1 input 2 outputs logic

Question

I have a system that can only supply very low current > 10mA at 12v. It has one output that will pull high or low depending on if it is powered on or not. I have another device I would like to control based on that output, it has an up, common, and down. When connecting up and common it turns on, when connecting down and common it turns off.

I was originally going to use a 5 pole relay to do the switching from common to the respective up and down (NC: common -> down) (NO: common -> up) but because the system is such low current I can't trigger the coil on the relay.

Could someone point me in the right direction for doing this with transistors? I have a very very basic understanding but I have been using circuit lab to try to build such a thing that sends output1 low, output2 high when given a high input, and then when given a low input sends output1 high, output2 low but I haven't had much luck.

For further clarification I have a controller that has a 12v power supply capable of 10mA (current limited internally), it has a switched output that can do 12v at 2.5mA (again current limited internally)

I then have a system that has a floating (meaning I can bring the voltage up or down it's not ground referenced) and it has "common", "up", and "down" terminals. To turn it on you need to allow current to flow from common to up, to turn it off you need to allow it to flow from common to down. This system is also limited, and a perfect 1mA flows from common to either up or down when connecting it.

So these systems are very low current and protected.

I am thinking I need something like a NOT Gate and maybe a regular Gate if such things exist to pull "up" or "down" to "common" depending on if the input (output of the first thing) is high or low.

Answer 1

If the relay would work for you and the only problem is that your control signal can't supply enough current to activate the relay, use this scheme to drive the relay:

Source: https://electronics.stackexchange.com/a/56097/95488

The "load" is your relay coil. +Vs is your 12V supply. Try 2K for RB. The transistor can be any common NPN type -- 2n3904, 2n2222. Put in the protection diode. See the source link for more details.

Update: Based on the comments, if the control signal will be either +12V or 0V you could do this:

^{simulate this circuit – Schematic created using CircuitLab}

If the control signal only outputs something like 0 or 5V you can use the circuit on the right to invert it.

The 2K resistor is picked for about 5mA of LED current.

Instead of using the NPN transistor on the left you might be able to drive the optocoupler LED directly from the control signal if it can supply enough current.

For the transistor side of the optocouplers, connect COM to the collectors and UP and DOWN to emitters. That will allow current to flow from COM to each of those terminals.

Answer 2

CMOS 4049 inverters have an output resistance around 300 ohms at Vcc=12V so tying then together reduces that to 100 ohms which at 10mA drops 1V so you can use a 13V supply if you like.

If you specify a tolerance on Vout , there are better FETs with much lower resistance.

But this is one solution that can be wired in an IC socket without a PCB. Just add a 0.1uF decoupling cap across 12V to 0V. So use the inverting 4049.

When the switch is closed 0V, the 1st stage out is Hi and 2nd stage is low. And visa versa.