# 12 DC OR Gate, 2 +12v inputs, 1 +12v output.

#### Steve Thames

Joined Oct 6, 2019
3
A) I have a circuit that activates at 1VDC input and deactivates at 12VDC input.
B) I have a 12VDC input that will start at 1VDC and, after a second or two, change to 12VDC. Sometimes it stays at 1VDC. This input is currently connected to A.
C) I have a 12VDC input that will start with an open circuit and, after a second or two, change to 12VDC.

I want to build a circuit that takes B and C as inputs and delivers 1V or 12V to A from either input. This seems like an OR gate to me, something like this: It looks like the diode model is the easiest but it's been many years since electronics class in high school so I don't understand how the circuit works.
1. Combining two 12V inputs will produce 24V, will it not. Why will this diode circuit not pass 24V to the output?
2. How do I determine the size of the diodes and resistor?
3. What is the size of the resistor and why do I need it?
4. Will this circuit pass the 1V input to the output as well as the 12V input? If not, how do I need to modify it?

Joined Mar 10, 2018
4,057
1. Combining two 12V inputs will produce 24V, will it not. Why will this diode circuit not pass 24V to the output?
2. How do I determine the size of the diodes and resistor?
3. What is the size of the resistor and why do I need it?
4. Will this circuit pass the 1V input to the output as well as the 12V input? If not, how do I need to modify it?
1) No, you will only see ~ 12 V - Vdiode (~.7V) on output. In fact you will see whichever
12 V is highest.
2) Diode should be ~ 25 V breakdown, rated at 2X the current you set with R. R
is determined by what is downstream load. If load is hi Z then diode could be a small
signal diode, 1N914 like. And R ~ 1K ohms would be fine. Need more info on what
3) The R may not be needed if your load is resistive, where the 12V is destined to go.
But having a R << load R makes the 12V look like a V source and the V relatively much
less sensitive to changes in load R.
4) No, the diode drops, nominal current 10's of mA, ~ .7V, so 1V in results in .3V out.

Regards, Dana.

#### Steve Thames

Joined Oct 6, 2019
3
Load is uncertain but it can’t be much. At 1v, it lights a dashboard led and sends a signal for a message to show on an LCD display. At 12v, light and msg are not activated So this is clearly some kind of signal rather than a power supply.

4) No, the diode drops, nominal current 10's of mA, ~ .7V, so 1V in results in .3V out.
I’m guessing the 1V is required so do you have another suggestion?

#### eetech00

Joined Jun 8, 2013
2,322
HI

You could try something like this.
The input to the voltage divider sets the input voltage at the base of Q2. If the input is 1v, the base is at .5v and Q1 is off.
If the input is 12v, the base is at 6v and Q1 turns on. The divider at the output of Q1 sets the output voltage level to 1v or 12v.
If In1 or In2 changes to 12v, Q1 turns on and A goes to 1v, otherwise A is at 12v

eT #### Steve Thames

Joined Oct 6, 2019
3
Ok, if I understand this, V1 is a constant 12v supplying current to A. I have some questions which will be stupid because, as I said, it's been many years since electronics class:
• What affect will R5 have on V1 while Q1 is off? What voltage will I see on A?
• What is the purpose of R6 and why is there not a resistor on V2?
A) I have a circuit that activates at 1VDC input and deactivates at 12VDC input.
B) I have a 12VDC input that will start at 1VDC and, after a second or two, change to 12VDC. Sometimes it stays at 1VDC. This input is currently connected to A.
• B (connected to A) rises to 1v for one second and then rises to 12v, deactivating the A circuit. Sometimes, it stays at 1v which keeps the A circuit in an activated state. I need A to rise to 12v from either V2 or V3 going to 12v.
Q1 will switch on when V2 or V3 rises to 12v. Will this not reduce the voltage on A to 1v?

#### eetech00

Joined Jun 8, 2013
2,322
Ok, if I understand this, V1 is a constant 12v supplying current to A. I have some questions which will be stupid because, as I said, it's been many years since electronics class:
• What affect will R5 have on V1 while Q1 is off? What voltage will I see on A?
Think of Q1 as a current controlled resistor. When Q1 is off the resistance between the collector and emitter is very large so almost no current flows. So A will equal 12v. When Q1 is on, R5 drops about 11 volts, so A is about 1v. You can adjust R5 and R4 for the correct voltage as needed.

• What is the purpose of R6 and why is there not a resistor on V2?
R6 is there to hold the input at a known state (voltage) when open.
• B (connected to A) rises to 1v for one second and then rises to 12v, deactivating the A circuit. Sometimes, it stays at 1v which keeps the A circuit in an activated state. I need A to rise to 12v from either V2 or V3 going to 12v.
That wasn't clear from your spec.

• Q1 will switch on when V2 or V3 rises to 12v. Will this not reduce the voltage on A to 1v?
Yes...A will be at 1v. But you can add another NPN between the base of Q1 and the input divider to invert the output voltage state.

eT

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