I'm building an 8 stage coil gun for a physics project, and I want to verify this circuit as I've never used a phototransistor before and I'm still pretty new to mosfets. Here is a schematic:

enter image description here

The idea is that when the projectile (a penny in this case) crosses between the ir led and the phototransistor, the coil triggers until the penny exits the photogate. It will also light the 6 2v red LED's for the duration. I'm not sure that Iwired the mosfets to the phototransistor properly. I know that the load should be attached between v+ and the collector of the phototransistor, but that would require routing the potentially 100 amps of coil current through the phototransistor and remove the purpose of even having the mosfet there. would putting a pullup resistor between the collector and 5v and attaching the Mosfets' gates to the emitter work as intended?

As a side note, the coil will only be on for a tiny fraction of a second. If I hook up the red LED's to the red channel of rgb LED's that have blue on constantly, will a purple wave travelling down the stages be visible, or will I need to extend the red channel pulse? I could use a capacitor charging circuit between two NAND gates I guess, but I want to minimize unnecessary circuitry so it can all fit in a small-ish package.

Thanks for the help; I'll probably be back shortly with some troubleshooting for the actual coils haha.

EDIT: this is the phototransistor I'm using: https://www.sparkfun.com/datasheets/Components/LTR-301.pdf

EDIT 2: could I hook up 12 v to the phototransistor, or would I risk overvolting the mosfet? I need to break the mosfet's activation voltage, which the pullup resistor might prevent from happening.... Also not having to route a 5v line helps.

  • \$\begingroup\$ Have you never heard about current limiting LEDs? How about load sharing on one transistor? \$\endgroup\$
    – Andy aka
    Commented Nov 26, 2015 at 21:05
  • \$\begingroup\$ I think you should put 2P 3S config for 6 red leds. Everything in series might just not work. Considering the voltage drop. Just a speculation. \$\endgroup\$
    – ammar.cma
    Commented Nov 26, 2015 at 22:37
  • \$\begingroup\$ @Andyaka What do you mean current limiting LED's? I decided against powering the LED's off of the same transistor, because that has a large voltage range, (It's a 6s LiPo) and I didn't want the brightness to vary that much. The extra fet doesn't take up too much room so I'm not too worried. \$\endgroup\$
    – Toms42
    Commented Nov 26, 2015 at 22:40
  • \$\begingroup\$ @ammar.cma I need the 6s configuration for the coils, and the LED's are rated for a 2v voltage drop. The series configuration should be more efficient than a bunch of resistors in parallel, so I did that. I might add a small resistor as per Wesley's comment. \$\endgroup\$
    – Toms42
    Commented Nov 26, 2015 at 22:47
  • \$\begingroup\$ So much for "thanks for the help" \$\endgroup\$
    – Andy aka
    Commented Nov 26, 2015 at 22:51

1 Answer 1


Is the penny blocking the light or reflecting? Depending on what its supposed to do you might have it reversed. When the phototransistor receives light and conducts the FETs would be ON.

Except that you have a voltage divider across the gate: R3 + transistor + 1k, then gate, and then 10k. So you'd be lucky if your gate saw 2.5V in this config, which is a bit low to turn mosfets on. I'd use smaller gate resistors and a smaller R3.

Also I'd add a small resistor in series with the LEDs just in case..

p.s.: yes you can use 12V at these FETs gates. Both are rated up to 20Vgs.

  • \$\begingroup\$ Oh you're right. It is blocking so I moved the connection for the gates to above the phototransistor, so the fets are off when the ir led is not blocked.I didn't realize they could handle 20v, so I hooked them up to 12v instead. Here is the new schematic: i.imgur.com/BPT8htj.png Thanks! \$\endgroup\$
    – Toms42
    Commented Nov 26, 2015 at 22:36

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