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Question on Wireless Key Fob and MOSFET Solenoid Control — Parallax Forums

Question on Wireless Key Fob and MOSFET Solenoid Control

GronkGronk Posts: 3
in Accessories
Greetings! I purchased a 700-10016 Wireless Key Fob, with the intention of having it control a solenoid using a MOSFET as a relay.

My issue is that I can't get the it to actually trigger the solenoid - or maybe I can't get it to trigger it long enough for it to move?

I've included a picture of my breadboard; the solenoid is connected across the diode shown between the MOSFET and the power supply board. When I move the red jumper wire from VT on the receiver to VCC, the solenoid is engaged through the MOSFET. But none of the VT/D1/D2/D3/D4 pins seem to be effective at what I need.

I saw another comment - http://forums.parallax.com/discussion/comment/1185507/#Comment_1185507 - stating that perhaps the receiver board needs to be configured to latch the signal on? I would like the solenoid to engage when the button is pressed and disengage when it is let up. I think I might need the "signal inter-locking (Latch )" mode of operation noted in that comment.

So, first of all: does my wiring make sense? I haven't breadboarded anything in 25 years. :)

And second of all: Will I need to connect a couple jumper pads to make it work the way I'm intending?

Thanks in advance for anyone who can help! :)

- Mike (Gronk)
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Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    What's the device number for your MOSFET?

    -Phil
  • GronkGronk Posts: 3
    Phil - It is a RFP30N06LE N-channel MOSFET.

    I based my circuit on guidance from a bildr.org MOSFET power tutorial, and I also modeled my circuit using Autodesk's 123D circuits lab.

    - Mike
  • JonMJonM Posts: 318
    I have something similar but using the key fob to trigger a relay switch. However, I have a 2N2222 transistor in the mix where a pin from the Receiver is connected to the base of the transistor via a 1K resistor. You might want to add a 1k or similar resistor between VT and Gate of your MOSFET device.

    This is what I have connected:
    VCC - 5V
    GND - Ground
    VT or D4 - 1K -> 2N2222 base.

    I did not connect the other ground so I am not sure if it is needed or not.

    With VT, you should get a signal from the receiver for any key pressed. Also, the LED on the Key Fob receiver should light when you press a key on the Transmitter. If none of this is happening, I would simplify your circuit and just supply ground and 5v to the receiver and see if it is receiving a signal from the transmitter; maybe put a LED on VT to see if that will light to ensure the device is working.

    Are the Red and White wires from your power supply board the same as the 5V and GND on the edges of the board? You might what to check that as well to ensure the correct voltage is coming out.

  • garyggaryg Posts: 420
    Hi
    You have maybe noticed this, but I thought I'd chime in anyway.
    In the photo, I would imagine that the red circuit board is a power supply?
    If it is, the diode connected to the center pin of the mosfet appears to be backwards to me.
    I'm thinking you are not getting the + voltage to your mosfet.
    ..
    Just a thought
  • JonMJonM Posts: 318
    It would depend on where the solenoid is plugged into. If it's positive is connected to where the red wire is plugged into the bread board and the other end is connected to where the white jumper from the drain of the MOSFET is connected to the diode, then the diode should be good. That is if you match the pic with the Arduino reference. However, not knowing anything about the red board in the pic, but if the Red and White wires from the board are for the solenoid, then I would move the White wire to where the diode and the white jumper wire from the drain of the MOSFET join on the BB.
  • GronkGronk Posts: 3
    GaryG and JonM -

    Thanks for writing back on this.

    The red board is indeed a power supply board - it's a 5V regulated board from SparkFun. The red and white wires are the unswitched/unregulated 13.8VDC coming in from the wall wart providing the power. The power rails on the breadboard are at 5V. So, this is intended to provide 5V for the receiver and MOSFET, while allowing 13.8V for the solenoid (it's a bigger one that takes up to 36V). The red wire is the +13.8V, and the white wire is tied to the common ground. That much of the circuit (including the diode) does seem to be working, as applying 5V to the MOSFET gate does indeed cause the solenoid to close, and removing the 5V causes it to open again.

    I have also tried a voltage divider for the MOSFET, putting a 1k resistor between VT and gate, and another 1k resistor between gate and ground. Again, no dice.

    I'm thinking that perhaps the receiver is actually pulsing instead of latching on, but I don't have an oscilloscope so I can't check the voltage to make sure. I tried shorting the jumpers on the receiver so that it would latch on, also to no effect. Perhaps I actually need to solder in a switch there instead of just trying to short the jumper with a small screwdriver? :)

    No, wait; I do have a scope - I have an old Heathkit IO-12 scope that I picked up as a toy at a HamFest. It doesn't have its own built-in sweep controls, but I wonder if I can get it to register a deflection for that 5V.

    - Mike
  • JonMJonM Posts: 318
    The Key Fob device I have, which I believe is the same the one you show, latches when a button on the transmitter is pressed and then releases with the release of the button. I have not see it pulse on me yet.
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