# Simple circuit, Need help

Discussion in 'General Electronics Discussion' started by JHarmon, Jul 5, 2012.

1. ### JHarmon

20
0
Jun 30, 2012
(This circuit is a small part of a graphics processing unit)
(Still not very good with this diagramming program...)

When input 1 goes to 5V, the output needs to go to 1V (working).
When input 2 goes to 5V, the output needs to go to .3V (working).
When input 3 goes to 5V, the output needs to go to 0V (stumped).

So... how could I alter the circuit to make the output go to 0V when input 3 goes to 5V?
Only one input will be on at a time.

Edit: Ignore those 0 ohm notations...

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Last edited: Jul 5, 2012
2. ### weird_dave

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May 9, 2012
a pull down resistor on the output would ensure it can reach 0v when inputs 1 and 2 are not on, leaving input 3 to be quite happy to allow 0v out whether it's at 5v or not

What's going to be connected to your output and what's it supposed to do with no inputs at 5v?

3. ### JHarmon

20
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Jun 30, 2012
So like a 100k resistor to ground?

And it's connected to a composite video cable. With no input, it should be ground.

4. ### CraziestOzzy

18
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Jul 4, 2012
assuming I am not allowed to use the easy option and not complete the circuit from input 3 to output to get the zero volts you need and assuming I am allowed to use any device to achieve the given the task of attaining zero volts and assuming I ignore amperage and current values for the input....

...simply connect a PNP such as 2n3906 with the base connected directly to input 3, the emitter connected to ground (throw in a 1k resister to make your teacher feel good) and have the collector connected to the output...and you will get zero volts.

...and to answer your question regards using a 100k value resistor to ground, it is common knowledge such a value will do squat to a 5 volt input.

I personally like the common sense idea of simply not connecting the circuit from input three to output and leaving it as it is in your first post circuit diagram..

5. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
651
May 8, 2012
Before I suggest or draw anything I don't see how 1 & 2 can be true unless the output is being loaded by the input to your graphics unit. What's the input impedance of the unit?