra bias to the gate somehow, to turn it on?
Any suggestions are really valuable!
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. That gives good immunity to impulse noise and continuity between words in speech.
Brian.
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.
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. 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. 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. You probably want loudspeaker simulation so it provides most output without amplification.
When you know what the source looks like, you can decide whether it needs amplifying or requires an offset. 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.
The method I use is based on a "SignaLink USB" box. Mine died after a while so I opened it up and traced the schematic. It uses an op-amp to boost the level then uses the comparator inside a PIC to trigger a timer. The timer period is set by a front panel control and read by the PIC ADC. The PIC had died so I wrote my own code to replicate the original function. With the delay at minimum it does exactly what you are trying to do but in a spectacularly complicated way!
Brian.
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.
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. 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. 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. You probably want loudspeaker simulation so it provides most output without amplification.
When you know what the source looks like, you can decide whether it needs amplifying or requires an offset. 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.
The method I use is based on a "SignaLink USB" box. Mine died after a while so I opened it up and traced the schematic. It uses an op-amp to boost the level then uses the comparator inside a PIC to trigger a timer. The timer period is set by a front panel control and read by the PIC ADC. The PIC had died so I wrote my own code to replicate the original function. With the delay at minimum it does exactly what you are trying to do but in a spectacularly complicated way!
Brian.
I think I did it. 6 components including the relay and if the relay has an internal diode, 5 components.
The advantage is that it does not use harder to find schottky diodes or germanium ones.
Also, there is no voltage drop since there are no diodes and the audio does not need to be switched all the way up. In my case it was about 70%.
Also the sensitivity can be varied a bit using the bias pot.
Here is how I see it working, which is different from the diode rectification.
The transistor acts as an AF amplifier. The pot, sets the bias so that the relay is just switched on. However the relay pulsates to the audio rate (tone) because there is no rectification. The capacitor on the relay contacts, delays the switching off of the relay and the relay no more pulsates.
I have tested it with 10uF at the relay and it works reliably from 200Hz tone and up and as said, on 70% volume only. CW speeds of 5wpm to 25wpm were tested reliably.
What do you think of my idea?
I think I did it. 6 components including the relay and if the relay has an internal diode, 5 components.
The advantage is that it does not use harder to find schottky diodes or germanium ones.
Also, there is no voltage drop since there are no diodes and the audio does not need to be switched all the way up. In my case it was about 70%.
Also the sensitivity can be varied a bit using the bias pot.
Here is how I see it working, which is different from the diode rectification.
The transistor acts as an AF amplifier. The pot, sets the bias so that the relay is just switched on. However the relay pulsates to the audio rate (tone) because there is no rectification. The capacitor on the relay contacts, delays the switching off of the relay and the relay no more pulsates.
I have tested it with 10uF at the relay and it works reliably from 200Hz tone and up and as said, on 70% volume only. CW speeds of 5wpm to 25wpm were tested reliably.
What do you think of my idea?
- - - Updated - - -
A JFET is shown but I have used the 2n7000
Mains hum on a high Z node can be many volts, and
MOSFET Cgs is nothing at 60Hz.
Line out on the audio card is probably 1V P-P max.
You probably want a gain block, envelope detect,
compare type lineup and I'd suggest rejecting both
very high (EMI) and very low (hum) frequencies.
Might consider a cheap PLL chip set up to detect
(say) 1kHz - bearing in mind >= 10 cycles to lock
and unlock so very slow actuation at audio freqs.
LM567 maybe? Saturated switch to GND, maybe
a switching PNP following that to get a default=
off, arbitrary high side drive swing (if you are
against a cheapo inverting MOSFET driver or in
the case of a logic level FET, maybe just all 6
channels of a 74HC14 paralleled).
- - - Updated - - -Well, it sounds too complicated. I 've done it in a few components. See the very end of this page https://192.168.2.50/allbandtrx/index.htm
Hi neazoi,
Hope you are well. Are the links working? I couldn't get either to open just now.
No it is not frequency sensitive (at least with this relay) it works fine from 100Hz to 3KHz.I have no doubt it works for you but it may not work for others. It is probably frequency sensitive and certainly level sensitive. Holding the relay coil at low current so it only takes a small extra kick to operate it is not efficient and you probably find it draws current even when the relay isn't energized. I appreciate it's simplicity but someone driving with a different sound card or at a different level may find it doesn't work or the relay contacts are suspended somewhere between open and closed which could cause erratic operation. Personally, I would still use a circuit that has a more positive on and off operation.
Brian.
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