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In the circuit below, I'm trying to replace the physical switch on the 'flipper opto switch board' (and 'power driver board') with a transistor, controlled by Arduino.

My understanding (please correct where I am wrong, learner here) is that I should be able to use a NPN transistor, with Collector tied to the 12v feed from the fliptronics board. The transistor needs to be able to pass 12V at ~12ma

I have tried using a 2n4401 that I have stock of, and I believe this has a hfe of 80, so I run the 5v arduino output through a 2k resistor, which should pass around 0.25ma to the base? (And be enough to switch the transistor 'on', dragging the 12v feed to the LM339 down to below 5v)

But nothing happens.

Entirely likely I'm making a fundamental error here, but I'm not sure what it is. Any help appreciated.

enter image description here

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schematic

simulate this circuit – Schematic created using CircuitLab

Peter Bennett is right.

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  • \$\begingroup\$ Thank you so much! Working nicely. Think my rookie mistake was not connecting arduino ground to anything! \$\endgroup\$ – PaulG May 20 '14 at 18:05
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At 12 mA wouldn't work if you directly connect X to Arduino?

Anyway this is what you get by redrawing the circuit without the LED:

schematic

simulate this circuit – Schematic created using CircuitLab

Any general purpose NPN should work (BC547, 2N3904).

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  • \$\begingroup\$ If I'm understanding correctly, you're suggesting I still need the 12v from the power driver board, even though it just comes in through 6, then out through 3 again? Apologies if this is a dumb question. \$\endgroup\$ – PaulG May 20 '14 at 14:37
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    \$\begingroup\$ As I see it, R1 and the connection to the power driver board are not needed. The Arduino ground must be connected to the Fliptronic board ground. \$\endgroup\$ – Peter Bennett May 20 '14 at 16:05
  • \$\begingroup\$ @PaulG 3 is the ground. There is a diode and a resistor across 6 and 3. \$\endgroup\$ – Cornelius May 20 '14 at 16:18

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