# 0 or 5V signal to reverse 180V DC neon bulb current using transistors?

Thread Starter

#### jwolfe138

Joined Mar 24, 2021
4
Thanks in advance for any help you guys can offer!

I've got a design idea that's driving me crazy. I'm trying to design a circuit that will reverse the current through an NE-2 type of neon bulb based on an input of 0 or 5V (say, from an Arduino pin). Functionally, I want to be able to input 5V into the circuit and make the current run "forward" through the NE-2 and light one of its two filaments, or input 0V and have the current run "backward" and light the other filament. Input voltage to the neon is 180V, with a 270k current limiting resistor.

I imagined that this would be an easy circuit to build with MPSA42 NPN transistors to open the "forward" circuit and MPSA92 PNP transistors to open the "reverse" circuit, but I'm tearing my hair out trying to figure out how to get all of the base to emitter voltages to work out correctly. I can provide a sketch and/or some of my failed attempts at this circuit if it would help to understand my idea.

Anyone know of a circuit design that can perform this function? My circuit design skills are not up to the task... Thank you!

#### ericgibbs

Joined Jan 29, 2010
13,652
hi 138,
Consider two High voltage NPN's, the neon is connected to across the Collectors.
Each Collector has a pull up resistor to the High voltage.
Switching either one or the other transistor will change the current direction thru the neon.
E

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#### wayneh

Joined Sep 9, 2010
17,153
That's clever. I was going to recommend a high-voltage H-bridge. You can probably find an off-the-shelf one on eBay and it'll be cheaper than you could build yourself.

#### djsfantasi

Joined Apr 11, 2010
7,839
Very clever... the TS didn’t specify if he was limited to one GPIO (Arduino?) pin. Using two pins as you did is the easiest way.

Thread Starter

#### jwolfe138

Joined Mar 24, 2021
4
Thanks for the idea! That circuit will work if I use 2 output pins from a controller.

What if I wanted to control both directions of the current with 1 pin, with "forward" when the signal is high and "reverse" when the pin is low? One idea I had was to get both sides of the neon to light (at least appear to be lit by persistence of vision) by sending an oscillating signal from a 555 circuit, which would make it oscillate between the two current directions. Or I could make it fade from one cathode to the other by fading a PWM signal up and down...

Last edited:

#### crutschow

Joined Mar 14, 2008
27,750
What if I wanted to control both directions of the current with 1 pin,
You can add an inverter circuit (a transistor or logic inverter) to generate the signal to the other side.

Thread Starter

#### jwolfe138

Joined Mar 24, 2021
4
Aha! This circuit should do the trick, correct?

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#### crutschow

Joined Mar 14, 2008
27,750
This circuit should do the trick, correct?
Yes, although you don't need a high voltage transistor for NPN3.

Thread Starter

#### jwolfe138

Joined Mar 24, 2021
4
Aha! This circuit should do the trick, correct?

EDIT: I breadboarded up the circuit and it works like a champ . Thanks for the help!