3 switched earth's to a decoder to produce 6 earth outputs

[Ozzy]

HEEELLLPP

Hi all I'm new here and am in need of some help.

I'm trying to design a component circuit that would allow me to use 3 switched earth's, that go through a decoder which outputs 6 earth outputs.

The outputs will be the earth for a display, I have been looking up 3-7 decoders but can't find the the chip set I would use!

I have a 12vdc to use for power too.

I have some ideas on a sheet that I will attach, please ignore the terrible imitation of relays I have drawn..

 

[dorke]

Welcome to EP.
You didn't find a 3 to 7 decoder cause there isn't a commercial one.
What you should use is a 3 to 8 one. like the 74HC138 etc.

 

[Ozzy]

Welcome to EP.
You didn't find a 3 to 7 decoder cause there isn't a commercial one.
What you should use is a 3 to 8 one. like the 74HC138 etc.

Yeah I was looking at those, thing is will I be able to input low and output low, will these low outputs be able to be used as an earth? Which ic would you recommend for that? Would the 74HC138 work? I used that on multisim but couldn't get it to operate.

 

[Ozzy]

Welcome to EP.
You didn't find a 3 to 7 decoder cause there isn't a commercial one.
What you should use is a 3 to 8 one. like the 74HC138 etc.

Thanks for the welcome.

I think I meant a 3-8 I only need a 3-6 but wont use two pins.

Will I need to use pull up resistors for the circuit? Which Voltage chip should I got for too?

 

[kellys_eye]

Which Voltage chip should I got for too?

5V if you use 74-series logic or 6V to 12V (or thereabouts) if you use the 4000-series CMOS.

It's usual (but not definitive) to use pull up resistors and ground the inputs if you are using switches as the input device. If you are driving the inputs from other devices you can omit pull ups and just go with the appropriate logic levels - assuming they 'match' of course.

 

[AnalogKid]

Note that most logic families (TTL, CMOS, whatever) have output stages that can source or sink only a few mA. You've given us almost zero information about your circuit's inputs, outputs, and the "display", but the odds that a logic chip output can drive the display directly, without some kind of driver component in between, is almost zero.

ak

 

[Ozzy]

Note that most logic families (TTL, CMOS, whatever) have output stages that can source or sink only a few mA. You've given us almost zero information about your circuit's inputs, outputs, and the "display", but the odds that a logic chip output can drive the display directly, without some kind of driver component in between, is almost zero.

ak

Yes forgive me for the ambiguous question.

So I have a switch that has an earth input and 3 outputs, depending on the position of the 6 state switch an earth is produced at the outputs of the switch (the truth table that is in the picture on the post) I would like to take these earth's decode them so that they can provide the earth needed to illuminate a display (basically converting the 3 inputs to 6 outputs) I believe that the display already has all the associated drivers in them as it's a fully assembled component, the instructions for this unit says that it only needs an earth at one of the 6 inputs to display the number wanted.

If the ic is not suitable for providing a suitable earth how would I use it to pull an earth when needed?

The project is for a motorbike, I have a gear selector switch and an aftermarket speedo, I am trying to make a component to put in-line as my bike has 3 output but the speedo has 6 inputs

 

[dorke]

Please provide a link or photo of your devices and instructions.

 

[Ozzy]

Please provide a link or photo of your devices and instructions.

I am trying to get hold of them, will post when I do

 

[Ozzy]

Please provide a link or photo of your devices and instructions.

Just as ambiguous as my question here is a rough schematic I have found, for each of the gear indicator lines it asks for a GND.

 

[Ozzy]

Just as ambiguous as my question here is a rough schematic I have found, for each of the gear indicator lines it asks for a GND.

The actual instructions im trying to look for atm, they are in very baddly written English and is hard to make sense of but I will do when I find them!

I do appreciate the help!

 

[AnalogKid]

I believe that the display already has all the associated drivers in them as it's a fully assembled component, the instructions for this unit says that it only needs an earth at one of the 6 inputs to display the number wanted.

The question is whether that is a low current signal ground or a power ground with all of the display current. Also (this may be a British/American thing), since no part of the bike's systems are tied to a copper stake driven into the dirt underneath it, a better term for the circuit reference potential is ground, not earth. Earth ground is a specific thing.

ak

 

[Ozzy]

The question is whether that is a low current signal ground or a power ground with all of the display current. Also (this may be a British/American thing), since no part of the bike's systems are tied to a copper stake driven into the dirt underneath it, a better term for the circuit reference potential is ground, not earth. Earth ground is a specific thing.

ak

Ahhh OK, so what would I do for both of those situations, I guess with a low current signal then I could hook it straight up to the IC, but if it was a power ground could I use transistors to switch in a power ground using the signal from the IC

 

[Ozzy]

Here is the wiring diagram for the actual bike, you will notice that the gear position switch is up at the top left and the original instrumentation on the very left, can any of this information help you?

 

[AnalogKid]

Ahhh OK, so what would I do for both of those situations, I guess with a low current signal then I could hook it straight up to the IC, but if it was a power ground could I use transistors to switch in a power ground using the signal from the IC

Yup.

 

[Ozzy]

Yup.

If I were to switch a power ground in how would I do that with transistors exactly?

 

[AnalogKid]

What is the open circuit voltage?
What is the closed circuit current?

ak

 

[Ozzy]

Just as ambiguous as my question here is a rough schematic I have found, for each of the gear indicator lines it asks for a GND.

As requested here are the Instructions