How to switch with 2.8 volts and very low amperage

[tomzarbo]

Tom,

What voltage does the preamp run off?

I'm making an assumption about the backlight circuit (that the backlight diode cathode is tied to ground), but even if the anode is tied to the rail voltage and the resistor is on the cathode side, it should still work. I'm also assuming the backlight is always on. In the circuit I've drawn, attaching the FET gate to the top side of the resistor should actually work better to guarantee that the FET is fully turned on and running in saturation.

The way this circuit works: The FET is open circuit between the Drain and Source, until the Gate voltage reaches a threshold above the Source. The 2N7002 has a very low threshold voltage of ~2.1 V, but the beauty of using a FET is that there is no steady-state current flow into the gate, so it should have minimal effect on your backlight, other than maybe taking a few nanoseconds longer to turn the backlight on. Once the voltage across the G-S reaches the threshold, it conducts current between the Drain and Source, effectively becoming a short in the circuit.

-Ben

Ben,

Thanks a million for working this up. I've been watching all the content I can on YouTube and reading up a ton on transistors and how to use them as switches. Your diagram helps a lot.

I ordered a multi-pack set of transistors, MOSFETS and even some diodes to play around with (read blow up) as I experiment.

Once they come in and things slow down for me in a day or two, I'll jump into this.

This particular preamp runs on 12 volts, but I have others that run on 5 volts. I'd have to say that 12 volts is more common in what I see.

More to come!

TomZ

[1100xxben]

Tom,

What voltage does the preamp run off?

I'm making an assumption about the backlight circuit (that the backlight diode cathode is tied to ground), but even if the anode is tied to the rail voltage and the resistor is on the cathode side, it should still work. I'm also assuming the backlight is always on. In the circuit I've drawn, attaching the FET gate to the top side of the resistor should actually work better to guarantee that the FET is fully turned on and running in saturation.

The way this circuit works: The FET is open circuit between the Drain and Source, until the Gate voltage reaches a threshold above the Source. The 2N7002 has a very low threshold voltage of ~2.1 V, but the beauty of using a FET is that there is no steady-state current flow into the gate, so it should have minimal effect on your backlight, other than maybe taking a few nanoseconds longer to turn the backlight on. Once the voltage across the G-S reaches the threshold, it conducts current between the Drain and Source, effectively becoming a short in the circuit.

-Ben

[tomzarbo]

I've been watching a lot of YouTube videos and looking at a lot of tutorials to try and understand the workings of transistors. I've picked up some good information, I'm not quite at the point of designing a working circuit, but I ordered an assortment of smaller transistors, Mosfets, and diodes to see what kind of trouble I can get into.

It seems entirely possible to use a transistor, resistor, and maybe a diode to take the 2.6 to 2.8 volts common to many of these preamp LED backlights, and use that voltage to trigger a transistor to flow enough voltage/current to trip the time delay relay I'm wanting to use in many of my projects.

Thanks for the guidance in this thread, and the other similar thread from several years back... several folks suggested using transistors then as well.

TomZ
Zarbo Audio Projects

[tomzarbo]

Thank you Devnull, Im reading those articles. It's interesting!


Tom

[devnull]

I have no idea. A little bit foggy this morning from last night's birthday celebration. Take a look at this web page, it has a simple circuit doing what you want without the boost board

https://www.petervis.com/GCSE_Design_and_Technology_Electronic_Products/Transistor_as_a_Switch/Transistor_as_a_Switch_for_Relay.html

[tomzarbo]

Devnull, I'm looking at transistors as well, thank you.

I saw this on another site:



It looks to be able to take 5 volts to do the switching. I would use the little voltage booster circuit I used previously dialed down to 5 volts, feed it into this, and use this to send the 12 volt (+) (The preamp board also needs 12 volts) to the time delay relay board to turn on the amp after a few seconds.

It's a mess of stuff to do what the preamp should already be designed to do from the get-go, but... Think it might work?

TomZ

[devnull]

Use a transistor as the relay, Google will have some simple schematics

[tomzarbo]

I just tried to adjust the little booster board so it's output was 12 volts to trigger a 12 volt time delay relay board which requires a touch less amperage to run.
Still no dice.

[tomzarbo]

Put a small 10uF cap across the LED leads and try it again. If it doesn't stabilize enough to kick on the relay, then I don't know about other options you'd be willing to try.
Wolf

It had an effect, but still no dice.
It made a little humming noise when the relay timed out, attempting to try and push the relay closed, but it still didn't work.
Really good idea, though, thanks!

TomZ

[Wolf]

Put a small 10uF cap across the LED leads and try it again. If it doesn't stabilize enough to kick on the relay, then I don't know about other options you'd be willing to try.
Wolf

[tomzarbo]

LEDs have current limiting resistors. That is why it's not the best source. If you can find the VDC for the LED prior the resistors, then you might have a shot. Remember that a small storage cap, 10uF even, might help in this case to stabilize the voltage.
Wolf

Good point, but I just don't find anything else that I can get to with switched voltage. Although the current is limited, it's still the only thing I can find that switches on when the power turns on. The bits are small surface mount stuff and I'd have a HECK of a time trying to fiddle with that, even with all my lights and magnification.

Even if the device I'm looking for is powered externally -- just detects a positive voltage, then switches on a relay to turn on an amp board -- that would be fine... as long as it's a fairly reasonably priced device. I know something like this must exist, I just don't know what it's called to find it. : )

TomZ

[Wolf]

LEDs have current limiting resistors. That is why it's not the best source. If you can find the VDC for the LED prior the resistors, then you might have a shot. Remember that a small storage cap, 10uF even, might help in this case to stabilize the voltage.
Wolf

[tomzarbo]

I'm at it again...

In search of a way to have one of these small preamp faceplate devices switch on an amp board using just the preamps power on/off function... instead of using a separate power on/off switch.

Here is my latest purchase, a whopping $12.50 from Ali... it's small, looks nice enough, and has a decent feel to the controls even... but still no way to automatically turn on/off the additional amplifier that is required to function as a bluetooth/USB/FM radio.

I'll save you the rant, but I can't believe all these little preamp board still don't have a method to switch on an amplifier... *#$%*&@# !!!!

The arrow is pointing to the leads that go to the LED backlight for the screen, 2.8 volts, and easy enough to solder to.



Anyway...

Here is the little doo-dad on the right I picked up awhile back next to the time delay relay board.



It takes as low as 2.6 volts and converts it to 5 volts. I wanted to take the 2.8 volts fed to the LED and feed that to the little voltage booster device, then feed that to the 5 volt time delay relay board to switch on a decent little amp board with a 1-2 second delay.

Didn't work, same as last time on the other board I tried this on. The booster board did have output, but it just doesn't have enough amperage to make the time delay relay function. I guess the little relay on the board needs more oomph than the little booster board/LED output can produce.

Someone mentioned that maybe some type of solid state relay or solid state relay board might get this to function? Some type of device that can take 2.6-2.9 volts with pretty low amperage... and then switch something on the order of 12 to 24 volts at 2, 3... heck maybe 5 or 6 amps to power a decent class D amp board?

Is there a device that I'm missing that can do what I need? Even in combo with one of the other doo-dads I already have?

I've gotta' keep trying... : )

Thanks for any advice...

TomZ

P.S. Yes, I did check the output at the USB solder pads. They output 5 volts even when the unit is turned off, that would have been easy, right? : )