How to convert positive signal to ground?

Thread Starter

Starbase101

Joined Apr 27, 2021
21
Hi, I have an Adafruit FX Mini board which requires grounding of the trigger inputs for playing recorded sounds. The problem is, my switches for turning on LEDs (12VDC) and kitchen digital timer buttons (3VDC) all have a common ground and activate on the "positive" side of the circuits. The switches are SPST (the ones I want to use aren't available as DPST), so what can I put between the switch's positive output and the Adafruit board for sending a ground signal to the trigger inputs? Thanks!
 

BobaMosfet

Joined Jul 1, 2009
1,780
Well, normally you would simply ground the input. use a dipswitch or a transistor. Just make sure you put a resistor in there so that you don't short positive to ground.

You may want to learn about pull-down and pull-up resistors.

Here, to help:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3
 

Thread Starter

Starbase101

Joined Apr 27, 2021
21
Thanks. Well, it won't be a dip-switch because this all goes inside a cabinet with only the push button and toggle switches on the outside. I expect some sort of transistor or inverter on the switch so when it's completing its positive circuit it's also triggering something else in parallel for grounding the sound board's input trigger. I just want to know what component to put between the switch and the sound board input to pull it down to ground.
 

Thread Starter

Starbase101

Joined Apr 27, 2021
21
It seems I'll need an NPN transistor on each switch with their gates connected through a 10K resistor to each 3V/12V switch, their sources connected to common ground, and their drains connected to the sound board's triggers. I know there exists resistor array and diode array ICs, so maybe there's an IC with multiple NPN transistors on one chip that could work for me? Or am I shopping for discrete transistors, one for each switch? Which NPN device is the correct one to use? Thanks
 

KeithWalker

Joined Jul 10, 2017
1,789
Hi, I have an Adafruit FX Mini board which requires grounding of the trigger inputs for playing recorded sounds. The problem is, my switches for turning on LEDs (12VDC) and kitchen digital timer buttons (3VDC) all have a common ground and activate on the "positive" side of the circuits. The switches are SPST (the ones I want to use aren't available as DPST), so what can I put between the switch's positive output and the Adafruit board for sending a ground signal to the trigger inputs? Thanks!
From your description.I don't understand what type of switches you have. Do you mean they all have have normally closed contacts that open when they are actuated or normally open contacts that close when they are actuated?
You need momentary acting switches for the trigger inputs, not toggle switches.
This is how you should wire normally closed and normally open switches to ground an input on the FX board.:

Switches.jpg
 
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sghioto

Joined Dec 31, 2017
2,363
multiple NPN transistors on one chip that could work for me?
General purpose NPN transistor like a 2N3904 or a ULN2002 chip. The ULN2002 has 7 separate transistor circuits with the 10K resistors built in with an inverting output. Mainly for driving relays (and certainly overkill for this project) it's outputs will only sink down to appx .7 volt. Not sure if that is low enough to trigger the Mini Board.
 

KeithWalker

Joined Jul 10, 2017
1,789
What supply voltage are you using? If it is close to 5V you can use a SN7404 inverting hex buffer. It contains six logic level inverters.
 

Thread Starter

Starbase101

Joined Apr 27, 2021
21
Sorry, I don't have schematics.

Most of the switches are momentary push-button normally open. One is a maintained push-button normally-open for turning on/off an LED strip light. I'd still like that one to trigger a sound when pushed to the On position, and perhaps also a different sound when released to the Off position. The toggle switch is normally-open and turns a motor on/off, and like the maintained push-button switch it would be preferred having a sound triggered for its two positions.

The LED strip light runs at 12VDC from an AC adapter. This 12V supply also powers the LEDs of push-button switches, a 5VDC regulator for Adafruit boards, and a 12V-to-3V converter for powering a timer board. So, besides the 12V LED voltage (which are separate terminals on the switches) there is a +12VDC supply, a +5VDC supply, and a +3VDC supply. I believe the FX Mini inputs are normally high and trigger the sounds when inputs are grounded.
 
There is a hard to read schematic on Addafruit's webiste and can;t spend the necessary time figuring it out.

Buttons could be scanned in a matrix form. Usually, you don't need a full contact closure. Sometimes even 1000 ohs works.
In any event, an OPTO FET, a optoMOS relay or PhotoMOS relay are your best options. They are effectively a LED and isolated FET(s). Contact resistance varies. They can be had using 1mA of current to turn on with whatever LED voltage drop. It's definately painless.
 

MrChips

Joined Oct 2, 2009
23,566
Usually you can choose and arrange your switch to get the desired logic action:

1) a switch can be normally open (NO) or normally closed (NC).
2) a switch can be wired on the low side (short to GND) or high side (short to supply voltage) with the appropriate pull-up or pull-down resistor.

Thus you have four possible combinations.

Failing that, you can use a single transistor as an inverter. Now you need to choose a low-side switch (NPN transistor such as 2N3904) or high-side switch (PNP transistor such as 2N3906).
 

KeithWalker

Joined Jul 10, 2017
1,789
Sorry, I don't have schematics.

Most of the switches are momentary push-button normally open. One is a maintained push-button normally-open for turning on/off an LED strip light. I'd still like that one to trigger a sound when pushed to the On position, and perhaps also a different sound when released to the Off position. The toggle switch is normally-open and turns a motor on/off, and like the maintained push-button switch it would be preferred having a sound triggered for its two positions.

The LED strip light runs at 12VDC from an AC adapter. This 12V supply also powers the LEDs of push-button switches, a 5VDC regulator for Adafruit boards, and a 12V-to-3V converter for powering a timer board. So, besides the 12V LED voltage (which are separate terminals on the switches) there is a +12VDC supply, a +5VDC supply, and a +3VDC supply. I believe the FX Mini inputs are normally high and trigger the sounds when inputs are grounded.
If your switches are normally open, you do not need to invert the signal. A momentary connection to ground is all that is required. No pull-up or pull-down resistors are required. To get a momentary signal from a toggle switch on both open and close will require some logic and pulse generation.
 

sghioto

Joined Dec 31, 2017
2,363
This works on the bench. Capacitor values should give the required pulse durations required by the FX inputs.
1619559972989.png
 
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Thread Starter

Starbase101

Joined Apr 27, 2021
21
Wow, that's a lot of components for each switch. Thank you for all the replies, I'm holding back with my replies until I get a schematic drawn up and posted.
 

Thread Starter

Starbase101

Joined Apr 27, 2021
21
Okay, I think I've got something to show (hopefully it's not all wrong - I'm a CAD engineer, not an electrical engineer):
Schematic.jpg

What I'm wanting to do is connect the sound signals S0 through S7, but they are positive from the switches and the FX Mini triggers need to be pulled to ground for playing a sound.

Switch Notes:
  • The maintained pushbutton switch has 5 terminals - common, on, off, LED plus, and LED minus. I would like it to trigger S4 when pressed on, and S5 when pressed off. Since the FX trigger pins can't be held at ground or the sounds will continue repeating, it needs some sort of transistor switching where it sends the ground signal then goes back to a high state.
  • The toggle switch has 3 terminals - common, on, and off. I would like it to trigger S6 in one position, and S7 in the other position - but again the ground signals can't be held low or the sounds will repeat.
  • The momentary pushbutton switches have 5 terminals, common, on, off, LED plus, and LED minus. They should simply trigger sounds S0 through S3 when pressed, but again - if a button is held down the sound plays only once.

So pretty much all of the buttons should activate a single ground "pulse" to the FX triggers.
 

ElectricSpidey

Joined Dec 2, 2017
1,681
So you have 8 sounds you want to use, so get yourself a ULN2803A and take each output from the mom switches and connect it to an input on the ULN2803A then take the output and connect it to a Mini trigger. S0-S3 (if the press is too short, the trigger will not engage)

On your maintained pushbutton and the toggle you will need a cap in series with the ULN2803A output or input, then a very high value resistor across the cap to discharge it.

I've used the FX Mini and I believe you need something like a 125 ms pulse,

It's not really that critical as long as it's longer than 125 ms,
 
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